go/types: fix error message for embedded non-interface types in interfaces

Fixes #10979.

Change-Id: Iac25645ba8181a56a75ddfcd29ff6d64c15c4f57
Reviewed-on: https://go-review.googlesource.com/10466Reviewed-by: Alan Donovan <adonovan@google.com>

src/go/types/testdata/issues.src

@@ -71,3 +71,27 @@ func issue9473(a []int, b ...int) {
 	append_(f0(), f1()... /* ERROR cannot use */ )
 	append_(f0(), f2()... /* ERROR cannot use */ )
 }
+
+// Check that embedding a non-interface type in an interface results in a good error message.
+func issue10979() {
+	type _ interface {
+		int /* ERROR int is not an interface */
+	}
+	type T struct{}
+	type _ interface {
+		T /* ERROR T is not an interface */
+	}
+	type _ interface {
+		nosuchtype /* ERROR undeclared name: nosuchtype */
+	}
+	type _ interface {
+		fmt /* ERROR Nosuchtype not declared by package fmt */ .Nosuchtype
+	}
+	type _ interface {
+		nosuchpkg /* ERROR undeclared name: nosuchpkg */ .Nosuchtype
+	}
+	type I interface {
+		I /* ERROR I\.m \(value of type func\(I\)\) is not a type */ .m
+		m()
+	}
+}

src/go/types/typexpr.go

@@ -525,20 +525,14 @@ func (check *Checker) interfaceType(iface *Interface, ityp *ast.InterfaceType, d
 	for _, e := range embedded {
 		pos := e.Pos()
 		typ := check.typExpr(e, nil, path)
+		// Determine underlying embedded (possibly incomplete) type
+		// by following its forward chain.
 		named, _ := typ.(*Named)
-		if named == nil {
-			if typ != Typ[Invalid] {
-				check.invalidAST(pos, "%s is not named type", typ)
-			}
-			continue
-		}
-		// determine underlying (possibly incomplete) type
-		// by following its forward chain
-		u := underlying(named)
-		embed, _ := u.(*Interface)
+		under := underlying(named)
+		embed, _ := under.(*Interface)
 		if embed == nil {
-			if u != Typ[Invalid] {
-				check.errorf(pos, "%s is not an interface", named)
+			if typ != Typ[Invalid] {
+				check.errorf(pos, "%s is not an interface", typ)
 			}
 			continue
 		}