Print unresolved types as _

Summary:
If we can't resolve a type variable, display it as "_" rather than "[unresolved]". This makes type errors easier to read.

For example, if a user writes `await 1` we used to show:

```
-> Expected Awaitable<[unresolved]>
-> But got int
```

The problem isn't that we couldn't find an inner type for `Awaitable`. The problem is that `1` isn't an awaitable value at all! We now show this:

```
-> Expected Awaitable<_>
-> But got int
```

This is much easier to read, especially in larger types. It's also consistent with `Terr`.

Reviewed By: hgoldstein

Differential Revision: D39067882

fbshipit-source-id: 0e758cf36fe9c73f3b6347d7f53572fabeadeca3

Author: Wilfred

hphp/hack/src/typing/typing_print.ml

@@ -653,7 +653,7 @@ module Full = struct
             if ISet.mem n' st then
               text "[rec]"
             else if !blank_tyvars then
-              text "[unresolved]"
+              text "_"
             else
               text ("#" ^ string_of_int n')
           in

hphp/hack/test/decl/yield_deeper.php.typecheck.exp

@@ -1,6 +1,6 @@
 File "yield_deeper.php", line 20, characters 3-45:
 `await` can only operate on an `Awaitable` (Typing[4110])
   File "yield_deeper.php", line 20, characters 3-45:
-  Expected `Awaitable<[unresolved]>`
+  Expected `Awaitable<_>`
   File "yield_deeper.php", line 20, characters 9-45:
   But got `AsyncGenerator<null, int, mixed>` (result of `async function` containing a `yield`)

hphp/hack/test/error_formatting_highlighted/position_within_position.php.exp

@@ -1,5 +1,5 @@
 Typing[4110] await can only operate on an Awaitable [1]
--> Expected Awaitable<[unresolved]> [1]
+-> Expected Awaitable<_> [1]
 -> But got int [2]
 
 position_within_position.php:4:3

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

@@ -13,7 +13,7 @@ Some type arguments violate their constraints (Typing[4323])
 File "generic_constraint_dynamic.bad.php", line 21, characters 14-20:
 Invalid argument (Typing[4110])
   File "generic_constraint_dynamic.bad.php", line 21, characters 12-12:
-  Expected `~[unresolved]` because type parameter `T` of `C` could not be determined. Please add explicit type parameters to the invocation of `C`
+  Expected `~_` because type parameter `T` of `C` could not be determined. Please add explicit type parameters to the invocation of `C`
   File "generic_constraint_dynamic.bad.php", line 6, characters 39-39:
     via this generic `T`
   File "generic_constraint_dynamic.bad.php", line 21, characters 18-18:

hphp/hack/test/sound_dynamic/typing/sound_dynamic_callable_1.bad.php.exp

@@ -5,4 +5,4 @@ Method  `foo` in class `C` is not dynamically callable. (Typing[4110])
   File "sound_dynamic_callable_1.bad.php", line 11, characters 27-34:
   Expected `dynamic` because method must be callable in a dynamic context
   File "sound_dynamic_callable_1.bad.php", line 12, characters 16-18:
-  Type `Box<[unresolved]>` is not a subtype of `dynamic` under dynamic-aware subtyping
+  Type `Box<_>` is not a subtype of `dynamic` under dynamic-aware subtyping

hphp/hack/test/sound_dynamic/typing/sound_dynamic_callable_1.bad.php.legacy_decl.exp

@@ -5,4 +5,4 @@ Method  `foo` in class `C` is not dynamically callable. (Typing[4110])
   File "sound_dynamic_callable_1.bad.php", line 11, characters 27-34:
   Expected `dynamic` because method must be callable in a dynamic context
   File "sound_dynamic_callable_1.bad.php", line 12, characters 16-18:
-  Type `Box<[unresolved]>` is not a subtype of `dynamic` under dynamic-aware subtyping
+  Type `Box<_>` is not a subtype of `dynamic` under dynamic-aware subtyping

hphp/hack/test/sound_dynamic/typing/sound_dynamic_callable_2.bad.php.exp

@@ -5,4 +5,4 @@ Method  `foo` in class `C` is not dynamically callable. (Typing[4110])
   File "sound_dynamic_callable_2.bad.php", line 10, characters 27-34:
   Expected `dynamic` because method must be callable in a dynamic context
   File "sound_dynamic_callable_2.bad.php", line 11, characters 16-18:
-  Type `Box<[unresolved]>` is not a subtype of `dynamic` under dynamic-aware subtyping
+  Type `Box<_>` is not a subtype of `dynamic` under dynamic-aware subtyping

hphp/hack/test/sound_dynamic/typing/sound_dynamic_callable_2.bad.php.legacy_decl.exp

@@ -5,4 +5,4 @@ Method  `foo` in class `C` is not dynamically callable. (Typing[4110])
   File "sound_dynamic_callable_2.bad.php", line 10, characters 27-34:
   Expected `dynamic` because method must be callable in a dynamic context
   File "sound_dynamic_callable_2.bad.php", line 11, characters 16-18:
-  Type `Box<[unresolved]>` is not a subtype of `dynamic` under dynamic-aware subtyping
+  Type `Box<_>` is not a subtype of `dynamic` under dynamic-aware subtyping

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

@@ -55,7 +55,7 @@ Invalid `foreach` (Typing[4110])
 File "supportdyn.bad.php", line 21, characters 3-11:
 `await` can only operate on an `Awaitable` (Typing[4110])
   File "supportdyn.bad.php", line 21, characters 3-11:
-  Expected `Awaitable<[unresolved]>`
+  Expected `Awaitable<_>`
   File "supportdyn.bad.php", line 18, characters 32-38:
   But got `nonnull`
   File "supportdynamic.hhi", line 6, characters 29-29:

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

@@ -55,7 +55,7 @@ Invalid `foreach` (Typing[4110])
 File "supportdynamic.bad.php", line 18, characters 3-11:
 `await` can only operate on an `Awaitable` (Typing[4110])
   File "supportdynamic.bad.php", line 18, characters 3-11:
-  Expected `Awaitable<[unresolved]>`
+  Expected `Awaitable<_>`
   File "supportdynamic.bad.php", line 15, characters 32-38:
   But got `nonnull`
   File "supportdynamic.hhi", line 6, characters 29-29: