src/cmd/compile/internal/typecheck/typecheck.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package typecheck

import (
	"fmt"
	"go/constant"
	"go/token"
	"strings"

	"cmd/compile/internal/base"
	"cmd/compile/internal/ir"
	"cmd/compile/internal/types"
	"cmd/internal/src"
)

// Function collecting autotmps generated during typechecking,
// to be included in the package-level init function.
var InitTodoFunc = ir.NewFunc(base.Pos)

var inimport bool // set during import

var TypecheckAllowed bool

var (
	NeedRuntimeType = func(*types.Type) {}
)

func AssignExpr(n ir.Node) ir.Node { return typecheck(n, ctxExpr|ctxAssign) }
func Expr(n ir.Node) ir.Node       { return typecheck(n, ctxExpr) }
func Stmt(n ir.Node) ir.Node       { return typecheck(n, ctxStmt) }

func Exprs(exprs []ir.Node) { typecheckslice(exprs, ctxExpr) }
func Stmts(stmts []ir.Node) { typecheckslice(stmts, ctxStmt) }

func Call(pos src.XPos, callee ir.Node, args []ir.Node, dots bool) ir.Node {
	call := ir.NewCallExpr(pos, ir.OCALL, callee, args)
	call.IsDDD = dots
	return typecheck(call, ctxStmt|ctxExpr)
}

func Callee(n ir.Node) ir.Node {
	return typecheck(n, ctxExpr|ctxCallee)
}

func FuncBody(n *ir.Func) {
	ir.CurFunc = n
	errorsBefore := base.Errors()
	Stmts(n.Body)
	CheckUnused(n)
	CheckReturn(n)
	if ir.IsBlank(n.Nname) || base.Errors() > errorsBefore {
		n.Body = nil // blank function or type errors; do not compile
	}
}

var importlist []*ir.Func

// AllImportedBodies reads in the bodies of all imported functions and typechecks
// them, if needed.
func AllImportedBodies() {
	for _, n := range importlist {
		if n.Inl != nil {
			ImportedBody(n)
		}
	}
}

var traceIndent []byte

func tracePrint(title string, n ir.Node) func(np *ir.Node) {
	indent := traceIndent

	// guard against nil
	var pos, op string
	var tc uint8
	if n != nil {
		pos = base.FmtPos(n.Pos())
		op = n.Op().String()
		tc = n.Typecheck()
	}

	types.SkipSizeForTracing = true
	defer func() { types.SkipSizeForTracing = false }()
	fmt.Printf("%s: %s%s %p %s %v tc=%d\n", pos, indent, title, n, op, n, tc)
	traceIndent = append(traceIndent, ". "...)

	return func(np *ir.Node) {
		traceIndent = traceIndent[:len(traceIndent)-2]

		// if we have a result, use that
		if np != nil {
			n = *np
		}

		// guard against nil
		// use outer pos, op so we don't get empty pos/op if n == nil (nicer output)
		var tc uint8
		var typ *types.Type
		if n != nil {
			pos = base.FmtPos(n.Pos())
			op = n.Op().String()
			tc = n.Typecheck()
			typ = n.Type()
		}

		types.SkipSizeForTracing = true
		defer func() { types.SkipSizeForTracing = false }()
		fmt.Printf("%s: %s=> %p %s %v tc=%d type=%L\n", pos, indent, n, op, n, tc, typ)
	}
}

const (
	ctxStmt    = 1 << iota // evaluated at statement level
	ctxExpr                // evaluated in value context
	ctxType                // evaluated in type context
	ctxCallee              // call-only expressions are ok
	ctxMultiOK             // multivalue function returns are ok
	ctxAssign              // assigning to expression
)

// type checks the whole tree of an expression.
// calculates expression types.
// evaluates compile time constants.
// marks variables that escape the local frame.
// rewrites n.Op to be more specific in some cases.

var typecheckdefstack []*ir.Name

// Resolve ONONAME to definition, if any.
func Resolve(n ir.Node) (res ir.Node) {
	if n == nil || n.Op() != ir.ONONAME {
		return n
	}

	// only trace if there's work to do
	if base.EnableTrace && base.Flag.LowerT {
		defer tracePrint("resolve", n)(&res)
	}

	if sym := n.Sym(); sym.Pkg != types.LocalPkg {
		// We might have an ir.Ident from oldname or importDot.
		if id, ok := n.(*ir.Ident); ok {
			if pkgName := DotImportRefs[id]; pkgName != nil {
				pkgName.Used = true
			}
		}

		return expandDecl(n)
	}

	r := ir.AsNode(n.Sym().Def)
	if r == nil {
		return n
	}

	if r.Op() == ir.OIOTA {
		if x := getIotaValue(); x >= 0 {
			return ir.NewInt(x)
		}
		return n
	}

	return r
}

func typecheckslice(l []ir.Node, top int) {
	for i := range l {
		l[i] = typecheck(l[i], top)
	}
}

var _typekind = []string{
	types.TINT:        "int",
	types.TUINT:       "uint",
	types.TINT8:       "int8",
	types.TUINT8:      "uint8",
	types.TINT16:      "int16",
	types.TUINT16:     "uint16",
	types.TINT32:      "int32",
	types.TUINT32:     "uint32",
	types.TINT64:      "int64",
	types.TUINT64:     "uint64",
	types.TUINTPTR:    "uintptr",
	types.TCOMPLEX64:  "complex64",
	types.TCOMPLEX128: "complex128",
	types.TFLOAT32:    "float32",
	types.TFLOAT64:    "float64",
	types.TBOOL:       "bool",
	types.TSTRING:     "string",
	types.TPTR:        "pointer",
	types.TUNSAFEPTR:  "unsafe.Pointer",
	types.TSTRUCT:     "struct",
	types.TINTER:      "interface",
	types.TCHAN:       "chan",
	types.TMAP:        "map",
	types.TARRAY:      "array",
	types.TSLICE:      "slice",
	types.TFUNC:       "func",
	types.TNIL:        "nil",
	types.TIDEAL:      "untyped number",
}

func typekind(t *types.Type) string {
	if t.IsUntyped() {
		return fmt.Sprintf("%v", t)
	}
	et := t.Kind()
	if int(et) < len(_typekind) {
		s := _typekind[et]
		if s != "" {
			return s
		}
	}
	return fmt.Sprintf("etype=%d", et)
}

func cycleFor(start ir.Node) []ir.Node {
	// Find the start node in typecheck_tcstack.
	// We know that it must exist because each time we mark
	// a node with n.SetTypecheck(2) we push it on the stack,
	// and each time we mark a node with n.SetTypecheck(2) we
	// pop it from the stack. We hit a cycle when we encounter
	// a node marked 2 in which case is must be on the stack.
	i := len(typecheck_tcstack) - 1
	for i > 0 && typecheck_tcstack[i] != start {
		i--
	}

	// collect all nodes with same Op
	var cycle []ir.Node
	for _, n := range typecheck_tcstack[i:] {
		if n.Op() == start.Op() {
			cycle = append(cycle, n)
		}
	}

	return cycle
}

func cycleTrace(cycle []ir.Node) string {
	var s string
	for i, n := range cycle {
		s += fmt.Sprintf("\n\t%v: %v uses %v", ir.Line(n), n, cycle[(i+1)%len(cycle)])
	}
	return s
}

var typecheck_tcstack []ir.Node

func Func(fn *ir.Func) {
	new := Stmt(fn)
	if new != fn {
		base.Fatalf("typecheck changed func")
	}
}

func typecheckNtype(n ir.Ntype) ir.Ntype {
	return typecheck(n, ctxType).(ir.Ntype)
}

// typecheck type checks node n.
// The result of typecheck MUST be assigned back to n, e.g.
// 	n.Left = typecheck(n.Left, top)
func typecheck(n ir.Node, top int) (res ir.Node) {
	// cannot type check until all the source has been parsed
	if !TypecheckAllowed {
		base.Fatalf("early typecheck")
	}

	if n == nil {
		return nil
	}

	// only trace if there's work to do
	if base.EnableTrace && base.Flag.LowerT {
		defer tracePrint("typecheck", n)(&res)
	}

	lno := ir.SetPos(n)

	// Skip over parens.
	for n.Op() == ir.OPAREN {
		n = n.(*ir.ParenExpr).X
	}

	// Resolve definition of name and value of iota lazily.
	n = Resolve(n)

