Consider type parameter defaults in TypeAppVarianceCheckT

Summary:
When we compare type typeapps with the same root, we take a shortcut and only
compare the type arguments according to the polarity of the type parameter.

This shortcut did not properly account for type parameter defaults. This diff
handles the default case by ensuring that all type arguments are constrained.

Reviewed By: jbrown215

Differential Revision: D15675738

fbshipit-source-id: 35cee8b2789ecc48a89717b557fea3bd177f3fa4

src/typing/flow_js.ml

@@ -3579,28 +3579,36 @@ let rec __flow cx ((l: Type.t), (u: Type.use_t)) trace =
         add_output cx ~trace
           (Error_message.ETooManyTypeArgs (reason_tapp, reason_arity, maximum_arity));
       ) else (
-        let unused_targs = Nel.fold_left (fun targs { name; default; polarity; reason; _ } ->
-          match default, targs with
-          | None, [] ->
-            (* fewer arguments than params but no default *)
-            add_output cx ~trace (Error_message.ETooFewTypeArgs
-              (reason_tapp, reason_arity, minimum_arity));
-            []
-          | _, [] -> []
-          | _, (t1, t2)::targs ->
+        let unused_targs, _, _ = Nel.fold_left (fun (targs, map1, map2) tparam->
+          let { name; default; polarity; reason; _ } = tparam in
+          let flow_targs t1 t2 =
             let use_op = Frame (TypeArgCompatibility {
               name;
               targ = reason;
               lower = reason_op;
               upper = reason_tapp;
               polarity;
             }, use_op) in
-            (match polarity with
+            match polarity with
             | Positive -> rec_flow cx trace (t1, UseT (use_op, t2))
             | Negative -> rec_flow cx trace (t2, UseT (use_op, t1))
-            | Neutral -> rec_unify cx trace ~use_op t1 t2);
-            targs
-        ) targs tparams in
+            | Neutral -> rec_unify cx trace ~use_op t1 t2;
+          in
+          match default, targs with
+          | None, [] ->
+            (* fewer arguments than params but no default *)
+            add_output cx ~trace (Error_message.ETooFewTypeArgs
+              (reason_tapp, reason_arity, minimum_arity));
+            [], map1, map2
+          | Some default, [] ->
+            let t1 = subst cx ~use_op map1 default in
+            let t2 = subst cx ~use_op map2 default in
+            flow_targs t1 t2;
+            [], SMap.add name t1 map1, SMap.add name t2 map2
+          | _, (t1, t2)::targs ->
+            flow_targs t1 t2;
+            targs, SMap.add name t1 map1, SMap.add name t2 map2
+        ) (targs, SMap.empty, SMap.empty) tparams in
         assert (unused_targs = []);
       )
 

tests/poly/inout.js

@@ -0,0 +1,7 @@
+declare opaque type A;
+declare opaque type B: A;
+
+class C<+Out, -In: Out = Out> {}
+
+declare var x: C<B>;
+(x: C<A>); // error: A ~> B in default-expanded type

tests/poly/poly.exp

@@ -173,6 +173,26 @@ References:
                   ^^^^^^ [2]
 
 
+Error ----------------------------------------------------------------------------------------------------- inout.js:7:2
+
+Cannot cast `x` to `C` because `B` [1] is incompatible with `A` [2] in type argument `In` [3].
+
+   inout.js:7:2
+   7| (x: C<A>); // error: A ~> B in default-expanded type
+       ^
+
+References:
+   inout.js:6:18
+   6| declare var x: C<B>;
+                       ^ [1]
+   inout.js:7:7
+   7| (x: C<A>); // error: A ~> B in default-expanded type
+            ^ [2]
+   inout.js:4:16
+   4| class C<+Out, -In: Out = Out> {}
+                     ^^ [3]
+
+
 Error --------------------------------------------------------------------------------------------------- phantom.js:7:2
 
 Cannot cast `a` to `B` because number [1] is incompatible with string [2] in type argument `Phantom` [3].
@@ -290,4 +310,4 @@ References:
 
 
 
-Found 19 errors
+Found 20 errors