go/types: copy embedded methods unchanged when completing interfaces

The existing code adjusted the receivers of embedded interface methods
to match the embedding interface type. That required cloning (shallow
copying) the embedded methods and destroyed their object identity in
the process. Don't do this anymore. The consequence to clients is that
they might see different methods of an interface having different
receiver types; they are always the type of the interface that explicitly
declared the method (which is what one usually would want, anyway).

Fixes #28282.

Change-Id: I2e6f1497f46affdf7510547a64601de3787367db
Reviewed-on: https://go-review.googlesource.com/c/143757
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/issues_test.go

@@ -422,3 +422,25 @@ func TestIssue28005(t *testing.T) {
 		}
 	}
 }
+
+func TestIssue28282(t *testing.T) {
+	// create type interface { error }
+	et := Universe.Lookup("error").Type()
+	it := NewInterfaceType(nil, []Type{et})
+	it.Complete()
+	// verify that after completing the interface, the embedded method remains unchanged
+	want := et.Underlying().(*Interface).Method(0)
+	got := it.Method(0)
+	if got != want {
+		t.Fatalf("%s.Method(0): got %q (%p); want %q (%p)", it, got, got, want, want)
+	}
+	// verify that lookup finds the same method in both interfaces (redundant check)
+	obj, _, _ := LookupFieldOrMethod(et, false, nil, "Error")
+	if obj != want {
+		t.Fatalf("%s.Lookup: got %q (%p); want %q (%p)", et, obj, obj, want, want)
+	}
+	obj, _, _ = LookupFieldOrMethod(it, false, nil, "Error")
+	if obj != want {
+		t.Fatalf("%s.Lookup: got %q (%p); want %q (%p)", it, obj, obj, want, want)
+	}
+}

src/go/types/stdlib_test.go

@@ -231,11 +231,6 @@ func typecheck(t *testing.T, path string, filenames []string) {
 
 	// Perform checks of API invariants.
 
-	// The code below fails at the moment - see issue #28282.
-	// Exit early for now to keep the longtest builder happy.
-	// TODO(gri) fix this ASAP and uncomment the code below.
-
-	/*
 	// All Objects have a package, except predeclared ones.
 	errorError := Universe.Lookup("error").Type().Underlying().(*Interface).ExplicitMethod(0) // (error).Error
 	for id, obj := range info.Uses {
@@ -249,7 +244,6 @@ func typecheck(t *testing.T, path string, filenames []string) {
 			}
 		}
 	}
-	*/
 }
 
 // pkgFilenames returns the list of package filenames for the given directory.

src/go/types/type.go

@@ -352,19 +352,14 @@ func (t *Interface) Complete() *Interface {
 		return t
 	}
 
+	// collect all methods
 	var allMethods []*Func
 	allMethods = append(allMethods, t.methods...)
 	for _, et := range t.embeddeds {
 		it := et.Underlying().(*Interface)
 		it.Complete()
-		for _, tm := range it.allMethods {
+		// copy embedded methods unchanged (see issue #28282)
-			// Make a copy of the method and adjust its receiver type.
+		allMethods = append(allMethods, it.allMethods...)
-			newm := *tm
-			newmtyp := *tm.typ.(*Signature)
-			newm.typ = &newmtyp
-			newmtyp.recv = NewVar(newm.pos, newm.pkg, "", t)
-			allMethods = append(allMethods, &newm)
-		}
 	}
 	sort.Sort(byUniqueMethodName(allMethods))