	// Skip typecheck if already done.
	// But re-typecheck ONAME/OTYPE/OLITERAL/OPACK node in case context has changed.
	if n.Typecheck() == 1 || n.Typecheck() == 3 {
		switch n.Op() {
		case ir.ONAME, ir.OTYPE, ir.OLITERAL, ir.OPACK:
			break

		default:
			base.Pos = lno
			return n
		}
	}

	if n.Typecheck() == 2 {
		// Typechecking loop. Trying printing a meaningful message,
		// otherwise a stack trace of typechecking.
		switch n.Op() {
		// We can already diagnose variables used as types.
		case ir.ONAME:
			n := n.(*ir.Name)
			if top&(ctxExpr|ctxType) == ctxType {
				base.Errorf("%v is not a type", n)
			}

		case ir.OTYPE:
			// Only report a type cycle if we are expecting a type.
			// Otherwise let other code report an error.
			if top&ctxType == ctxType {
				// A cycle containing only alias types is an error
				// since it would expand indefinitely when aliases
				// are substituted.
				cycle := cycleFor(n)
				for _, n1 := range cycle {
					if n1.Name() != nil && !n1.Name().Alias() {
						// Cycle is ok. But if n is an alias type and doesn't
						// have a type yet, we have a recursive type declaration
						// with aliases that we can't handle properly yet.
						// Report an error rather than crashing later.
						if n.Name() != nil && n.Name().Alias() && n.Type() == nil {
							base.Pos = n.Pos()
							base.Fatalf("cannot handle alias type declaration (issue #25838): %v", n)
						}
						base.Pos = lno
						return n
					}
				}
				base.ErrorfAt(n.Pos(), "invalid recursive type alias %v%s", n, cycleTrace(cycle))
			}

		case ir.OLITERAL:
			if top&(ctxExpr|ctxType) == ctxType {
				base.Errorf("%v is not a type", n)
				break
			}
			base.ErrorfAt(n.Pos(), "constant definition loop%s", cycleTrace(cycleFor(n)))
		}

		if base.Errors() == 0 {
			var trace string
			for i := len(typecheck_tcstack) - 1; i >= 0; i-- {
				x := typecheck_tcstack[i]
				trace += fmt.Sprintf("\n\t%v %v", ir.Line(x), x)
			}
			base.Errorf("typechecking loop involving %v%s", n, trace)
		}

		base.Pos = lno
		return n
	}

	typecheck_tcstack = append(typecheck_tcstack, n)

	n.SetTypecheck(2)
	n = typecheck1(n, top)
	n.SetTypecheck(1)

	last := len(typecheck_tcstack) - 1
	typecheck_tcstack[last] = nil
	typecheck_tcstack = typecheck_tcstack[:last]

	_, isExpr := n.(ir.Expr)
	_, isStmt := n.(ir.Stmt)
	isMulti := false
	switch n.Op() {
	case ir.OCALLFUNC, ir.OCALLINTER, ir.OCALLMETH:
		n := n.(*ir.CallExpr)
		if t := n.X.Type(); t != nil && t.Kind() == types.TFUNC {
			nr := t.NumResults()
			isMulti = nr > 1
			if nr == 0 {
				isExpr = false
			}
		}
	case ir.OAPPEND:
		// Must be used (and not BinaryExpr/UnaryExpr).
		isStmt = false
	case ir.OCLOSE, ir.ODELETE, ir.OPANIC, ir.OPRINT, ir.OPRINTN, ir.OVARKILL, ir.OVARLIVE:
		// Must not be used.
		isExpr = false
		isStmt = true
	case ir.OCOPY, ir.ORECOVER, ir.ORECV:
		// Can be used or not.
		isStmt = true
	}

	t := n.Type()
	if t != nil && !t.IsFuncArgStruct() && n.Op() != ir.OTYPE {
		switch t.Kind() {
		case types.TFUNC, // might have TANY; wait until it's called
			types.TANY, types.TFORW, types.TIDEAL, types.TNIL, types.TBLANK:
			break

		default:
			types.CheckSize(t)
		}
	}
	if t != nil {
		n = EvalConst(n)
		t = n.Type()
	}

	// TODO(rsc): Lots of the complexity here is because typecheck can
	// see OTYPE, ONAME, and OLITERAL nodes multiple times.
	// Once we make the IR a proper tree, we should be able to simplify
	// this code a bit, especially the final case.
	switch {
	case top&(ctxStmt|ctxExpr) == ctxExpr && !isExpr && n.Op() != ir.OTYPE && !isMulti:
		if !n.Diag() {
			base.Errorf("%v used as value", n)
			n.SetDiag(true)
		}
		if t != nil {
			n.SetType(nil)
		}

	case top&ctxType == 0 && n.Op() == ir.OTYPE && t != nil:
		if !n.Type().Broke() {
			base.Errorf("type %v is not an expression", n.Type())
			n.SetDiag(true)
		}

	case top&(ctxStmt|ctxExpr) == ctxStmt && !isStmt && t != nil:
		if !n.Diag() {
			base.Errorf("%v evaluated but not used", n)
			n.SetDiag(true)
		}
		n.SetType(nil)

	case top&(ctxType|ctxExpr) == ctxType && n.Op() != ir.OTYPE && n.Op() != ir.ONONAME && (t != nil || n.Op() == ir.ONAME):
		base.Errorf("%v is not a type", n)
		if t != nil {
			if n.Op() == ir.ONAME {
				t.SetBroke(true)
			} else {
				n.SetType(nil)
			}
		}

	}

	base.Pos = lno
	return n
}

// indexlit implements typechecking of untyped values as
// array/slice indexes. It is almost equivalent to DefaultLit
// but also accepts untyped numeric values representable as
// value of type int (see also checkmake for comparison).
// The result of indexlit MUST be assigned back to n, e.g.
// 	n.Left = indexlit(n.Left)
func indexlit(n ir.Node) ir.Node {
	if n != nil && n.Type() != nil && n.Type().Kind() == types.TIDEAL {
		return DefaultLit(n, types.Types[types.TINT])
	}
	return n
}

// typecheck1 should ONLY be called from typecheck.
func typecheck1(n ir.Node, top int) ir.Node {
	if n, ok := n.(*ir.Name); ok {
		typecheckdef(n)
	}

	switch n.Op() {
	default:
		ir.Dump("typecheck", n)
		base.Fatalf("typecheck %v", n.Op())
		panic("unreachable")

	case ir.OLITERAL:
		if n.Sym() == nil && n.Type() == nil {
			if !n.Diag() {
				base.Fatalf("literal missing type: %v", n)
			}
		}
		return n

	case ir.ONIL:
		return n

	// names
	case ir.ONONAME:
		if !n.Diag() {
			// Note: adderrorname looks for this string and
			// adds context about the outer expression
			base.ErrorfAt(n.Pos(), "undefined: %v", n.Sym())
			n.SetDiag(true)
		}
		n.SetType(nil)
		return n

	case ir.ONAME:
		n := n.(*ir.Name)
		if n.BuiltinOp != 0 {
			if top&ctxCallee == 0 {
				base.Errorf("use of builtin %v not in function call", n.Sym())
				n.SetType(nil)
				return n
			}
			return n
		}
		if top&ctxAssign == 0 {
			// not a write to the variable
			if ir.IsBlank(n) {
				base.Errorf("cannot use _ as value")
				n.SetType(nil)
				return n
			}
			n.SetUsed(true)
		}
		return n

	case ir.OLINKSYMOFFSET:
		// type already set
		return n

	case ir.OPACK:
		n := n.(*ir.PkgName)
		base.Errorf("use of package %v without selector", n.Sym())
		n.SetDiag(true)
		return n

	// types (ODEREF is with exprs)
	case ir.OTYPE:
		return n

	case ir.OTSLICE:
		n := n.(*ir.SliceType)
		return tcSliceType(n)

	case ir.OTARRAY:
		n := n.(*ir.ArrayType)
		return tcArrayType(n)

	case ir.OTMAP:
		n := n.(*ir.MapType)
		return tcMapType(n)

	case ir.OTCHAN:
		n := n.(*ir.ChanType)
		return tcChanType(n)

	case ir.OTSTRUCT:
		n := n.(*ir.StructType)
		return tcStructType(n)

	case ir.OTINTER:
		n := n.(*ir.InterfaceType)
		return tcInterfaceType(n)

	case ir.OTFUNC:
		n := n.(*ir.FuncType)
		return tcFuncType(n)
	// type or expr
	case ir.ODEREF:
		n := n.(*ir.StarExpr)
		return tcStar(n, top)

