go/types, types2: operand.convertibleTo must consider Alias types

Fixes regression from Go 1.22.

Fixes #67540.
For #67547.

Change-Id: I61f642970c6a9bd8567654bb5ecf645ae77b3bcc
Reviewed-on: https://go-review.googlesource.com/c/go/+/587159
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Cuong Manh Le <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>

Author: griesemer

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

@@ -139,13 +139,16 @@ func (x *operand) convertibleTo(check *Checker, T Type, cause *string) bool {
 		return true
 	}
 
-	// "V and T have identical underlying types if tags are ignored
-	// and V and T are not type parameters"
-	V := x.typ
+	origT := T
+	V := Unalias(x.typ)
+	T = Unalias(T)
 	Vu := under(V)
 	Tu := under(T)
 	Vp, _ := V.(*TypeParam)
 	Tp, _ := T.(*TypeParam)
+
+	// "V and T have identical underlying types if tags are ignored
+	// and V and T are not type parameters"
 	if IdenticalIgnoreTags(Vu, Tu) && Vp == nil && Tp == nil {
 		return true
 	}
@@ -267,7 +270,7 @@ func (x *operand) convertibleTo(check *Checker, T Type, cause *string) bool {
 			}
 			x.typ = V.typ
 			if !x.convertibleTo(check, T, cause) {
-				errorf("cannot convert %s (in %s) to type %s", V.typ, Vp, T)
+				errorf("cannot convert %s (in %s) to type %s", V.typ, Vp, origT)
 				return false
 			}
 			return true

src/go/types/conversions.go

@@ -26,3 +26,43 @@ func _[P map[int]int]() {
 	var m A
 	clear(m) // don't report an error for m
 }
+
+type S1 struct {
+	x int "S1.x"
+}
+
+type S2 struct {
+	x int "S2.x"
+}
+
+func _[P1 S1, P2 S2]() {
+	type A = P1
+	var p A
+	_ = P2(p) // conversion must be valid
+}
+
+func _[P1 S1, P2 S2]() {
+	var p P1
+	type A = P2
+	_ = A(p) // conversion must be valid
+}
+
+func _[P int | string]() {
+	var p P
+	type A = int
+	// preserve target type name A in error messages when using Alias types
+	// (test are run with and without Alias types enabled, so we need to
+	// keep both A and int in the error message)
+	_ = A(p /* ERRORx "cannot convert string .* to type (A|int)" */)
+}
+
+// Test case for go.dev/issue/67540.
+func _() {
+	type (
+		S struct{}
+		A = *S
+		T S
+	)
+	var p A
+	_ = (*T)(p)
+}