go/types, types2: add missing Unalias call to type string functionality

For #67547.

Change-Id: I999cd31f9a01f91e7984b4e7012c81e8bd9c6b06
Reviewed-on: https://go-review.googlesource.com/c/go/+/587940
Reviewed-by: Robert Griesemer <[email protected]>
Auto-Submit: Robert Griesemer <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: griesemer

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

@@ -186,7 +186,7 @@ func operandString(x *operand, qf Qualifier) string {
 			}
 			buf.WriteString(intro)
 			WriteType(&buf, x.typ, qf)
-			if tpar, _ := x.typ.(*TypeParam); tpar != nil {
+			if tpar, _ := Unalias(x.typ).(*TypeParam); tpar != nil {
 				buf.WriteString(" constrained by ")
 				WriteType(&buf, tpar.bound, qf) // do not compute interface type sets here
 				// If we have the type set and it's empty, say so for better error messages.

src/go/types/operand.go

@@ -77,6 +77,13 @@ func _[P []int | struct{}]() {
 	_ = a
 }
 
+func _[P any]() {
+	type A = P
+	var x A
+	// keep "constrained by" for aliased type parameters in error messages
+	var _ int = x // ERRORx `cannot use x \(variable of type (A|P) constrained by any\) as int value in variable declaration`
+}
+
 // Test case for go.dev/issue/67540.
 func _() {
 	type (