[parser] Add parser support for _ in constructors and calls

Summary:
This diff adds support for parsing `_`, which will be used to specify that a type parameter should be implicitly instantiated.

We add a new ast node to the Expression module, `TypeParameterInstantiation`. This is reminiscent of `Type`'s `ParameterInstantiation`, except that instead of a list of `Type.t`s, it contains a list of `Implicit of Loc.t | Explicit of Type.t`. When parsing `New` and `Call` type arguments, we parse `_` as an `Implicit` and all other types `t` as `Explicit t`.

Creating the new ast node comes with a number of benefits:
1. External consumers of our AST currently consume `ParameterInstantiation` as it was originally designed: for type arguments to polymorphic types. Creating a new AST node makes sure we don't need to update all of those external consumers.
2. This new AST node allows OCaml to enforce the invariant that `_` can only appear in `Call` and `New`.
3. Related to 2., this more faithfully models the structure of our AST. `Call` and `New` have type arguments, but the fact that we used the same representation as we did for `Type` was just because they happened to have the same internal representation. These types are types we come across when parsing an expression, not a type annotation.

We also now say that `_` is a reserved keyword when it's found in type positions, so it cannot be used unless it means implicit instantiation.

We need to update babel to consider "_" to be a reserved identifier in type positions and to parse `_` as an `ImplicitInstantiationAnnotation`.

Reviewed By: samwgoldman

Differential Revision: D10105957

fbshipit-source-id: 3003b0cadbb1f9163a07bae1957710e9fbdeccdd

src/parser/estree_translator.ml

@@ -450,7 +450,7 @@ end with type t = Impl.t) = struct
     | loc, New _new -> New.(
         node "NewExpression" loc [
           "callee", expression _new.callee;
-          "typeArguments", option type_parameter_instantiation _new.targs;
+          "typeArguments", option type_parameter_instantiation_with_implicit _new.targs;
           "arguments", array_of_list expression_or_spread _new.arguments;
         ]
       )
@@ -1030,6 +1030,13 @@ end with type t = Impl.t) = struct
     | Exists -> exists_type loc
   )
 
+  and implicit loc = generic_type (loc,
+    {Type.Generic.id = Type.Generic.Identifier.Unqualified (loc, "_"); targs = None})
+
+  and explicit_or_implicit_targ x = match x with
+  | Expression.TypeParameterInstantiation.Explicit t -> _type t
+  | Expression.TypeParameterInstantiation.Implicit loc -> implicit loc
+
   and any_type loc = node "AnyTypeAnnotation" loc []
 
   and mixed_type loc = node "MixedTypeAnnotation" loc []
@@ -1273,6 +1280,11 @@ end with type t = Impl.t) = struct
       "params", array_of_list _type targs;
     ]
 
+  and type_parameter_instantiation_with_implicit (loc, targs) =
+    node "TypeParameterInstantiation" loc [
+      "params", array_of_list explicit_or_implicit_targ targs;
+    ]
+
   and jsx_element (loc, (element: (Loc.t, Loc.t) JSX.element)) = JSX.(
     node "JSXElement" loc [
       "openingElement", jsx_opening element.openingElement;
@@ -1458,7 +1470,7 @@ end with type t = Impl.t) = struct
 
   and call_node_properties call = Expression.Call.([
     "callee", expression call.callee;
-    "typeArguments", option type_parameter_instantiation call.targs;
+    "typeArguments", option type_parameter_instantiation_with_implicit call.targs;
     "arguments", array_of_list expression_or_spread call.arguments;
   ])
 

src/parser/expression_parser.ml

@@ -572,7 +572,7 @@ module Expression
         (* Parameterized call syntax is ambiguous, so we fall back to
            standard parsing if it fails. *)
         Try.or_else env ~fallback:left (fun env ->
-          let targs = Type.type_parameter_instantiation env in
+          let targs = type_parameter_instantiation env in
           arguments ?targs env
         )
     | _ -> left
@@ -622,7 +622,7 @@ module Expression
              standard parsing if it fails. *)
           let error_callback _ _ = raise Try.Rollback in
           let env = env |> with_error_callback error_callback in
-          Try.or_else env ~fallback:None Type.type_parameter_instantiation
+          Try.or_else env ~fallback:None type_parameter_instantiation
         else
           None
       in
@@ -637,6 +637,35 @@ module Expression
         arguments;
       }))
 
