Avoid generating expression-dependent type when it's safe to do so

Summary:
Expression-dependent types are necessary only for classes that EITHER define abstract type constants (which can be realized by distinct types in subclasses) OR have methods that make use of `this` in their signatures (because these can be realized by distinct types in subclasses). Unfortunately, there is nothing at the class level that flags this property, but for well-known built-in classes such as `vec` and `dict` we can avoid generating an expression-dependent type.

Comments welcome on how we might generalize this.

Reviewed By: mheiber, madgen

Differential Revision: D53920324

fbshipit-source-id: e68445d1155cbc40c364faba7a0536da25e4a6f0

hphp/hack/src/typing/typing_dependent_type.ml

@@ -102,6 +102,12 @@ module ExprDepTy = struct
       let (env, ty) = Env.expand_type env ty in
       match deref ty with
       | (_, Tclass (_, Exact, _)) -> (env, ty)
+      (* Special case for known final generic types that don't use this in their API *)
+      | (_, Tclass ((_, x), _, _))
+        when String.equal x Naming_special_names.Collections.cVec
+             || String.equal x Naming_special_names.Collections.cDict
+             || String.equal x Naming_special_names.Collections.cKeyset ->
+        (env, ty)
       | (_, Tclass (((_, x) as c), Nonexact _, tyl)) ->
         let class_ = Env.get_class env x in
         (* If a class is both final and variant, we must treat it as non-final

hphp/hack/test/sound_dynamic/typing/subtype_supportdynamic.bad.php.exp

@@ -33,25 +33,25 @@ Invalid argument (Typing[4110])
   File "subtype_supportdynamic.bad.php", line 3, characters 12-30:
   Expected `dynamic`
   File "subtype_supportdynamic.bad.php", line 13, characters 39-44:
-  Type `<expr#3> as vec<C>` is not a subtype of `dynamic` under dynamic-aware subtyping
+  Type `vec<C>` is not a subtype of `dynamic` under dynamic-aware subtyping
 File "subtype_supportdynamic.bad.php", line 18, characters 5-7:
 Invalid argument (Typing[4110])
   File "subtype_supportdynamic.bad.php", line 3, characters 12-30:
   Expected `dynamic`
   File "subtype_supportdynamic.bad.php", line 13, characters 50-64:
-  Type `<expr#4> as dict<string, C>` is not a subtype of `dynamic` under dynamic-aware subtyping
+  Type `dict<string, C>` is not a subtype of `dynamic` under dynamic-aware subtyping
 File "subtype_supportdynamic.bad.php", line 19, characters 5-7:
 Invalid argument (Typing[4110])
   File "subtype_supportdynamic.bad.php", line 3, characters 12-30:
   Expected `dynamic`
   File "subtype_supportdynamic.bad.php", line 14, characters 23-41:
-  Type `<expr#5> as dict<arraykey, C>` is not a subtype of `dynamic` under dynamic-aware subtyping
+  Type `dict<arraykey, C>` is not a subtype of `dynamic` under dynamic-aware subtyping
 File "subtype_supportdynamic.bad.php", line 20, characters 5-7:
 Invalid argument (Typing[4110])
   File "subtype_supportdynamic.bad.php", line 3, characters 12-30:
   Expected `dynamic`
   File "subtype_supportdynamic.bad.php", line 15, characters 23-31:
-  Type `<expr#6> as vec<C>` is not a subtype of `dynamic` under dynamic-aware subtyping
+  Type `vec<C>` is not a subtype of `dynamic` under dynamic-aware subtyping
 File "subtype_supportdynamic.bad.php", line 24, characters 5-6:
 Invalid argument (Typing[4110])
   File "subtype_supportdynamic.bad.php", line 3, characters 12-30:
@@ -63,13 +63,13 @@ Invalid argument (Typing[4110])
   File "subtype_supportdynamic.bad.php", line 3, characters 12-30:
   Expected `dynamic`
   File "subtype_supportdynamic.bad.php", line 28, characters 33-38:
-  Type `<expr#2> as vec<C>` is not a subtype of `dynamic` under dynamic-aware subtyping
+  Type `vec<C>` is not a subtype of `dynamic` under dynamic-aware subtyping
 File "subtype_supportdynamic.bad.php", line 33, characters 5-6:
 Invalid argument (Typing[4110])
   File "subtype_supportdynamic.bad.php", line 3, characters 12-30:
   Expected `dynamic`
   File "subtype_supportdynamic.bad.php", line 29, characters 33-44:
-  Type `<expr#3> as dict<int, C>` is not a subtype of `dynamic` under dynamic-aware subtyping
+  Type `dict<int, C>` is not a subtype of `dynamic` under dynamic-aware subtyping
 File "subtype_supportdynamic.bad.php", line 37, characters 5-6:
 Invalid argument (Typing[4110])
   File "subtype_supportdynamic.bad.php", line 3, characters 12-30:

hphp/hack/test/typecheck/new_inference/simplify_unions.php.exp

@@ -1,3 +1,3 @@
 File "simplify_unions.php", line 10, characters 3-13:
-  (<expr#2> as vec<C> | vec<nothing>)
+  vec<C>
 No errors

hphp/hack/test/typecheck/new_inference/simplify_unions.php.imp_pess_exp

@@ -1,3 +1,3 @@
 File "simplify_unions.php", line 10, characters 3-13:
-  ~(<expr#2> as vec<C> | vec<nothing>)
+  ~vec<C>
 No errors

hphp/hack/test/typecheck/typedef17.php.imp_pess_exp

@@ -3,6 +3,4 @@ Invalid argument (Typing[4110])
   File "typedef17.php", line 14, characters 26-29:
   Expected `~vec<_>`
   File "typedef17.php", line 22, characters 13-20:
-  But got `<expr#2> as vec<int>`
-  File "typedef17.php", line 23, characters 6-7:
-    where `<expr#2>` is a reference to this expression
+  But got `vec<int>`