go/types: use correct (file) scopes when computing interface method sets

This was already partially fixed by commit 99843e22e8
(https://go-review.googlesource.com/c/go/+/96376); but
we missed a couple of places where we also need to
propagate the scope.

Fixes #25008.

Change-Id: I041fa74d1f6d3b5a8edb922efa126ff1dacd7900
Reviewed-on: https://go-review.googlesource.com/109139
Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/check_test.go

@@ -90,6 +90,7 @@ var tests = [][]string{
 	{"testdata/labels.src"},
 	{"testdata/issues.src"},
 	{"testdata/blank.src"},
+	{"testdata/issue25008b.src", "testdata/issue25008a.src"}, // order (b before a) is crucial!
 }
 
 var fset = token.NewFileSet()

src/go/types/interfaces.go

@@ -221,9 +221,9 @@ func (check *Checker) infoFromTypeLit(scope *Scope, iface *ast.InterfaceType, tn
 				var e *ifaceInfo
 				switch ename := f.Type.(type) {
 				case *ast.Ident:
-					e = check.infoFromTypeName(ename, path)
+					e = check.infoFromTypeName(scope, ename, path)
 				case *ast.SelectorExpr:
-					e = check.infoFromQualifiedTypeName(ename)
+					e = check.infoFromQualifiedTypeName(scope, ename)
 				default:
 					// The parser makes sure we only see one of the above.
 					// Constructed ASTs may contain other (invalid) nodes;
@@ -262,7 +262,7 @@ func (check *Checker) infoFromTypeLit(scope *Scope, iface *ast.InterfaceType, tn
 // which must denote a type whose underlying type is an interface.
 // The same result qualifications apply as for infoFromTypeLit.
 // infoFromTypeName should only be called from infoFromTypeLit.
-func (check *Checker) infoFromTypeName(name *ast.Ident, path []*TypeName) *ifaceInfo {
+func (check *Checker) infoFromTypeName(scope *Scope, name *ast.Ident, path []*TypeName) *ifaceInfo {
 	// A single call of infoFromTypeName handles a sequence of (possibly
 	// recursive) type declarations connected via unqualified type names.
 	// Each type declaration leading to another typename causes a "tail call"
@@ -291,7 +291,7 @@ func (check *Checker) infoFromTypeName(name *ast.Ident, path []*TypeName) *iface
 
 typenameLoop:
 	// name must be a type name denoting a type whose underlying type is an interface
-	obj := check.lookup(name.Name)
+	_, obj := scope.LookupParent(name.Name, check.pos)
 	if obj == nil {
 		return nil
 	}
@@ -333,7 +333,7 @@ typenameLoop:
 			goto typenameLoop
 		case *ast.SelectorExpr:
 			// type tname p.T
-			return check.infoFromQualifiedTypeName(typ)
+			return check.infoFromQualifiedTypeName(decl.file, typ)
 		case *ast.InterfaceType:
 			// type tname interface{...}
 			return check.infoFromTypeLit(decl.file, typ, tname, path)
@@ -360,13 +360,13 @@ typenameLoop:
 }
 
 // infoFromQualifiedTypeName computes the method set for the given qualified type name, or nil.
-func (check *Checker) infoFromQualifiedTypeName(qname *ast.SelectorExpr) *ifaceInfo {
+func (check *Checker) infoFromQualifiedTypeName(scope *Scope, qname *ast.SelectorExpr) *ifaceInfo {
 	// see also Checker.selector
 	name, _ := qname.X.(*ast.Ident)
 	if name == nil {
 		return nil
 	}
-	obj1 := check.lookup(name.Name)
+	_, obj1 := scope.LookupParent(name.Name, check.pos)
 	if obj1 == nil {
 		return nil
 	}

src/go/types/testdata/issue25008a.src

@@ -0,0 +1,15 @@
+// Copyright 2018 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
+
+import "io"
+
+type A interface {
+        io.Reader
+}
+
+func f(a A) {
+        a.Read(nil)
+}

src/go/types/testdata/issue25008b.src

@@ -0,0 +1,9 @@
+// Copyright 2018 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
+
+type B interface {
+    A
+}