+  and type_parameter_instantiation =
+    let args env acc =
+      let rec args_helper env acc =
+        match Peek.token env with
+      | T_EOF
+      | T_GREATER_THAN -> List.rev acc
+      | _ ->
+          let t = match Peek.token env with
+          | T_IDENTIFIER {value = "_"; _} ->
+              let loc = Peek.loc env in
+              Expect.identifier env "_";
+              Expression.TypeParameterInstantiation.Implicit loc
+          | _ -> Expression.TypeParameterInstantiation.Explicit(Type._type env)
+          in
+          let acc = t::acc in
+          if Peek.token env <> T_GREATER_THAN
+          then Expect.token env T_COMMA;
+          args_helper env acc
+      in args_helper env acc
+
+    in fun env -> if Peek.token env = T_LESS_THAN then
+      Some (with_loc (fun env ->
+        Expect.token env T_LESS_THAN;
+        let args = args env [] in
+        Expect.token env T_GREATER_THAN;
+        args
+      ) env)
+    else None
+
   and arguments =
     let argument env =
       match Peek.token env with

src/parser/flow_ast.ml

@@ -249,6 +249,7 @@ and Type : sig
     and ('M, 'T) t' = ('M, 'T) TypeParam.t list
     [@@deriving show]
   end
+
   module ParameterInstantiation : sig
     type ('M, 'T) t = 'M * ('M, 'T) t'
     and ('M, 'T) t' = ('M, 'T) Type.t list
@@ -627,6 +628,14 @@ and Statement : sig
 end = Statement
 
 and Expression : sig
+
+  module TypeParameterInstantiation : sig
+    type ('M, 'T) t = 'M * ('M, 'T) t'
+    and ('M, 'T) type_parameter_instantiation = Explicit of ('M, 'T) Type.t | Implicit of 'T
+    and ('M, 'T) t' = ('M, 'T) type_parameter_instantiation list
+    [@@deriving show]
+  end
+
   module SpreadElement : sig
     type ('M, 'T) t = 'M * ('M, 'T) t'
     and ('M, 'T) t' = {
@@ -838,15 +847,15 @@ and Expression : sig
   module New : sig
     type ('M, 'T) t = {
       callee: ('M, 'T) Expression.t;
-      targs: ('M, 'T) Type.ParameterInstantiation.t option;
+      targs: ('M, 'T) Expression.TypeParameterInstantiation.t option;
       arguments: ('M, 'T) expression_or_spread list;
     }
     [@@deriving show]
   end
   module Call : sig
     type ('M, 'T) t = {
       callee: ('M, 'T) Expression.t;
-      targs: ('M, 'T) Type.ParameterInstantiation.t option;
+      targs: ('M, 'T) Expression.TypeParameterInstantiation.t option;
       arguments: ('M, 'T) expression_or_spread list;
     }
     [@@deriving show]

src/parser/parser_env.ml

@@ -440,7 +440,7 @@ let is_reserved str_val =
 let is_reserved_type str_val =
   match str_val with
   | "any" | "bool" | "boolean" | "empty" | "false" | "mixed" | "null" | "number"
-  | "static" | "string" | "true" | "typeof" | "void" | "interface" | "extends"
+  | "static" | "string" | "true" | "typeof" | "void" | "interface" | "extends" | "_"
     -> true
   | _ -> false
 

src/parser/test/flow/types/annotations/underscore_is_allowed_trailing_commas.js

@@ -0,0 +1,7 @@
+test<
+  _,
+  _,
+  number,
+  _,
+  _,
+>();

src/parser/test/flow/types/annotations/underscore_is_allowed_trailing_commas.tree.json

