Don't create default constructor for subclasses

This behavior prevented classes from correctly inheriting the
constructor function from their superclass.

Fixes facebook#214

src/typing/type_inference_js.ml

@@ -4578,14 +4578,17 @@ and body_loc = Ast.Statement.FunctionDeclaration.(function
 )
 
 (* Makes signatures for fields and methods in a class. *)
-and mk_signature cx reason_c c_type_params_map body = Ast.Class.(
+and mk_signature cx reason_c c_type_params_map superClass body = Ast.Class.(
   let _, { Body.body = elements } = body in
 
   (* In case there is no constructor, we create one. *)
-  let default_methods =
-    SMap.singleton "constructor"
-      (replace_reason "default constructor" reason_c, [], SMap.empty,
-       ([], [], VoidT.t, SMap.empty, SMap.empty))
+  let default_methods = match superClass with
+    | None ->
+        SMap.singleton "constructor"
+          (replace_reason "default constructor" reason_c, [], SMap.empty,
+           ([], [], VoidT.t, SMap.empty, SMap.empty))
+    | Some _ ->
+        SMap.empty
   in
   (* NOTE: We used to mine field declarations from field assignments in a
      constructor as a convenience, but it was not worth it: often, all that did
@@ -4765,7 +4768,7 @@ and mk_class cx loc reason_c = Ast.Class.(function { id=_; body; superClass;
 
   (* fields: { f: X }, methods_: { m<Y: X>(x: Y): X } *)
   let sfields, smethods_, fields, methods_ =
-    mk_signature cx reason_c type_params_map body
+    mk_signature cx reason_c type_params_map superClass body
   in
 
   let id = Flow_js.mk_nominal cx in

tests/class_subtyping/class_subtyping.exp

@@ -1,4 +1,6 @@
 
+test2.js:7:14,20: constructor call
+Error:
 test2.js:7:18,18: C
 This type is incompatible with
 test2.js:7:8,10: B

tests/generics/generics.exp

@@ -7,6 +7,10 @@ generics.js:18:18,18: number
 This type is incompatible with
 generics.js:19:7,12: string
 
+generics.js:27:9,15: undefined (too few arguments, expected default/rest parameters)
+This type is incompatible with
+generics.js:28:7,12: string
+
 generics.js:28:22,22: number
 This type is incompatible with
 generics.js:28:7,12: string
@@ -24,4 +28,4 @@ Incorrect number of type parameters (expected 0)
 generics.js:43:10,25: 
 Incorrect number of type parameters (expected 0)
 
-Found 7 errors
+Found 8 errors

tests/generics/generics.js

@@ -24,7 +24,7 @@ class E<X> extends C<X> {
     set(x:X):X { /*return x;*/ this.x = x; return /*this.x; */this.get(); }
 }
 
-var e = new E();
+var e = new E(); // error: too few arguments to inherited constructor
 var x:string = e.set(0);
 
 class F<X> { }