[flow] Specialize TypeAppT at the declaration site

Summary:
Similar to `EvalT`, let's force it at the declaration site. Similar to the challenges to `EvalT`, forcing it everywhere causes too many errors, which points to some gaps in the language design.

This diff alone will not fix the issue of misplaced error, but at least the error will now consistently appear at the declaration site no matter whether it's used, so users can see it in the IDE without being overridden. It's also a step towards getting rid of all misplaced errors. In a future when we can eagerly validate all these things that tend to cause misplaced errors, we can just filter out these errors in the future.

Changelog: [errors] Flow will now error on invalid type applications at the declaration site, if the type application does not use another generic type from an outer scope. [example](https://flow.org/try/#1N4Igxg9gdgZglgcxALlAIwIZoKYBsD6uEEAztvhgE6UYCe+JADpdhgCYowa5kA0I2KAFcAtiRQAXSkOz9sADwxgJ+NPTbYuQ3BMnTZA+Y2yU4IwRO4A6SFBIrGVDGM7c+IFkolXpUCWewUEAwhCQgRDH8wEH4hMnwROHlsNnw4KHwwSLAAC3wANyo4LFxscWQuHgMNZmwsiRSAWglaY1cq-hIAa2wJXNpG4Vxcdvdu3v7B0RxKUYMhKDBSqmbWwIq3eagoOrKSKgH0wtMMPznY7d2SfcoBiEZ-aG5G3Ix085AF-ZhsRoRehqUEiNMgSQHlSruBZxJrMcJwMhzAC+-EgGiCLWMAAIACpYgC8WPspigCAA3AAdPxrLEAQQJuIAPMT0ggAHxkrEAei5WJMlAglCxUAgAHcYiB8iYSHBoEEIvYTCAkUA)

Reviewed By: panagosg7

Differential Revision: D55456495

fbshipit-source-id: 03527deab4a2054d4659c1e8481a9df3f921ed64

src/typing/type_annotation.ml

@@ -2597,9 +2597,17 @@ module Make (ConsGen : Type_annotation_sig.ConsGen) (Statement : Statement_sig.S
       (ConsGen.mk_instance env.cx ~type_t_kind reason c, None)
     | Some (loc, { Ast.Type.TypeArgs.arguments = targs; comments }) ->
       let (targs, targs_ast) = convert_list env targs in
-      ( typeapp_annot ~from_value:false ~use_desc:false annot_loc c targs,
-        Some (loc, { Ast.Type.TypeArgs.arguments = targs_ast; comments })
-      )
+      let t = typeapp_annot ~from_value:false ~use_desc:false annot_loc c targs in
+      ( if Subst_name.Set.is_empty (Type_subst.free_var_finder env.cx t) then
+        match t with
+        | TypeAppT { reason; use_op; type_; targs; from_value = _; use_desc = _ } ->
+          Context.add_post_inference_validation_flow
+            env.cx
+            type_
+            (SpecializeT (use_op, reason, reason, true, Some targs, Tvar.mk env.cx reason))
+        | _ -> failwith "typeapp_annot should create a TypeAppT"
+      );
+      (t, Some (loc, { Ast.Type.TypeArgs.arguments = targs_ast; comments }))
 
   and mk_type_param env ~from_infer_type (loc, type_param) =
     let { cx; tparams_map; _ } = env in

tests/badly_positioned_polyt/badly_positioned_polyt.exp

@@ -1,3 +1,18 @@
+Error ----------------------------------------------------------------------------------------------- concretize.js:7:47
+
+Cannot instantiate `SyntheticKeyboardEvent` because `C1` [1] is incompatible with `EventTarget` [2] in type argument
+`T`. [incompatible-type-arg]
+
+   concretize.js:7:47
+    7| function _onKeyDown(e: SyntheticKeyboardEvent<C1>): void {};
+                                                     ^^ [1]
+
+References:
+   <BUILTINS>/react-dom.js:74:42
+   74| declare class SyntheticKeyboardEvent<+T: EventTarget = EventTarget>
+                                                ^^^^^^^^^^^ [2]
+
+
 Error ----------------------------------------------------------------------------------------------- concretize.js:8:16
 