	// x op= y
	case ir.OASOP:
		n := n.(*ir.AssignOpStmt)
		n.X, n.Y = Expr(n.X), Expr(n.Y)
		checkassign(n, n.X)
		if n.IncDec && !okforarith[n.X.Type().Kind()] {
			base.Errorf("invalid operation: %v (non-numeric type %v)", n, n.X.Type())
			return n
		}
		switch n.AsOp {
		case ir.OLSH, ir.ORSH:
			n.X, n.Y, _ = tcShift(n, n.X, n.Y)
		case ir.OADD, ir.OAND, ir.OANDNOT, ir.ODIV, ir.OMOD, ir.OMUL, ir.OOR, ir.OSUB, ir.OXOR:
			n.X, n.Y, _ = tcArith(n, n.AsOp, n.X, n.Y)
		default:
			base.Fatalf("invalid assign op: %v", n.AsOp)
		}
		return n

	// logical operators
	case ir.OANDAND, ir.OOROR:
		n := n.(*ir.LogicalExpr)
		n.X, n.Y = Expr(n.X), Expr(n.Y)
		if n.X.Type() == nil || n.Y.Type() == nil {
			n.SetType(nil)
			return n
		}
		// For "x == x && len(s)", it's better to report that "len(s)" (type int)
		// can't be used with "&&" than to report that "x == x" (type untyped bool)
		// can't be converted to int (see issue #41500).
		if !n.X.Type().IsBoolean() {
			base.Errorf("invalid operation: %v (operator %v not defined on %s)", n, n.Op(), typekind(n.X.Type()))
			n.SetType(nil)
			return n
		}
		if !n.Y.Type().IsBoolean() {
			base.Errorf("invalid operation: %v (operator %v not defined on %s)", n, n.Op(), typekind(n.Y.Type()))
			n.SetType(nil)
			return n
		}
		l, r, t := tcArith(n, n.Op(), n.X, n.Y)
		n.X, n.Y = l, r
		n.SetType(t)
		return n

	// shift operators
	case ir.OLSH, ir.ORSH:
		n := n.(*ir.BinaryExpr)
		n.X, n.Y = Expr(n.X), Expr(n.Y)
		l, r, t := tcShift(n, n.X, n.Y)
		n.X, n.Y = l, r
		n.SetType(t)
		return n

	// comparison operators
	case ir.OEQ, ir.OGE, ir.OGT, ir.OLE, ir.OLT, ir.ONE:
		n := n.(*ir.BinaryExpr)
		n.X, n.Y = Expr(n.X), Expr(n.Y)
		l, r, t := tcArith(n, n.Op(), n.X, n.Y)
		if t != nil {
			n.X, n.Y = l, r
			n.SetType(types.UntypedBool)
			if con := EvalConst(n); con.Op() == ir.OLITERAL {
				return con
			}
			n.X, n.Y = defaultlit2(l, r, true)
		}
		return n

	// binary operators
	case ir.OADD, ir.OAND, ir.OANDNOT, ir.ODIV, ir.OMOD, ir.OMUL, ir.OOR, ir.OSUB, ir.OXOR:
		n := n.(*ir.BinaryExpr)
		n.X, n.Y = Expr(n.X), Expr(n.Y)
		l, r, t := tcArith(n, n.Op(), n.X, n.Y)
		if t != nil && t.Kind() == types.TSTRING && n.Op() == ir.OADD {
			// create or update OADDSTR node with list of strings in x + y + z + (w + v) + ...
			var add *ir.AddStringExpr
			if l.Op() == ir.OADDSTR {
				add = l.(*ir.AddStringExpr)
				add.SetPos(n.Pos())
			} else {
				add = ir.NewAddStringExpr(n.Pos(), []ir.Node{l})
			}
			if r.Op() == ir.OADDSTR {
				r := r.(*ir.AddStringExpr)
				add.List.Append(r.List.Take()...)
			} else {
				add.List.Append(r)
			}
			add.SetType(t)
			return add
		}
		n.X, n.Y = l, r
		n.SetType(t)
		return n

	case ir.OBITNOT, ir.ONEG, ir.ONOT, ir.OPLUS:
		n := n.(*ir.UnaryExpr)
		return tcUnaryArith(n)

	// exprs
	case ir.OADDR:
		n := n.(*ir.AddrExpr)
		return tcAddr(n)

	case ir.OCOMPLIT:
		return tcCompLit(n.(*ir.CompLitExpr))

	case ir.OXDOT, ir.ODOT:
		n := n.(*ir.SelectorExpr)
		return tcDot(n, top)

	case ir.ODOTTYPE:
		n := n.(*ir.TypeAssertExpr)
		return tcDotType(n)

	case ir.OINDEX:
		n := n.(*ir.IndexExpr)
		return tcIndex(n)

	case ir.ORECV:
		n := n.(*ir.UnaryExpr)
		return tcRecv(n)

	case ir.OSEND:
		n := n.(*ir.SendStmt)
		return tcSend(n)

	case ir.OSLICEHEADER:
		n := n.(*ir.SliceHeaderExpr)
		return tcSliceHeader(n)

	case ir.OMAKESLICECOPY:
		n := n.(*ir.MakeExpr)
		return tcMakeSliceCopy(n)

	case ir.OSLICE, ir.OSLICE3:
		n := n.(*ir.SliceExpr)
		return tcSlice(n)

	// call and call like
	case ir.OCALL:
		n := n.(*ir.CallExpr)
		return tcCall(n, top)

	case ir.OALIGNOF, ir.OOFFSETOF, ir.OSIZEOF:
		n := n.(*ir.UnaryExpr)
		n.SetType(types.Types[types.TUINTPTR])
		return n

	case ir.OCAP, ir.OLEN:
		n := n.(*ir.UnaryExpr)
		return tcLenCap(n)

	case ir.OREAL, ir.OIMAG:
		n := n.(*ir.UnaryExpr)
		return tcRealImag(n)

	case ir.OCOMPLEX:
		n := n.(*ir.BinaryExpr)
		return tcComplex(n)

	case ir.OCLOSE:
		n := n.(*ir.UnaryExpr)
		return tcClose(n)

	case ir.ODELETE:
		n := n.(*ir.CallExpr)
		return tcDelete(n)

	case ir.OAPPEND:
		n := n.(*ir.CallExpr)
		return tcAppend(n)

	case ir.OCOPY:
		n := n.(*ir.BinaryExpr)
		return tcCopy(n)

	case ir.OCONV:
		n := n.(*ir.ConvExpr)
		return tcConv(n)

	case ir.OMAKE:
		n := n.(*ir.CallExpr)
		return tcMake(n)

	case ir.ONEW:
		n := n.(*ir.UnaryExpr)
		return tcNew(n)

	case ir.OPRINT, ir.OPRINTN:
		n := n.(*ir.CallExpr)
		return tcPrint(n)

	case ir.OPANIC:
		n := n.(*ir.UnaryExpr)
		return tcPanic(n)

	case ir.ORECOVER:
		n := n.(*ir.CallExpr)
		return tcRecover(n)

	case ir.ORECOVERFP:
		n := n.(*ir.CallExpr)
		return tcRecoverFP(n)

	case ir.OUNSAFEADD:
		n := n.(*ir.BinaryExpr)
		return tcUnsafeAdd(n)

	case ir.OUNSAFESLICE:
		n := n.(*ir.BinaryExpr)
		return tcUnsafeSlice(n)

	case ir.OCLOSURE:
		n := n.(*ir.ClosureExpr)
		return tcClosure(n, top)

	case ir.OITAB:
		n := n.(*ir.UnaryExpr)
		return tcITab(n)

	case ir.OIDATA:
		// Whoever creates the OIDATA node must know a priori the concrete type at that moment,
		// usually by just having checked the OITAB.
		n := n.(*ir.UnaryExpr)
		base.Fatalf("cannot typecheck interface data %v", n)
		panic("unreachable")

	case ir.OSPTR:
		n := n.(*ir.UnaryExpr)
		return tcSPtr(n)

	case ir.OCFUNC:
		n := n.(*ir.UnaryExpr)
		n.X = Expr(n.X)
		n.SetType(types.Types[types.TUINTPTR])
		return n

	case ir.OGETCALLERPC, ir.OGETCALLERSP:
		n := n.(*ir.CallExpr)
		if len(n.Args) != 0 {
			base.FatalfAt(n.Pos(), "unexpected arguments: %v", n)
		}
		n.SetType(types.Types[types.TUINTPTR])
		return n

	case ir.OCONVNOP:
		n := n.(*ir.ConvExpr)
		n.X = Expr(n.X)
		return n

	// statements
	case ir.OAS:
		n := n.(*ir.AssignStmt)
		tcAssign(n)

