go.tools/go/types: fix interface vs interface type assertion

R=adonovan
CC=golang-dev
https://golang.org/cl/12378043

go/types/lookup.go

@@ -225,22 +225,18 @@ func consolidateMultiples(list []embeddedType) []embeddedType {
 // is missing or simply has the wrong type.
 //
 func MissingMethod(typ Type, T *Interface) (method *Func, wrongType bool) {
-	// an interface type implements T if it has no methods with conflicting signatures
-	// Note: This is stronger than the current spec. Should the spec require this?
-
 	// fast path for common case
 	if T.NumMethods() == 0 {
 		return
 	}
 
-	// An interface type implements T if it has at least the methods of T.
+	// The dynamic type of a value stored in an interface may implement T,
+	// but only if all the shared interface methods have matching signatures.
+	// Note: This is stronger than the current spec. Should the spec require this?
 	if ityp, _ := typ.Underlying().(*Interface); ityp != nil {
 		for _, m := range T.methods {
 			_, obj := lookupMethod(ityp.methods, m.pkg, m.name)
-			if obj == nil {
+			if obj != nil && !IsIdentical(obj.Type(), m.typ) {
-				return m, false
-			}
-			if !IsIdentical(obj.Type(), m.typ) {
 				return m, true
 			}
 		}

go/types/stdlib_test.go

@@ -108,6 +108,7 @@ func TestStdfixed(t *testing.T) {
 		"bug050.go", "bug088.go", "bug106.go", // TODO(gri) parser loses comments when bailing out early
 		"bug222.go", "bug282.go", "bug306.go", // TODO(gri) parser loses comments when bailing out early
 		"bug136.go", "bug179.go", "bug344.go", // TODO(gri) implement missing label checks
+		"bug251.go",                           // TODO(gri) incorrect cycle checks for interface types
 		"bug165.go",                           // TODO(gri) isComparable not working for incomplete struct type
 		"bug176.go",                           // TODO(gri) composite literal array index must be non-negative constant
 		"bug200.go",                           // TODO(gri) complete duplicate checking in expr switches

go/types/testdata/conversions.src

@@ -44,12 +44,19 @@ func interface_conversions() {
 
 	type I2 interface{
 		m1()
-		m2()
+		m2(x int)
+	}
+
+	type I3 interface{
+		m1()
+		m2() int
 	}
 
 	var e E
 	var i1 I1
 	var i2 I2
+	var i3 I3
+
 	_ = E(0)
 	_ = E(nil)
 	_ = E(e)
@@ -59,10 +66,18 @@ func interface_conversions() {
 	_ = I1 /* ERROR "cannot convert" */ (0)
 	_ = I1(nil)
 	_ = I1(i1)
-	_ = I1 /* ERROR "cannot convert" */ (e)
+	_ = I1(e)
 	_ = I1(i2)
 
-	_ = I2 /* ERROR "cannot convert" */ (i1)
+	_ = I2(nil)
+	_ = I2(i1)
+	_ = I2(i2)
+	_ = I2 /* ERROR "cannot convert" */ (i3)
+
+	_ = I3(nil)
+	_ = I3(i1)
+	_ = I3 /* ERROR "cannot convert" */ (i2)
+	_ = I3(i3)
 
 	// TODO(gri) add more tests, improve error message
 }

go/types/testdata/stmt0.src

@@ -298,6 +298,19 @@ func typeswitch1() {
 	}
 }
 
+// Test correct typeswitch against interface types.
+type A interface { a() }
+type B interface { b() }
+type C interface { a(int) }
+
+func typeswitch2() {
+   switch A(nil).(type) {
+   case A:
+   case B:
+   case C /* ERROR "cannot have dynamic type" */:
+   }
+}
+
 func rangeloops() {
 	var (
 		x int