 Cannot create `C1` element because `EventTarget` [1] is incompatible with `C1` [2] in type argument `T` [3] of the first
@@ -74,4 +89,4 @@ References:
 
 
 
-Found 4 errors
+Found 5 errors

tests/contextual_typing/contextual_typing.exp

@@ -530,6 +530,22 @@ References:
                                ^^^^ [1]
 
 
+Error --------------------------------------------------------------------------------- implicit_instantiation.js:211:21
+
+Cannot instantiate `Record` because interface type [1] is not a subtype of object type [2]. Class instances are not
+subtypes of object types; consider rewriting object type [2] as an interface in type argument `T`.
+[class-object-subtyping]
+
+   implicit_instantiation.js:211:21
+   211|     records: Record<interface {}>,
+                            ^^^^^^^^^^^^ [1]
+
+References:
+   implicit_instantiation.js:203:18
+   203|   type Record<T: {...}> = { ...T, ... };
+                         ^^^^^ [2]
+
+
 Error -------------------------------------------------------------------------------------------- intersections.js:7:24
 
 Cannot cast `x` to number because string [1] is incompatible with number [2]. [incompatible-cast]
@@ -1221,4 +1237,4 @@ References:
 
 
 
-Found 88 errors
+Found 89 errors

tests/relay/relay.exp

@@ -331,6 +331,36 @@ References:
                                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [2]
 
 
+Error --------------------------------------------------------------------------------------------------- more.js:140:34
+
+Cannot instantiate `FragmentReference` because object type [1] is incompatible with `FragmentTypeof` [2] in type
+argument `T`. [incompatible-type-arg]
+
+   more.js:140:34
+   140|   __fragments: FragmentReference<{thing1: true}> &
+                                         ^^^^^^^^^^^^^^ [1]
+
+References:
+   more.js:13:42
+    13| declare opaque type FragmentReference<T: FragmentTypeof>;
+                                                 ^^^^^^^^^^^^^^ [2]
+
+
+Error --------------------------------------------------------------------------------------------------- more.js:142:23
+
+Cannot instantiate `FragmentReference` because object type [1] is incompatible with `FragmentTypeof` [2] in type
+argument `T`. [incompatible-type-arg]
+
+   more.js:142:23
+   142|     FragmentReference<{thing2: true}>,
+                              ^^^^^^^^^^^^^^ [1]
+
+References:
+   more.js:13:42
+    13| declare opaque type FragmentReference<T: FragmentTypeof>;
+                                                 ^^^^^^^^^^^^^^ [2]
+
+
 Error --------------------------------------------------------------------------------------------------- more.js:153:20
 
 Cannot create `PluralTest` element because null [1] is incompatible with read-only array type [2] in property `users`.
@@ -428,4 +458,4 @@ References:
 
 
 
-Found 26 errors
+Found 28 errors

tests/types_first_reasons/types_first_reasons.exp

@@ -13,6 +13,20 @@ References:
                                     ^^ [2]
 
 
+Error ---------------------------------------------------------------------------------------- instantiation_err.js:2:17
+
+Cannot instantiate `T` because `T` [1] is not a polymorphic type. [incompatible-use]
+
+   instantiation_err.js:2:17
+   2| export type U = T<string>; // Err: cannot instantiate
+                      ^^^^^^^^^
+
+References:
+   type_exports.js:1:13
+   1| export type T = string;
+                  ^ [1]
+
+
 Error ---------------------------------------------------------------------------------------- instantiation_err.js:2:17
 
 Cannot instantiate `T` because `T` [1] is not a polymorphic type. (FLOW BUG: This is a misplaced error. The original
@@ -139,4 +153,4 @@ References:
 
 
 
-Found 9 errors
+Found 10 errors