		// Code that creates temps does not bother to set defn, so do it here.
		if n.X.Op() == ir.ONAME && ir.IsAutoTmp(n.X) {
			n.X.Name().Defn = n
		}
		return n

	case ir.OAS2:
		tcAssignList(n.(*ir.AssignListStmt))
		return n

	case ir.OBREAK,
		ir.OCONTINUE,
		ir.ODCL,
		ir.OGOTO,
		ir.OFALL,
		ir.OVARKILL,
		ir.OVARLIVE:
		return n

	case ir.OBLOCK:
		n := n.(*ir.BlockStmt)
		Stmts(n.List)
		return n

	case ir.OLABEL:
		if n.Sym().IsBlank() {
			// Empty identifier is valid but useless.
			// Eliminate now to simplify life later.
			// See issues 7538, 11589, 11593.
			n = ir.NewBlockStmt(n.Pos(), nil)
		}
		return n

	case ir.ODEFER, ir.OGO:
		n := n.(*ir.GoDeferStmt)
		n.Call = typecheck(n.Call, ctxStmt|ctxExpr)
		if !n.Call.Diag() {
			tcGoDefer(n)
		}
		return n

	case ir.OFOR, ir.OFORUNTIL:
		n := n.(*ir.ForStmt)
		return tcFor(n)

	case ir.OIF:
		n := n.(*ir.IfStmt)
		return tcIf(n)

	case ir.ORETURN:
		n := n.(*ir.ReturnStmt)
		return tcReturn(n)

	case ir.OTAILCALL:
		n := n.(*ir.TailCallStmt)
		n.Call = typecheck(n.Call, ctxStmt|ctxExpr).(*ir.CallExpr)
		return n

	case ir.OCHECKNIL:
		n := n.(*ir.UnaryExpr)
		return tcCheckNil(n)

	case ir.OSELECT:
		tcSelect(n.(*ir.SelectStmt))
		return n

	case ir.OSWITCH:
		tcSwitch(n.(*ir.SwitchStmt))
		return n

	case ir.ORANGE:
		tcRange(n.(*ir.RangeStmt))
		return n

	case ir.OTYPESW:
		n := n.(*ir.TypeSwitchGuard)
		base.Errorf("use of .(type) outside type switch")
		n.SetDiag(true)
		return n

	case ir.ODCLFUNC:
		tcFunc(n.(*ir.Func))
		return n

	case ir.ODCLCONST:
		n := n.(*ir.Decl)
		n.X = Expr(n.X).(*ir.Name)
		return n

	case ir.ODCLTYPE:
		n := n.(*ir.Decl)
		n.X = typecheck(n.X, ctxType).(*ir.Name)
		types.CheckSize(n.X.Type())
		return n
	}

	// No return n here!
	// Individual cases can type-assert n, introducing a new one.
	// Each must execute its own return n.
}

func typecheckargs(n ir.InitNode) {
	var list []ir.Node
	switch n := n.(type) {
	default:
		base.Fatalf("typecheckargs %+v", n.Op())
	case *ir.CallExpr:
		list = n.Args
		if n.IsDDD {
			Exprs(list)
			return
		}
	case *ir.ReturnStmt:
		list = n.Results
	}
	if len(list) != 1 {
		Exprs(list)
		return
	}

	typecheckslice(list, ctxExpr|ctxMultiOK)
	t := list[0].Type()
	if t == nil || !t.IsFuncArgStruct() {
		return
	}

	// Save n as n.Orig for fmt.go.
	if ir.Orig(n) == n {
		n.(ir.OrigNode).SetOrig(ir.SepCopy(n))
	}

	// Rewrite f(g()) into t1, t2, ... = g(); f(t1, t2, ...).
	RewriteMultiValueCall(n, list[0])
}

// RewriteMultiValueCall rewrites multi-valued f() to use temporaries,
// so the backend wouldn't need to worry about tuple-valued expressions.
func RewriteMultiValueCall(n ir.InitNode, call ir.Node) {
	// If we're outside of function context, then this call will
	// be executed during the generated init function. However,
	// init.go hasn't yet created it. Instead, associate the
	// temporary variables with  InitTodoFunc for now, and init.go
	// will reassociate them later when it's appropriate.
	static := ir.CurFunc == nil
	if static {
		ir.CurFunc = InitTodoFunc
	}

	as := ir.NewAssignListStmt(base.Pos, ir.OAS2, nil, []ir.Node{call})
	results := call.Type().FieldSlice()
	list := make([]ir.Node, len(results))
	for i, result := range results {
		tmp := Temp(result.Type)
		as.PtrInit().Append(ir.NewDecl(base.Pos, ir.ODCL, tmp))
		as.Lhs.Append(tmp)
		list[i] = tmp
	}
	if static {
		ir.CurFunc = nil
	}

	n.PtrInit().Append(Stmt(as))

	switch n := n.(type) {
	default:
		base.Fatalf("rewriteMultiValueCall %+v", n.Op())
	case *ir.CallExpr:
		n.Args = list
	case *ir.ReturnStmt:
		n.Results = list
	case *ir.AssignListStmt:
		if n.Op() != ir.OAS2FUNC {
			base.Fatalf("rewriteMultiValueCall: invalid op %v", n.Op())
		}
		as.SetOp(ir.OAS2FUNC)
		n.SetOp(ir.OAS2)
		n.Rhs = make([]ir.Node, len(list))
		for i, tmp := range list {
			n.Rhs[i] = AssignConv(tmp, n.Lhs[i].Type(), "assignment")
		}
	}
}

func checksliceindex(l ir.Node, r ir.Node, tp *types.Type) bool {
	t := r.Type()
	if t == nil {
		return false
	}
	if !t.IsInteger() {
		base.Errorf("invalid slice index %v (type %v)", r, t)
		return false
	}

	if r.Op() == ir.OLITERAL {
		x := r.Val()
		if constant.Sign(x) < 0 {
			base.Errorf("invalid slice index %v (index must be non-negative)", r)
			return false
		} else if tp != nil && tp.NumElem() >= 0 && constant.Compare(x, token.GTR, constant.MakeInt64(tp.NumElem())) {
			base.Errorf("invalid slice index %v (out of bounds for %d-element array)", r, tp.NumElem())
			return false
		} else if ir.IsConst(l, constant.String) && constant.Compare(x, token.GTR, constant.MakeInt64(int64(len(ir.StringVal(l))))) {
			base.Errorf("invalid slice index %v (out of bounds for %d-byte string)", r, len(ir.StringVal(l)))
			return false
		} else if ir.ConstOverflow(x, types.Types[types.TINT]) {
			base.Errorf("invalid slice index %v (index too large)", r)
			return false
		}
	}

	return true
}

func checksliceconst(lo ir.Node, hi ir.Node) bool {
	if lo != nil && hi != nil && lo.Op() == ir.OLITERAL && hi.Op() == ir.OLITERAL && constant.Compare(lo.Val(), token.GTR, hi.Val()) {
		base.Errorf("invalid slice index: %v > %v", lo, hi)
		return false
	}

	return true
}

// The result of implicitstar MUST be assigned back to n, e.g.
// 	n.Left = implicitstar(n.Left)
func implicitstar(n ir.Node) ir.Node {
	// insert implicit * if needed for fixed array
	t := n.Type()
	if t == nil || !t.IsPtr() {
		return n
	}
	t = t.Elem()
	if t == nil {
		return n
	}
	if !t.IsArray() {
		return n
	}
	star := ir.NewStarExpr(base.Pos, n)
	star.SetImplicit(true)
	return Expr(star)
}

func needOneArg(n *ir.CallExpr, f string, args ...interface{}) (ir.Node, bool) {
	if len(n.Args) == 0 {
		p := fmt.Sprintf(f, args...)
		base.Errorf("missing argument to %s: %v", p, n)
		return nil, false
	}

	if len(n.Args) > 1 {
		p := fmt.Sprintf(f, args...)
		base.Errorf("too many arguments to %s: %v", p, n)
		return n.Args[0], false
	}

	return n.Args[0], true
}

func needTwoArgs(n *ir.CallExpr) (ir.Node, ir.Node, bool) {
	if len(n.Args) != 2 {
		if len(n.Args) < 2 {
			base.Errorf("not enough arguments in call to %v", n)
		} else {
			base.Errorf("too many arguments in call to %v", n)
		}
		return nil, nil, false
	}
	return n.Args[0], n.Args[1], true
}

