go/types, types2: don't lose alias information during unification

While at it, rename asTypeParam to asBoundTypeParam for clarity.

For #67547.
Fixes #67628.

Change-Id: I2f447c4cd4d72f5315fe9323d82fcb9bf33657c6
Reviewed-on: https://go-review.googlesource.com/c/go/+/588797
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Robert Griesemer <gri@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

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

@@ -205,10 +205,10 @@ func (u *unifier) join(x, y *TypeParam) bool {
 	return true
 }
 
-// asTypeParam returns x.(*TypeParam) if x is a type parameter recorded with u.
+// asBoundTypeParam returns x.(*TypeParam) if x is a type parameter recorded with u.
 // Otherwise, the result is nil.
-func (u *unifier) asTypeParam(x Type) *TypeParam {
-	if x, _ := x.(*TypeParam); x != nil {
+func (u *unifier) asBoundTypeParam(x Type) *TypeParam {
+	if x, _ := Unalias(x).(*TypeParam); x != nil {
 		if _, found := u.handles[x]; found {
 			return x
 		}
@@ -269,7 +269,7 @@ func (u *unifier) inferred(tparams []*TypeParam) []Type {
 // asInterface returns the underlying type of x as an interface if
 // it is a non-type parameter interface. Otherwise it returns nil.
 func asInterface(x Type) (i *Interface) {
-	if _, ok := x.(*TypeParam); !ok {
+	if _, ok := Unalias(x).(*TypeParam); !ok {
 		i, _ = under(x).(*Interface)
 	}
 	return i
@@ -291,11 +291,8 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 		u.depth--
 	}()
 
-	x = Unalias(x)
-	y = Unalias(y)
-
 	// nothing to do if x == y
-	if x == y {
+	if x == y || Unalias(x) == Unalias(y) {
 		return true
 	}
 
@@ -314,7 +311,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 	// Ensure that if we have at least one
 	// - defined type, make sure one is in y
 	// - type parameter recorded with u, make sure one is in x
-	if asNamed(x) != nil || u.asTypeParam(y) != nil {
+	if asNamed(x) != nil || u.asBoundTypeParam(y) != nil {
 		if traceInference {
 			u.tracef("%s ≡ %s\t// swap", y, x)
 		}
@@ -358,7 +355,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 	// isTypeLit(x) is false and y was not changed above. In other
 	// words, if y was a defined type, it is still a defined type
 	// (relevant for the logic below).
-	switch px, py := u.asTypeParam(x), u.asTypeParam(y); {
+	switch px, py := u.asBoundTypeParam(x), u.asBoundTypeParam(y); {
 	case px != nil && py != nil:
 		// both x and y are type parameters
 		if u.join(px, py) {
@@ -449,7 +446,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 	}
 
 	// x != y if we get here
-	assert(x != y)
+	assert(x != y && Unalias(x) != Unalias(y))
 
 	// If u.EnableInterfaceInference is set and we don't require exact unification,
 	// if both types are interfaces, one interface must have a subset of the
@@ -573,6 +570,10 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 		emode |= exact
 	}
 
+	// Continue with unaliased types but don't lose original alias names, if any (go.dev/issue/67628).
+	xorig, x := x, Unalias(x)
+	yorig, y := y, Unalias(y)
+
 	switch x := x.(type) {
 	case *Basic:
 		// Basic types are singletons except for the rune and byte
@@ -751,7 +752,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 	case *TypeParam:
 		// x must be an unbound type parameter (see comment above).
 		if debug {
-			assert(u.asTypeParam(x) == nil)
+			assert(u.asBoundTypeParam(x) == nil)
 		}
 		// By definition, a valid type argument must be in the type set of
 		// the respective type constraint. Therefore, the type argument's
@@ -774,13 +775,13 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 			// need to take care of that case separately.
 			if cx := coreType(x); cx != nil {
 				if traceInference {
-					u.tracef("core %s ≡ %s", x, y)
+					u.tracef("core %s ≡ %s", xorig, yorig)
 				}
 				// If y is a defined type, it may not match against cx which
 				// is an underlying type (incl. int, string, etc.). Use assign
 				// mode here so that the unifier automatically takes under(y)
 				// if necessary.
-				return u.nify(cx, y, assign, p)
+				return u.nify(cx, yorig, assign, p)
 			}
 		}
 		// x != y and there's nothing to do
@@ -789,7 +790,7 @@ func (u *unifier) nify(x, y Type, mode unifyMode, p *ifacePair) (result bool) {
 		// avoid a crash in case of nil type
 
 	default:
-		panic(sprintf(nil, true, "u.nify(%s, %s, %d)", x, y, mode))
+		panic(sprintf(nil, true, "u.nify(%s, %s, %d)", xorig, yorig, mode))
 	}
 
 	return false

src/internal/types/testdata/fixedbugs/issue67628.go

@@ -0,0 +1,17 @@
+// -gotypesalias=1
+
+// Copyright 2024 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 any](x P) P { return x }
+
+func _() {
+	type A = int
+	var a A
+	b := f(a) // type of b is A
+	// error should report type of b as A, not int
+	_ = b /* ERROR "mismatched types A and untyped string" */ + "foo"
+}