go/types, types2: fix incorrect logic in assignability check

Do not return prematurely in assignableTo.

Fixes #54424.

Change-Id: I769b3b4ad9d79b7bce60b92cc59c3564117e36db
Reviewed-on: https://go-review.googlesource.com/c/go/+/498400
Run-TryBot: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -290,13 +290,21 @@ func (x *operand) assignableTo(check *Checker, T Type, cause *string) (bool, Cod
 		return true, 0
 	}
 
-	// T is an interface type and x implements T and T is not a type parameter.
-	// Also handle the case where T is a pointer to an interface.
+	// T is an interface type, but not a type parameter, and V implements T.
+	// Also handle the case where T is a pointer to an interface so that we get
+	// the Checker.implements error cause.
 	if _, ok := Tu.(*Interface); ok && Tp == nil || isInterfacePtr(Tu) {
-		if !check.implements(x.Pos(), V, T, false, cause) {
+		if check.implements(x.Pos(), V, T, false, cause) {
+			return true, 0
+		}
+		// V doesn't implement T but V may still be assignable to T if V
+		// is a type parameter; do not report an error in that case yet.
+		if Vp == nil {
 			return false, InvalidIfaceAssign
 		}
-		return true, 0
+		if cause != nil {
+			*cause = ""
+		}
 	}
 
 	// If V is an interface, check if a missing type assertion is the problem.

src/go/types/operand.go

@@ -277,13 +277,21 @@ func (x *operand) assignableTo(check *Checker, T Type, cause *string) (bool, Cod
 		return true, 0
 	}
 
-	// T is an interface type and x implements T and T is not a type parameter.
-	// Also handle the case where T is a pointer to an interface.
+	// T is an interface type, but not a type parameter, and V implements T.
+	// Also handle the case where T is a pointer to an interface so that we get
+	// the Checker.implements error cause.
 	if _, ok := Tu.(*Interface); ok && Tp == nil || isInterfacePtr(Tu) {
-		if !check.implements(x.Pos(), V, T, false, cause) {
+		if check.implements(x.Pos(), V, T, false, cause) {
+			return true, 0
+		}
+		// V doesn't implement T but V may still be assignable to T if V
+		// is a type parameter; do not report an error in that case yet.
+		if Vp == nil {
 			return false, InvalidIfaceAssign
 		}
-		return true, 0
+		if cause != nil {
+			*cause = ""
+		}
 	}
 
 	// If V is an interface, check if a missing type assertion is the problem.

src/internal/types/testdata/fixedbugs/issue54424.go

@@ -0,0 +1,12 @@
+// Copyright 2023 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
+
+func f[P ~*T, T any]() {
+	var p P
+	var tp *T
+	tp = p // this assignment is valid
+	_ = tp
+}