// Lookdot1 looks up the specified method s in the list fs of methods, returning
// the matching field or nil. If dostrcmp is 0, it matches the symbols. If
// dostrcmp is 1, it matches by name exactly. If dostrcmp is 2, it matches names
// with case folding.
func Lookdot1(errnode ir.Node, s *types.Sym, t *types.Type, fs *types.Fields, dostrcmp int) *types.Field {
	var r *types.Field
	for _, f := range fs.Slice() {
		if dostrcmp != 0 && f.Sym.Name == s.Name {
			return f
		}
		if dostrcmp == 2 && strings.EqualFold(f.Sym.Name, s.Name) {
			return f
		}
		if f.Sym != s {
			continue
		}
		if r != nil {
			if errnode != nil {
				base.Errorf("ambiguous selector %v", errnode)
			} else if t.IsPtr() {
				base.Errorf("ambiguous selector (%v).%v", t, s)
			} else {
				base.Errorf("ambiguous selector %v.%v", t, s)
			}
			break
		}

		r = f
	}

	return r
}

// typecheckMethodExpr checks selector expressions (ODOT) where the
// base expression is a type expression (OTYPE).
func typecheckMethodExpr(n *ir.SelectorExpr) (res ir.Node) {
	if base.EnableTrace && base.Flag.LowerT {
		defer tracePrint("typecheckMethodExpr", n)(&res)
	}

	t := n.X.Type()

	// Compute the method set for t.
	var ms *types.Fields
	if t.IsInterface() {
		ms = t.AllMethods()
	} else {
		mt := types.ReceiverBaseType(t)
		if mt == nil {
			base.Errorf("%v undefined (type %v has no method %v)", n, t, n.Sel)
			n.SetType(nil)
			return n
		}
		CalcMethods(mt)
		ms = mt.AllMethods()

		// The method expression T.m requires a wrapper when T
		// is different from m's declared receiver type. We
		// normally generate these wrappers while writing out
		// runtime type descriptors, which is always done for
		// types declared at package scope. However, we need
		// to make sure to generate wrappers for anonymous
		// receiver types too.
		if mt.Sym() == nil {
			NeedRuntimeType(t)
		}
	}

	s := n.Sel
	m := Lookdot1(n, s, t, ms, 0)
	if m == nil {
		if Lookdot1(n, s, t, ms, 1) != nil {
			base.Errorf("%v undefined (cannot refer to unexported method %v)", n, s)
		} else if _, ambig := dotpath(s, t, nil, false); ambig {
			base.Errorf("%v undefined (ambiguous selector)", n) // method or field
		} else {
			base.Errorf("%v undefined (type %v has no method %v)", n, t, s)
		}
		n.SetType(nil)
		return n
	}

	if !types.IsMethodApplicable(t, m) {
		base.Errorf("invalid method expression %v (needs pointer receiver: (*%v).%S)", n, t, s)
		n.SetType(nil)
		return n
	}

	n.SetOp(ir.OMETHEXPR)
	n.Selection = m
	n.SetType(NewMethodType(m.Type, n.X.Type()))
	return n
}

func derefall(t *types.Type) *types.Type {
	for t != nil && t.IsPtr() {
		t = t.Elem()
	}
	return t
}

// Lookdot looks up field or method n.Sel in the type t and returns the matching
// field. It transforms the op of node n to ODOTINTER or ODOTMETH, if appropriate.
// It also may add a StarExpr node to n.X as needed for access to non-pointer
// methods. If dostrcmp is 0, it matches the field/method with the exact symbol
// as n.Sel (appropriate for exported fields). If dostrcmp is 1, it matches by name
// exactly. If dostrcmp is 2, it matches names with case folding.
func Lookdot(n *ir.SelectorExpr, t *types.Type, dostrcmp int) *types.Field {
	s := n.Sel

	types.CalcSize(t)
	var f1 *types.Field
	if t.IsStruct() {
		f1 = Lookdot1(n, s, t, t.Fields(), dostrcmp)
	} else if t.IsInterface() {
		f1 = Lookdot1(n, s, t, t.AllMethods(), dostrcmp)
	}

	var f2 *types.Field
	if n.X.Type() == t || n.X.Type().Sym() == nil {
		mt := types.ReceiverBaseType(t)
		if mt != nil {
			f2 = Lookdot1(n, s, mt, mt.Methods(), dostrcmp)
		}
	}

	if f1 != nil {
		if dostrcmp > 1 || f1.Broke() {
			// Already in the process of diagnosing an error.
			return f1
		}
		if f2 != nil {
			base.Errorf("%v is both field and method", n.Sel)
		}
		if f1.Offset == types.BADWIDTH {
			base.Fatalf("Lookdot badwidth t=%v, f1=%v@%p", t, f1, f1)
		}
		n.Selection = f1
		n.SetType(f1.Type)
		if t.IsInterface() {
			if n.X.Type().IsPtr() {
				star := ir.NewStarExpr(base.Pos, n.X)
				star.SetImplicit(true)
				n.X = Expr(star)
			}

			n.SetOp(ir.ODOTINTER)
		}
		return f1
	}

	if f2 != nil {
		if dostrcmp > 1 {
			// Already in the process of diagnosing an error.
			return f2
		}
		orig := n.X
		tt := n.X.Type()
		types.CalcSize(tt)
		rcvr := f2.Type.Recv().Type
		if !types.Identical(rcvr, tt) {
			if rcvr.IsPtr() && types.Identical(rcvr.Elem(), tt) {
				checklvalue(n.X, "call pointer method on")
				addr := NodAddr(n.X)
				addr.SetImplicit(true)
				n.X = typecheck(addr, ctxType|ctxExpr)
			} else if tt.IsPtr() && (!rcvr.IsPtr() || rcvr.IsPtr() && rcvr.Elem().NotInHeap()) && types.Identical(tt.Elem(), rcvr) {
				star := ir.NewStarExpr(base.Pos, n.X)
				star.SetImplicit(true)
				n.X = typecheck(star, ctxType|ctxExpr)
			} else if tt.IsPtr() && tt.Elem().IsPtr() && types.Identical(derefall(tt), derefall(rcvr)) {
				base.Errorf("calling method %v with receiver %L requires explicit dereference", n.Sel, n.X)
				for tt.IsPtr() {
					// Stop one level early for method with pointer receiver.
					if rcvr.IsPtr() && !tt.Elem().IsPtr() {
						break
					}
					star := ir.NewStarExpr(base.Pos, n.X)
					star.SetImplicit(true)
					n.X = typecheck(star, ctxType|ctxExpr)
					tt = tt.Elem()
				}
			} else {
				base.Fatalf("method mismatch: %v for %v", rcvr, tt)
			}
		}

		// Check that we haven't implicitly dereferenced any defined pointer types.
		for x := n.X; ; {
			var inner ir.Node
			implicit := false
			switch x := x.(type) {
			case *ir.AddrExpr:
				inner, implicit = x.X, x.Implicit()
			case *ir.SelectorExpr:
				inner, implicit = x.X, x.Implicit()
			case *ir.StarExpr:
				inner, implicit = x.X, x.Implicit()
			}
			if !implicit {
				break
			}
			if inner.Type().Sym() != nil && (x.Op() == ir.ODEREF || x.Op() == ir.ODOTPTR) {
				// Found an implicit dereference of a defined pointer type.
				// Restore n.X for better error message.
				n.X = orig
				return nil
			}
			x = inner
		}

		n.Selection = f2
		n.SetType(f2.Type)
		n.SetOp(ir.ODOTMETH)

		return f2
	}

	return nil
}

func nokeys(l ir.Nodes) bool {
	for _, n := range l {
		if n.Op() == ir.OKEY || n.Op() == ir.OSTRUCTKEY {
			return false
		}
	}
	return true
}

func hasddd(t *types.Type) bool {
	for _, tl := range t.Fields().Slice() {
		if tl.IsDDD() {
			return true
		}
	}

