go/types, cmd/compile: fix composite literal structural typing

For a composite literal expression like []T{{f: 1}}, we allow T to be
a pointer to struct type, so it's consistent to allow T to also be a
type parameter whose structural type is a pointer to struct type.

Fixes #50833.

Change-Id: Ib0781ec4a4f327c875ea25b97740ff2c0c86b916
Reviewed-on: https://go-review.googlesource.com/c/go/+/381075
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Robert Griesemer <gri@golang.org>
Trust: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/noder/expr.go

@@ -332,7 +332,7 @@ func (g *irgen) exprs(exprs []syntax.Expr) []ir.Node {
 }
 
 func (g *irgen) compLit(typ types2.Type, lit *syntax.CompositeLit) ir.Node {
-	if ptr, ok := typ.Underlying().(*types2.Pointer); ok {
+	if ptr, ok := types2.StructuralType(typ).(*types2.Pointer); ok {
 		n := ir.NewAddrExpr(g.pos(lit), g.compLit(ptr.Elem(), lit))
 		n.SetOp(ir.OPTRLIT)
 		return typed(g.typ(typ), n)

src/cmd/compile/internal/noder/writer.go

@@ -1338,8 +1338,7 @@ func (w *writer) compLit(lit *syntax.CompositeLit) {
 	w.typ(tv.Type)
 
 	typ := tv.Type
-	// TODO(mdempsky): Use types2.StructuralType here too? See #50833.
-	if ptr, ok := typ.Underlying().(*types2.Pointer); ok {
+	if ptr, ok := types2.StructuralType(typ).(*types2.Pointer); ok {
 		typ = ptr.Elem()
 	}
 	str, isStruct := types2.StructuralType(typ).(*types2.Struct)

src/cmd/compile/internal/types2/expr.go

@@ -1262,11 +1262,7 @@ func (check *Checker) exprInternal(x *operand, e syntax.Expr, hint Type) exprKin
 		case hint != nil:
 			// no composite literal type present - use hint (element type of enclosing type)
 			typ = hint
-			base = typ
-			if !isTypeParam(typ) {
-				base = under(typ)
-			}
-			base, _ = deref(base) // *T implies &T{}
+			base, _ = deref(structuralType(typ)) // *T implies &T{}
 
 		default:
 			// TODO(gri) provide better error messages depending on context

src/cmd/compile/internal/types2/testdata/fixedbugs/issue50833.go2

@@ -0,0 +1,16 @@
+// 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 p
+
+type (
+	S  struct{ f int }
+	PS *S
+)
+
+func a() []*S { return []*S{{f: 1}} }
+func b() []PS { return []PS{{f: 1}} }
+
+func c[P *S]() []P { return []P{{f: 1}} }
+func d[P PS]() []P { return []P{{f: 1}} }

src/go/types/expr.go

@@ -1241,11 +1241,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 		case hint != nil:
 			// no composite literal type present - use hint (element type of enclosing type)
 			typ = hint
-			base = typ
-			if !isTypeParam(typ) {
-				base = under(typ)
-			}
-			base, _ = deref(base) // *T implies &T{}
+			base, _ = deref(structuralType(typ)) // *T implies &T{}
 
 		default:
 			// TODO(gri) provide better error messages depending on context

src/go/types/testdata/fixedbugs/issue50833.go2

@@ -0,0 +1,16 @@
+// 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 p
+
+type (
+	S  struct{ f int }
+	PS *S
+)
+
+func a() []*S { return []*S{{f: 1}} }
+func b() []PS { return []PS{{f: 1}} }
+
+func c[P *S]() []P { return []P{{f: 1}} }
+func d[P PS]() []P { return []P{{f: 1}} }

test/typeparam/issue50833.go

@@ -0,0 +1,23 @@
+// run -gcflags=-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 (
+	S  struct{ f int }
+	PS *S
+)
+
+func a() []*S { return []*S{{f: 1}} }
+func b() []PS { return []PS{{f: 1}} }
+
+func c[P *S]() []P { return []P{{f: 1}} }
+func d[P PS]() []P { return []P{{f: 1}} }
+
+func main() {
+	c[*S]()
+	d[PS]()
+}