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

This reverts CL 405436 (commit e1b14f53c13c7ab809ea1a33d2ec7f787efdafa8).

Fixes #53883.
Updates #51840.

Change-Id: Ide5a9568a7ae5b449ef154c29b69699a7e4b3f6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/417616
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Cherry Mui <cherryyz@google.com>
Reviewed-by: Jenny Rakoczy <jenny@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>

2 files changed, 4 insertions, 50 deletions

diff --git a/src/cmd/compile/internal/typecheck/stmt.go b/src/cmd/compile/internal/typecheck/stmt.go
index 7ab9a1df96..9a02c1752c 100644
--- a/src/cmd/compile/internal/typecheck/stmt.go
+++ b/src/cmd/compile/internal/typecheck/stmt.go
@@ -127,23 +127,13 @@ func assign(stmt ir.Node, lhs, rhs []ir.Node) {
 
 	checkLHS := func(i int, typ *types.Type) {
 		lhs[i] = Resolve(lhs[i])
-		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")
+		if n := lhs[i]; typ != nil && ir.DeclaredBy(n, stmt) && n.Name().Ntype == nil {
+			if typ.Kind() != types.TNIL {
 				n.SetType(defaultType(typ))
-			}
-		} 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")
-				}
+			} else {
+				base.Errorf("use of untyped nil")
 			}
 		}
-
 		if lhs[i].Typecheck() == 0 {
 			lhs[i] = AssignExpr(lhs[i])
 		}
diff --git a/test/typeparam/issue51840.go b/test/typeparam/issue51840.go
deleted file mode 100644
index f0ca9d1a1d..0000000000
--- a/test/typeparam/issue51840.go
+++ /dev/null
@@ -1,36 +0,0 @@
-// 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})
-}