	return false
}

// typecheck assignment: type list = expression list
func typecheckaste(op ir.Op, call ir.Node, isddd bool, tstruct *types.Type, nl ir.Nodes, desc func() string) {
	var t *types.Type
	var i int

	lno := base.Pos
	defer func() { base.Pos = lno }()

	if tstruct.Broke() {
		return
	}

	var n ir.Node
	if len(nl) == 1 {
		n = nl[0]
	}

	n1 := tstruct.NumFields()
	n2 := len(nl)
	if !hasddd(tstruct) {
		if isddd {
			goto invalidddd
		}
		if n2 > n1 {
			goto toomany
		}
		if n2 < n1 {
			goto notenough
		}
	} else {
		if !isddd {
			if n2 < n1-1 {
				goto notenough
			}
		} else {
			if n2 > n1 {
				goto toomany
			}
			if n2 < n1 {
				goto notenough
			}
		}
	}

	i = 0
	for _, tl := range tstruct.Fields().Slice() {
		t = tl.Type
		if tl.IsDDD() {
			if isddd {
				if i >= len(nl) {
					goto notenough
				}
				if len(nl)-i > 1 {
					goto toomany
				}
				n = nl[i]
				ir.SetPos(n)
				if n.Type() != nil {
					nl[i] = assignconvfn(n, t, desc)
				}
				return
			}

			// TODO(mdempsky): Make into ... call with implicit slice.
			for ; i < len(nl); i++ {
				n = nl[i]
				ir.SetPos(n)
				if n.Type() != nil {
					nl[i] = assignconvfn(n, t.Elem(), desc)
				}
			}
			return
		}

		if i >= len(nl) {
			goto notenough
		}
		n = nl[i]
		ir.SetPos(n)
		if n.Type() != nil {
			nl[i] = assignconvfn(n, t, desc)
		}
		i++
	}

	if i < len(nl) {
		goto toomany
	}

invalidddd:
	if isddd {
		if call != nil {
			base.Errorf("invalid use of ... in call to %v", call)
		} else {
			base.Errorf("invalid use of ... in %v", op)
		}
	}
	return

notenough:
	if n == nil || (!n.Diag() && n.Type() != nil) {
		details := errorDetails(nl, tstruct, isddd)
		if call != nil {
			// call is the expression being called, not the overall call.
			// Method expressions have the form T.M, and the compiler has
			// rewritten those to ONAME nodes but left T in Left.
			if call.Op() == ir.OMETHEXPR {
				call := call.(*ir.SelectorExpr)
				base.Errorf("not enough arguments in call to method expression %v%s", call, details)
			} else {
				base.Errorf("not enough arguments in call to %v%s", call, details)
			}
		} else {
			base.Errorf("not enough arguments to %v%s", op, details)
		}
		if n != nil {
			n.SetDiag(true)
		}
	}
	return

toomany:
	details := errorDetails(nl, tstruct, isddd)
	if call != nil {
		base.Errorf("too many arguments in call to %v%s", call, details)
	} else {
		base.Errorf("too many arguments to %v%s", op, details)
	}
}

func errorDetails(nl ir.Nodes, tstruct *types.Type, isddd bool) string {
	// Suppress any return message signatures if:
	//
	// (1) We don't know any type at a call site (see #19012).
	// (2) Any node has an unknown type.
	// (3) Invalid type for variadic parameter (see #46957).
	if tstruct == nil {
		return "" // case 1
	}

	if isddd && !nl[len(nl)-1].Type().IsSlice() {
		return "" // case 3
	}

	for _, n := range nl {
		if n.Type() == nil {
			return "" // case 2
		}
	}
	return fmt.Sprintf("\n\thave %s\n\twant %v", fmtSignature(nl, isddd), tstruct)
}

// sigrepr is a type's representation to the outside world,
// in string representations of return signatures
// e.g in error messages about wrong arguments to return.
func sigrepr(t *types.Type, isddd bool) string {
	switch t {
	case types.UntypedString:
		return "string"
	case types.UntypedBool:
		return "bool"
	}

	if t.Kind() == types.TIDEAL {
		// "untyped number" is not commonly used
		// outside of the compiler, so let's use "number".
		// TODO(mdempsky): Revisit this.
		return "number"
	}

	// Turn []T... argument to ...T for clearer error message.
	if isddd {
		if !t.IsSlice() {
			base.Fatalf("bad type for ... argument: %v", t)
		}
		return "..." + t.Elem().String()
	}
	return t.String()
}

// sigerr returns the signature of the types at the call or return.
func fmtSignature(nl ir.Nodes, isddd bool) string {
	if len(nl) < 1 {
		return "()"
	}

	var typeStrings []string
	for i, n := range nl {
		isdddArg := isddd && i == len(nl)-1
		typeStrings = append(typeStrings, sigrepr(n.Type(), isdddArg))
	}

	return fmt.Sprintf("(%s)", strings.Join(typeStrings, ", "))
}

// type check composite
func fielddup(name string, hash map[string]bool) {
	if hash[name] {
		base.Errorf("duplicate field name in struct literal: %s", name)
		return
	}
	hash[name] = true
}

// iscomptype reports whether type t is a composite literal type.
func iscomptype(t *types.Type) bool {
	switch t.Kind() {
	case types.TARRAY, types.TSLICE, types.TSTRUCT, types.TMAP:
		return true
	default:
		return false
	}
}

// pushtype adds elided type information for composite literals if
// appropriate, and returns the resulting expression.
func pushtype(nn ir.Node, t *types.Type) ir.Node {
	if nn == nil || nn.Op() != ir.OCOMPLIT {
		return nn
	}
	n := nn.(*ir.CompLitExpr)
	if n.Ntype != nil {
		return n
	}

	switch {
	case iscomptype(t):
		// For T, return T{...}.
		n.Ntype = ir.TypeNode(t)

	case t.IsPtr() && iscomptype(t.Elem()):
		// For *T, return &T{...}.
		n.Ntype = ir.TypeNode(t.Elem())

		addr := NodAddrAt(n.Pos(), n)
		addr.SetImplicit(true)
		return addr
	}
	return n
}

// typecheckarraylit type-checks a sequence of slice/array literal elements.
func typecheckarraylit(elemType *types.Type, bound int64, elts []ir.Node, ctx string) int64 {
	// If there are key/value pairs, create a map to keep seen
	// keys so we can check for duplicate indices.
	var indices map[int64]bool
	for _, elt := range elts {
		if elt.Op() == ir.OKEY {
			indices = make(map[int64]bool)
			break
		}
	}

	var key, length int64
	for i, elt := range elts {
		ir.SetPos(elt)
		r := elts[i]
		var kv *ir.KeyExpr
		if elt.Op() == ir.OKEY {
			elt := elt.(*ir.KeyExpr)
			elt.Key = Expr(elt.Key)
			key = IndexConst(elt.Key)
			if key < 0 {
				if !elt.Key.Diag() {
					if key == -2 {
						base.Errorf("index too large")
					} else {
						base.Errorf("index must be non-negative integer constant")
					}
					elt.Key.SetDiag(true)
				}
				key = -(1 << 30) // stay negative for a while
			}
			kv = elt
			r = elt.Value
		}

		r = pushtype(r, elemType)
		r = Expr(r)
		r = AssignConv(r, elemType, ctx)
		if kv != nil {
			kv.Value = r
		} else {
			elts[i] = r
		}

		if key >= 0 {
			if indices != nil {
				if indices[key] {
					base.Errorf("duplicate index in %s: %d", ctx, key)
				} else {
					indices[key] = true
				}
			}

			if bound >= 0 && key >= bound {
				base.Errorf("array index %d out of bounds [0:%d]", key, bound)
				bound = -1
			}
		}

		key++
		if key > length {
			length = key
		}
	}

	return length
}

// visible reports whether sym is exported or locally defined.
func visible(sym *types.Sym) bool {
	return sym != nil && (types.IsExported(sym.Name) || sym.Pkg == types.LocalPkg)
}

