[release-branch.go1.18] cmd/compile: backport fix for #51840

This CL is a manual backport of CLs 403837 and 404914 to Go 1.18.

CL 403837 was intended just as a simplification CL, but evidently it
also fixed #51840. However, for backporting to Go 1.18, the existing
logic needs to be preserved to support -G=0 mode (which still relies
on Ntype).

Fixes #51849.

Change-Id: Ib060b0bc67ecf26de8a65d5b4d2f8a65cd547517
Reviewed-on: https://go-review.googlesource.com/c/go/+/405436
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>

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

@@ -127,13 +127,23 @@ func assign(stmt ir.Node, lhs, rhs []ir.Node) {
 
 	checkLHS := func(i int, typ *types.Type) {
 		lhs[i] = Resolve(lhs[i])
-		if n := lhs[i]; typ != nil && ir.DeclaredBy(n, stmt) && n.Name().Ntype == nil {
-			if typ.Kind() != types.TNIL {
+		if base.Flag.G != 0 || base.Debug.Unified != 0 {
+			// New logic added in CL 403837 for Go 1.19, which only has -G=3 and unified IR.
+			if n := lhs[i]; typ != nil && ir.DeclaredBy(n, stmt) && n.Type() == nil {
+				base.Assertf(typ.Kind() == types.TNIL, "unexpected untyped nil")
 				n.SetType(defaultType(typ))
-			} else {
-				base.Errorf("use of untyped nil")
+			}
+		} else {
+			// Original logic from Go 1.18, which is still needed for -G=0.
+			if n := lhs[i]; typ != nil && ir.DeclaredBy(n, stmt) && n.Name().Ntype == nil {
+				if typ.Kind() != types.TNIL {
+					n.SetType(defaultType(typ))
+				} else {
+					base.Errorf("use of untyped nil")
+				}
 			}
 		}
+
 		if lhs[i].Typecheck() == 0 {
 			lhs[i] = AssignExpr(lhs[i])
 		}

test/typeparam/issue51840.go

@@ -0,0 +1,36 @@
+// compile -G=3
+
+// Copyright 2022 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 main
+
+type Addr struct {
+	hi uint64
+	lo uint64
+	z  *byte
+}
+
+func EqualMap[M1, M2 ~map[K]V, K, V comparable](m1 M1, m2 M2) bool {
+	for k, v1 := range m1 {
+		if v2, ok := m2[k]; !ok || v1 != v2 {
+			return false
+		}
+	}
+	return true
+}
+
+type Set[T comparable] map[T]struct{}
+
+func NewSet[T comparable](items ...T) Set[T] {
+	return nil
+}
+
+func (s Set[T]) Equals(other Set[T]) bool {
+	return EqualMap(s, other)
+}
+
+func main() {
+	NewSet[Addr](Addr{0, 0, nil})
+}