@@ -0,0 +1,96 @@
+{
+  "type":"Program",
+  "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":7,"column":4}},
+  "range":[0,40],
+  "body":[
+    {
+      "type":"ExpressionStatement",
+      "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":7,"column":4}},
+      "range":[0,40],
+      "expression":{
+        "type":"CallExpression",
+        "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":7,"column":3}},
+        "range":[0,39],
+        "callee":{
+          "type":"Identifier",
+          "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":4}},
+          "range":[0,4],
+          "name":"test",
+          "typeAnnotation":null,
+          "optional":false
+        },
+        "typeArguments":{
+          "type":"TypeParameterInstantiation",
+          "loc":{"source":null,"start":{"line":1,"column":4},"end":{"line":7,"column":1}},
+          "range":[4,37],
+          "params":[
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":2,"column":2},"end":{"line":2,"column":3}},
+              "range":[8,9],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":2,"column":2},"end":{"line":2,"column":3}},
+                "range":[8,9],
+                "name":"_",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            },
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":3,"column":2},"end":{"line":3,"column":3}},
+              "range":[13,14],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":3,"column":2},"end":{"line":3,"column":3}},
+                "range":[13,14],
+                "name":"_",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            },
+            {
+              "type":"NumberTypeAnnotation",
+              "loc":{"source":null,"start":{"line":4,"column":2},"end":{"line":4,"column":8}},
+              "range":[18,24]
+            },
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":5,"column":2},"end":{"line":5,"column":3}},
+              "range":[28,29],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":5,"column":2},"end":{"line":5,"column":3}},
+                "range":[28,29],
+                "name":"_",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            },
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":6,"column":2},"end":{"line":6,"column":3}},
+              "range":[33,34],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":6,"column":2},"end":{"line":6,"column":3}},
+                "range":[33,34],
+                "name":"_",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            }
+          ]
+        },
+        "arguments":[]
+      },
+      "directive":null
+    }
+  ],
+  "comments":[]
+}

src/parser/test/flow/types/annotations/underscore_is_implicit_anywhere_in_list.js

@@ -0,0 +1 @@
+test<number, _, string, _, _, _, Foo, Bar, Baz>();

src/parser/test/flow/types/annotations/underscore_is_implicit_anywhere_in_list.tree.json

@@ -0,0 +1,143 @@
+{
+  "type":"Program",
+  "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":50}},
+  "range":[0,50],
+  "body":[
+    {
+      "type":"ExpressionStatement",
+      "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":50}},
+      "range":[0,50],
+      "expression":{
+        "type":"CallExpression",
+        "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":49}},
+        "range":[0,49],
+        "callee":{
+          "type":"Identifier",
+          "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":4}},
+          "range":[0,4],
+          "name":"test",
+          "typeAnnotation":null,
+          "optional":false
+        },
+        "typeArguments":{
+          "type":"TypeParameterInstantiation",
+          "loc":{"source":null,"start":{"line":1,"column":4},"end":{"line":1,"column":47}},
+          "range":[4,47],
+          "params":[
+            {
+              "type":"NumberTypeAnnotation",
+              "loc":{"source":null,"start":{"line":1,"column":5},"end":{"line":1,"column":11}},
+              "range":[5,11]
+            },
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":1,"column":13},"end":{"line":1,"column":14}},
+              "range":[13,14],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":1,"column":13},"end":{"line":1,"column":14}},
+                "range":[13,14],
+                "name":"_",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            },
+            {
+              "type":"StringTypeAnnotation",
+              "loc":{"source":null,"start":{"line":1,"column":16},"end":{"line":1,"column":22}},
+              "range":[16,22]
+            },
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":1,"column":24},"end":{"line":1,"column":25}},
+              "range":[24,25],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":1,"column":24},"end":{"line":1,"column":25}},
+                "range":[24,25],
+                "name":"_",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            },
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":1,"column":27},"end":{"line":1,"column":28}},
+              "range":[27,28],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":1,"column":27},"end":{"line":1,"column":28}},
+                "range":[27,28],
+                "name":"_",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            },
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":1,"column":30},"end":{"line":1,"column":31}},
+              "range":[30,31],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":1,"column":30},"end":{"line":1,"column":31}},
+                "range":[30,31],
+                "name":"_",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            },
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":1,"column":33},"end":{"line":1,"column":36}},
+              "range":[33,36],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":1,"column":33},"end":{"line":1,"column":36}},
+                "range":[33,36],
+                "name":"Foo",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            },
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":1,"column":38},"end":{"line":1,"column":41}},
+              "range":[38,41],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":1,"column":38},"end":{"line":1,"column":41}},
+                "range":[38,41],
+                "name":"Bar",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            },
+            {
+              "type":"GenericTypeAnnotation",
+              "loc":{"source":null,"start":{"line":1,"column":43},"end":{"line":1,"column":46}},
+              "range":[43,46],
+              "id":{
+                "type":"Identifier",
+                "loc":{"source":null,"start":{"line":1,"column":43},"end":{"line":1,"column":46}},
+                "range":[43,46],
+                "name":"Baz",
+                "typeAnnotation":null,
+                "optional":false
+              },
+              "typeParameters":null
+            }
+          ]
+        },
+        "arguments":[]
+      },
+      "directive":null
+    }
+  ],
+  "comments":[]
+}