// nonexported reports whether sym is an unexported field.
func nonexported(sym *types.Sym) bool {
	return sym != nil && !types.IsExported(sym.Name)
}

func checklvalue(n ir.Node, verb string) {
	if !ir.IsAddressable(n) {
		base.Errorf("cannot %s %v", verb, n)
	}
}

func checkassign(stmt ir.Node, n ir.Node) {
	// have already complained about n being invalid
	if n.Type() == nil {
		if base.Errors() == 0 {
			base.Fatalf("expected an error about %v", n)
		}
		return
	}

	if ir.IsAddressable(n) {
		return
	}
	if n.Op() == ir.OINDEXMAP {
		n := n.(*ir.IndexExpr)
		n.Assigned = true
		return
	}

	defer n.SetType(nil)
	if n.Diag() {
		return
	}
	switch {
	case n.Op() == ir.ODOT && n.(*ir.SelectorExpr).X.Op() == ir.OINDEXMAP:
		base.Errorf("cannot assign to struct field %v in map", n)
	case (n.Op() == ir.OINDEX && n.(*ir.IndexExpr).X.Type().IsString()) || n.Op() == ir.OSLICESTR:
		base.Errorf("cannot assign to %v (strings are immutable)", n)
	case n.Op() == ir.OLITERAL && n.Sym() != nil && ir.IsConstNode(n):
		base.Errorf("cannot assign to %v (declared const)", n)
	default:
		base.Errorf("cannot assign to %v", n)
	}
}

func checkassignto(src *types.Type, dst ir.Node) {
	// TODO(mdempsky): Handle all untyped types correctly.
	if src == types.UntypedBool && dst.Type().IsBoolean() {
		return
	}

	if op, why := Assignop(src, dst.Type()); op == ir.OXXX {
		base.Errorf("cannot assign %v to %L in multiple assignment%s", src, dst, why)
		return
	}
}

// The result of stringtoruneslit MUST be assigned back to n, e.g.
// 	n.Left = stringtoruneslit(n.Left)
func stringtoruneslit(n *ir.ConvExpr) ir.Node {
	if n.X.Op() != ir.OLITERAL || n.X.Val().Kind() != constant.String {
		base.Fatalf("stringtoarraylit %v", n)
	}

	var l []ir.Node
	i := 0
	for _, r := range ir.StringVal(n.X) {
		l = append(l, ir.NewKeyExpr(base.Pos, ir.NewInt(int64(i)), ir.NewInt(int64(r))))
		i++
	}

	nn := ir.NewCompLitExpr(base.Pos, ir.OCOMPLIT, ir.TypeNode(n.Type()), nil)
	nn.List = l
	return Expr(nn)
}

var mapqueue []*ir.MapType

func CheckMapKeys() {
	for _, n := range mapqueue {
		k := n.Type().MapType().Key
		if !k.Broke() && !types.IsComparable(k) {
			base.ErrorfAt(n.Pos(), "invalid map key type %v", k)
		}
	}
	mapqueue = nil
}

func typecheckdeftype(n *ir.Name) {
	if base.EnableTrace && base.Flag.LowerT {
		defer tracePrint("typecheckdeftype", n)(nil)
	}

	t := types.NewNamed(n)
	if n.Curfn != nil {
		t.SetVargen()
	}

	if n.Pragma()&ir.NotInHeap != 0 {
		t.SetNotInHeap(true)
	}

	n.SetType(t)
	n.SetTypecheck(1)
	n.SetWalkdef(1)

	types.DeferCheckSize()
	errorsBefore := base.Errors()
	n.Ntype = typecheckNtype(n.Ntype)
	if underlying := n.Ntype.Type(); underlying != nil {
		t.SetUnderlying(underlying)
	} else {
		n.SetDiag(true)
		n.SetType(nil)
	}
	if t.Kind() == types.TFORW && base.Errors() > errorsBefore {
		// Something went wrong during type-checking,
		// but it was reported. Silence future errors.
		t.SetBroke(true)
	}
	types.ResumeCheckSize()
}

func typecheckdef(n *ir.Name) {
	if base.EnableTrace && base.Flag.LowerT {
		defer tracePrint("typecheckdef", n)(nil)
	}

	if n.Walkdef() == 1 {
		return
	}

	if n.Type() != nil { // builtin
		// Mark as Walkdef so that if n.SetType(nil) is called later, we
		// won't try walking again.
		if got := n.Walkdef(); got != 0 {
			base.Fatalf("unexpected walkdef: %v", got)
		}
		n.SetWalkdef(1)
		return
	}

	lno := ir.SetPos(n)
	typecheckdefstack = append(typecheckdefstack, n)
	if n.Walkdef() == 2 {
		base.FlushErrors()
		fmt.Printf("typecheckdef loop:")
		for i := len(typecheckdefstack) - 1; i >= 0; i-- {
			n := typecheckdefstack[i]
			fmt.Printf(" %v", n.Sym())
		}
		fmt.Printf("\n")
		base.Fatalf("typecheckdef loop")
	}

	n.SetWalkdef(2)

	switch n.Op() {
	default:
		base.Fatalf("typecheckdef %v", n.Op())

	case ir.OLITERAL:
		if n.Ntype != nil {
			n.Ntype = typecheckNtype(n.Ntype)
			n.SetType(n.Ntype.Type())
			n.Ntype = nil
			if n.Type() == nil {
				n.SetDiag(true)
				goto ret
			}
		}

		e := n.Defn
		n.Defn = nil
		if e == nil {
			ir.Dump("typecheckdef nil defn", n)
			base.ErrorfAt(n.Pos(), "xxx")
		}

		e = Expr(e)
		if e.Type() == nil {
			goto ret
		}
		if !ir.IsConstNode(e) {
			if !e.Diag() {
				if e.Op() == ir.ONIL {
					base.ErrorfAt(n.Pos(), "const initializer cannot be nil")
				} else {
					base.ErrorfAt(n.Pos(), "const initializer %v is not a constant", e)
				}
				e.SetDiag(true)
			}
			goto ret
		}

		t := n.Type()
		if t != nil {
			if !ir.OKForConst[t.Kind()] {
				base.ErrorfAt(n.Pos(), "invalid constant type %v", t)
				goto ret
			}

			if !e.Type().IsUntyped() && !types.Identical(t, e.Type()) {
				base.ErrorfAt(n.Pos(), "cannot use %L as type %v in const initializer", e, t)
				goto ret
			}

			e = convlit(e, t)
		}

		n.SetType(e.Type())
		if n.Type() != nil {
			n.SetVal(e.Val())
		}

	case ir.ONAME:
		if n.Ntype != nil {
			n.Ntype = typecheckNtype(n.Ntype)
			n.SetType(n.Ntype.Type())
			if n.Type() == nil {
				n.SetDiag(true)
				goto ret
			}
		}

		if n.Type() != nil {
			break
		}
		if n.Defn == nil {
			if n.BuiltinOp != 0 { // like OPRINTN
				break
			}
			if base.Errors() > 0 {
				// Can have undefined variables in x := foo
				// that make x have an n.name.Defn == nil.
				// If there are other errors anyway, don't
				// bother adding to the noise.
				break
			}

			base.Fatalf("var without type, init: %v", n.Sym())
		}

		if n.Defn.Op() == ir.ONAME {
			n.Defn = Expr(n.Defn)
			n.SetType(n.Defn.Type())
			break
		}

		n.Defn = Stmt(n.Defn) // fills in n.Type

	case ir.OTYPE:
		if n.Alias() {
			// Type alias declaration: Simply use the rhs type - no need
			// to create a new type.
			// If we have a syntax error, name.Ntype may be nil.
			if n.Ntype != nil {
				n.Ntype = typecheckNtype(n.Ntype)
				n.SetType(n.Ntype.Type())
				if n.Type() == nil {
					n.SetDiag(true)
					goto ret
				}
			}
			break
		}

		// regular type declaration
		typecheckdeftype(n)
	}

ret:
	if n.Op() != ir.OLITERAL && n.Type() != nil && n.Type().IsUntyped() {
		base.Fatalf("got %v for %v", n.Type(), n)
	}
	last := len(typecheckdefstack) - 1
	if typecheckdefstack[last] != n {
		base.Fatalf("typecheckdefstack mismatch")
	}
	typecheckdefstack[last] = nil
	typecheckdefstack = typecheckdefstack[:last]

	base.Pos = lno
	n.SetWalkdef(1)
}

func checkmake(t *types.Type, arg string, np *ir.Node) bool {
	n := *np
	if !n.Type().IsInteger() && n.Type().Kind() != types.TIDEAL {
		base.Errorf("non-integer %s argument in make(%v) - %v", arg, t, n.Type())
		return false
	}

