internal/apidiff: support materialized aliases

(The previous CL 559935 was insufficient.)

Also, improve test output.

Updates golang/go#65294

Change-Id: I05cafadce0dd6f18ff66d2ca462a3eb546c4ca81
Reviewed-on: https://go-review.googlesource.com/c/tools/+/577576
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Robert Findley <rfindley@google.com>

go.mod

@@ -10,6 +10,8 @@ require (
 
 require golang.org/x/sync v0.7.0
 
+require github.com/google/go-cmp v0.6.0 // indirect
+
 require (
 	golang.org/x/sys v0.19.0 // indirect
 	golang.org/x/telemetry v0.0.0-20240228155512-f48c80bd79b2

go.sum

@@ -1,3 +1,5 @@
+github.com/google/go-cmp v0.6.0 h1:ofyhxvXcZhMsU5ulbFiLKl/XBFqE1GSq7atu8tAmTRI=
+github.com/google/go-cmp v0.6.0/go.mod h1:17dUlkBOakJ0+DkrSSNjCkIjxS6bF9zb3elmeNGIjoY=
 github.com/yuin/goldmark v1.4.13 h1:fVcFKWvrslecOb/tg+Cc05dkeYx540o0FuFt3nUVDoE=
 github.com/yuin/goldmark v1.4.13/go.mod h1:6yULJ656Px+3vBD8DxQVa3kxgyrAnzto9xy5taEt/CY=
 golang.org/x/mod v0.17.0 h1:zY54UmvipHiNd+pm+m0x9KhZ9hl1/7QNMyxXbc6ICqA=

internal/apidiff/apidiff.go

@@ -208,7 +208,8 @@ func (d *differ) typeChanged(obj types.Object, part string, old, new types.Type)
 // Since these can change without affecting compatibility, we don't want users to
 // be distracted by them, so we remove them.
 func removeNamesFromSignature(t types.Type) types.Type {
-	sig, ok := aliases.Unalias(t).(*types.Signature)
+	t = aliases.Unalias(t)
+	sig, ok := t.(*types.Signature)
 	if !ok {
 		return t
 	}
@@ -217,7 +218,7 @@ func removeNamesFromSignature(t types.Type) types.Type {
 		var vars []*types.Var
 		for i := 0; i < p.Len(); i++ {
 			v := p.At(i)
-			vars = append(vars, types.NewVar(v.Pos(), v.Pkg(), "", v.Type()))
+			vars = append(vars, types.NewVar(v.Pos(), v.Pkg(), "", aliases.Unalias(v.Type())))
 		}
 		return types.NewTuple(vars...)
 	}

internal/apidiff/apidiff_test.go

@@ -10,11 +10,11 @@ import (
 	"go/types"
 	"os"
 	"path/filepath"
-	"reflect"
 	"sort"
 	"strings"
 	"testing"
 
+	"github.com/google/go-cmp/cmp"
 	"golang.org/x/tools/go/packages"
 	"golang.org/x/tools/internal/testenv"
 )
@@ -42,12 +42,12 @@ func TestChanges(t *testing.T) {
 	report := Changes(oldpkg.Types, newpkg.Types)
 
 	got := report.messages(false)
-	if !reflect.DeepEqual(got, wanti) {
-		t.Errorf("incompatibles: got %v\nwant %v\n", got, wanti)
+	if diff := cmp.Diff(wanti, got); diff != "" {
+		t.Errorf("incompatibles (-want +got):\n%s", diff)
 	}
 	got = report.messages(true)
-	if !reflect.DeepEqual(got, wantc) {
-		t.Errorf("compatibles: got %v\nwant %v\n", got, wantc)
+	if diff := cmp.Diff(wantc, got); diff != "" {
+		t.Errorf("compatibles (-want +got):\n%s", diff)
 	}
 }