go.tools/go/types: fix logic error in MissingMethod and type assert code

R=adonovan
CC=golang-dev
https://golang.org/cl/10448046

go/types/expr.go

@@ -1564,7 +1564,7 @@ func (check *checker) rawExpr(x *operand, e ast.Expr, hint Type, iota int, cycle
 		}
 		// x.(type) expressions are handled explicitly in type switches
 		if e.Type == nil {
-			check.errorf(e.Pos(), "use of .(type) outside type switch")
+			check.invalidAST(e.Pos(), "use of .(type) outside type switch")
 			goto Error
 		}
 		typ := check.typ(e.Type, false)
@@ -1575,10 +1575,15 @@ func (check *checker) rawExpr(x *operand, e ast.Expr, hint Type, iota int, cycle
 			var msg string
 			if wrongType {
 				msg = "%s cannot have dynamic type %s (wrong type for method %s)"
-			} else {
+			} else if _, ok := typ.Underlying().(*Interface); !ok {
+				// Concrete types must have all methods of T. Interfaces
+				// may not have all methods of T statically, but may have
+				// them dynamically; don't report an error in that case.
 				msg = "%s cannot have dynamic type %s (missing method %s)"
 			}
-			check.errorf(e.Type.Pos(), msg, x, typ, method.name)
+			if msg != "" {
+				check.errorf(e.Type.Pos(), msg, x, typ, method.name)
+			}
 			// ok to continue
 		}
 		x.mode = valueok

go/types/lookup.go

@@ -188,17 +188,21 @@ func MissingMethod(typ Type, T *Interface) (method *Func, wrongType bool) {
 		return
 	}
 
+	// An interface type implements T if it has at least the methods of T.
 	if ityp, _ := typ.Underlying().(*Interface); ityp != nil {
 		for _, m := range T.methods {
 			_, obj := lookupMethod(ityp.methods, m.pkg, m.name)
-			if obj != nil && !IsIdentical(obj.Type(), m.typ) {
+			if obj == nil {
+				return m, false
+			}
+			if !IsIdentical(obj.Type(), m.typ) {
 				return m, true
 			}
 		}
 		return
 	}
 
-	// a concrete type implements T if it implements all methods of T.
+	// A concrete type implements T if it implements all methods of T.
 	for _, m := range T.methods {
 		obj, _, indirect := LookupFieldOrMethod(typ, m.pkg, m.name)
 		if obj == nil {
@@ -222,6 +226,7 @@ func MissingMethod(typ Type, T *Interface) (method *Func, wrongType bool) {
 			return m, true
 		}
 	}
+
 	return
 }
 

go/types/testdata/conversions.src

@@ -33,4 +33,36 @@ func string_conversions() {
 // 
 var (
 	_ = int8(0)
-)
+)
+
+func interface_conversions() {
+	type E interface{}
+
+	type I1 interface{
+		m1()
+	}
+
+	type I2 interface{
+		m1()
+		m2()
+	}
+
+	var e E
+	var i1 I1
+	var i2 I2
+	_ = E(0)
+	_ = E(nil)
+	_ = E(e)
+	_ = E(i1)
+	_ = E(i2)
+
+	_ = I1 /* ERROR "cannot convert" */ (0)
+	_ = I1(nil)
+	_ = I1(i1)
+	_ = I1 /* ERROR "cannot convert" */ (e)
+	_ = I1(i2)
+
+	_ = I2 /* ERROR "cannot convert" */ (i1)
+
+	// TODO(gri) add more tests, improve error message
+}

go/types/testdata/expr3.src

@@ -281,6 +281,9 @@ func type_asserts() {
 	_ = t.(T1 /* ERROR "missing method m" */ )
 	_ = t.(T2 /* ERROR "wrong type for method m" */ )
 	_ = t.(I2 /* ERROR "wrong type for method m" */ )
+
+	// e doesn't statically have an m, but may have one dynamically.
+	_ = e.(I2)
 }
 
 func f0() {}