	// Do range checks for constants before DefaultLit
	// to avoid redundant "constant NNN overflows int" errors.
	if n.Op() == ir.OLITERAL {
		v := toint(n.Val())
		if constant.Sign(v) < 0 {
			base.Errorf("negative %s argument in make(%v)", arg, t)
			return false
		}
		if ir.ConstOverflow(v, types.Types[types.TINT]) {
			base.Errorf("%s argument too large in make(%v)", arg, t)
			return false
		}
	}

	// DefaultLit is necessary for non-constants too: n might be 1.1<<k.
	// TODO(gri) The length argument requirements for (array/slice) make
	// are the same as for index expressions. Factor the code better;
	// for instance, indexlit might be called here and incorporate some
	// of the bounds checks done for make.
	n = DefaultLit(n, types.Types[types.TINT])
	*np = n

	return true
}

// checkunsafeslice is like checkmake but for unsafe.Slice.
func checkunsafeslice(np *ir.Node) bool {
	n := *np
	if !n.Type().IsInteger() && n.Type().Kind() != types.TIDEAL {
		base.Errorf("non-integer len argument in unsafe.Slice - %v", n.Type())
		return false
	}

	// Do range checks for constants before DefaultLit
	// to avoid redundant "constant NNN overflows int" errors.
	if n.Op() == ir.OLITERAL {
		v := toint(n.Val())
		if constant.Sign(v) < 0 {
			base.Errorf("negative len argument in unsafe.Slice")
			return false
		}
		if ir.ConstOverflow(v, types.Types[types.TINT]) {
			base.Errorf("len argument too large in unsafe.Slice")
			return false
		}
	}

	// DefaultLit is necessary for non-constants too: n might be 1.1<<k.
	n = DefaultLit(n, types.Types[types.TINT])
	*np = n

	return true
}

// markBreak marks control statements containing break statements with SetHasBreak(true).
func markBreak(fn *ir.Func) {
	var labels map[*types.Sym]ir.Node
	var implicit ir.Node

	var mark func(ir.Node) bool
	mark = func(n ir.Node) bool {
		switch n.Op() {
		default:
			ir.DoChildren(n, mark)

		case ir.OBREAK:
			n := n.(*ir.BranchStmt)
			if n.Label == nil {
				setHasBreak(implicit)
			} else {
				setHasBreak(labels[n.Label])
			}

		case ir.OFOR, ir.OFORUNTIL, ir.OSWITCH, ir.OSELECT, ir.ORANGE:
			old := implicit
			implicit = n
			var sym *types.Sym
			switch n := n.(type) {
			case *ir.ForStmt:
				sym = n.Label
			case *ir.RangeStmt:
				sym = n.Label
			case *ir.SelectStmt:
				sym = n.Label
			case *ir.SwitchStmt:
				sym = n.Label
			}
			if sym != nil {
				if labels == nil {
					// Map creation delayed until we need it - most functions don't.
					labels = make(map[*types.Sym]ir.Node)
				}
				labels[sym] = n
			}
			ir.DoChildren(n, mark)
			if sym != nil {
				delete(labels, sym)
			}
			implicit = old
		}
		return false
	}

	mark(fn)
}

func controlLabel(n ir.Node) *types.Sym {
	switch n := n.(type) {
	default:
		base.Fatalf("controlLabel %+v", n.Op())
		return nil
	case *ir.ForStmt:
		return n.Label
	case *ir.RangeStmt:
		return n.Label
	case *ir.SelectStmt:
		return n.Label
	case *ir.SwitchStmt:
		return n.Label
	}
}

func setHasBreak(n ir.Node) {
	switch n := n.(type) {
	default:
		base.Fatalf("setHasBreak %+v", n.Op())
	case nil:
		// ignore
	case *ir.ForStmt:
		n.HasBreak = true
	case *ir.RangeStmt:
		n.HasBreak = true
	case *ir.SelectStmt:
		n.HasBreak = true
	case *ir.SwitchStmt:
		n.HasBreak = true
	}
}

// isTermNodes reports whether the Nodes list ends with a terminating statement.
func isTermNodes(l ir.Nodes) bool {
	s := l
	c := len(s)
	if c == 0 {
		return false
	}
	return isTermNode(s[c-1])
}

// isTermNode reports whether the node n, the last one in a
// statement list, is a terminating statement.
func isTermNode(n ir.Node) bool {
	switch n.Op() {
	// NOTE: OLABEL is treated as a separate statement,
	// not a separate prefix, so skipping to the last statement
	// in the block handles the labeled statement case by
	// skipping over the label. No case OLABEL here.

	case ir.OBLOCK:
		n := n.(*ir.BlockStmt)
		return isTermNodes(n.List)

	case ir.OGOTO, ir.ORETURN, ir.OTAILCALL, ir.OPANIC, ir.OFALL:
		return true

	case ir.OFOR, ir.OFORUNTIL:
		n := n.(*ir.ForStmt)
		if n.Cond != nil {
			return false
		}
		if n.HasBreak {
			return false
		}
		return true

	case ir.OIF:
		n := n.(*ir.IfStmt)
		return isTermNodes(n.Body) && isTermNodes(n.Else)

	case ir.OSWITCH:
		n := n.(*ir.SwitchStmt)
		if n.HasBreak {
			return false
		}
		def := false
		for _, cas := range n.Cases {
			if !isTermNodes(cas.Body) {
				return false
			}
			if len(cas.List) == 0 { // default
				def = true
			}
		}
		return def

	case ir.OSELECT:
		n := n.(*ir.SelectStmt)
		if n.HasBreak {
			return false
		}
		for _, cas := range n.Cases {
			if !isTermNodes(cas.Body) {
				return false
			}
		}
		return true
	}

	return false
}

// CheckUnused checks for any declared variables that weren't used.
func CheckUnused(fn *ir.Func) {
	// Only report unused variables if we haven't seen any type-checking
	// errors yet.
	if base.Errors() != 0 {
		return
	}

	// Propagate the used flag for typeswitch variables up to the NONAME in its definition.
	for _, ln := range fn.Dcl {
		if ln.Op() == ir.ONAME && ln.Class == ir.PAUTO && ln.Used() {
			if guard, ok := ln.Defn.(*ir.TypeSwitchGuard); ok {
				guard.Used = true
			}
		}
	}

	for _, ln := range fn.Dcl {
		if ln.Op() != ir.ONAME || ln.Class != ir.PAUTO || ln.Used() {
			continue
		}
		if defn, ok := ln.Defn.(*ir.TypeSwitchGuard); ok {
			if defn.Used {
				continue
			}
			base.ErrorfAt(defn.Tag.Pos(), "%v declared but not used", ln.Sym())
			defn.Used = true // suppress repeats
		} else {
			base.ErrorfAt(ln.Pos(), "%v declared but not used", ln.Sym())
		}
	}
}

// CheckReturn makes sure that fn terminates appropriately.
func CheckReturn(fn *ir.Func) {
	if fn.Type() != nil && fn.Type().NumResults() != 0 && len(fn.Body) != 0 {
		markBreak(fn)
		if !isTermNodes(fn.Body) {
			base.ErrorfAt(fn.Endlineno, "missing return at end of function")
		}
	}
}

// getIotaValue returns the current value for "iota",
// or -1 if not within a ConstSpec.
func getIotaValue() int64 {
	if i := len(typecheckdefstack); i > 0 {
		if x := typecheckdefstack[i-1]; x.Op() == ir.OLITERAL {
			return x.Iota()
		}
	}

	if ir.CurFunc != nil && ir.CurFunc.Iota >= 0 {
		return ir.CurFunc.Iota
	}

	return -1
}

// curpkg returns the current package, based on Curfn.
func curpkg() *types.Pkg {
	fn := ir.CurFunc
	if fn == nil {
		// Initialization expressions for package-scope variables.
		return types.LocalPkg
	}
	return fnpkg(fn.Nname)
}

func Conv(n ir.Node, t *types.Type) ir.Node {
	if types.Identical(n.Type(), t) {
		return n
	}
	n = ir.NewConvExpr(base.Pos, ir.OCONV, nil, n)
	n.SetType(t)
	n = Expr(n)
	return n
}

// ConvNop converts node n to type t using the OCONVNOP op
// and typechecks the result with ctxExpr.
func ConvNop(n ir.Node, t *types.Type) ir.Node {
	if types.Identical(n.Type(), t) {
		return n
	}
	n = ir.NewConvExpr(base.Pos, ir.OCONVNOP, nil, n)
	n.SetType(t)
	n = Expr(n)
	return n
}