Allow function types inside tuples inside arrow return types

Summary:
To avoid ambiguity, Flow disallows using a function type in the return type, like

  const f = (): boolean => string => { return (b: boolean): string => "hello"; }

unless you wrap it in parens

  const f = (): (boolean => string) => { return (b: boolean): string => "hello"; }

We forgot to allow it inside tuples too, like

  const f = (): [boolean => string] => { return [(b: boolean): string => "hello"]; }

Reviewed By: dsainati1

Differential Revision: D15150181

fbshipit-source-id: dbc9292ac0935ea79373437501b22373e538979a

src/parser/test/flow/arrow_function/tuple_return_type.js

@@ -0,0 +1 @@
+(): [number => void] => {}

src/parser/test/flow/arrow_function/tuple_return_type.tree.json

@@ -0,0 +1,70 @@
+{
+  "type":"Program",
+  "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":26}},
+  "range":[0,26],
+  "body":[
+    {
+      "type":"ExpressionStatement",
+      "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":26}},
+      "range":[0,26],
+      "expression":{
+        "type":"ArrowFunctionExpression",
+        "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":26}},
+        "range":[0,26],
+        "id":null,
+        "params":[],
+        "body":{
+          "type":"BlockStatement",
+          "loc":{"source":null,"start":{"line":1,"column":24},"end":{"line":1,"column":26}},
+          "range":[24,26],
+          "body":[]
+        },
+        "async":false,
+        "generator":false,
+        "predicate":null,
+        "expression":false,
+        "returnType":{
+          "type":"TypeAnnotation",
+          "loc":{"source":null,"start":{"line":1,"column":2},"end":{"line":1,"column":20}},
+          "range":[2,20],
+          "typeAnnotation":{
+            "type":"TupleTypeAnnotation",
+            "loc":{"source":null,"start":{"line":1,"column":4},"end":{"line":1,"column":20}},
+            "range":[4,20],
+            "types":[
+              {
+                "type":"FunctionTypeAnnotation",
+                "loc":{"source":null,"start":{"line":1,"column":5},"end":{"line":1,"column":19}},
+                "range":[5,19],
+                "params":[
+                  {
+                    "type":"FunctionTypeParam",
+                    "loc":{"source":null,"start":{"line":1,"column":5},"end":{"line":1,"column":11}},
+                    "range":[5,11],
+                    "name":null,
+                    "typeAnnotation":{
+                      "type":"NumberTypeAnnotation",
+                      "loc":{"source":null,"start":{"line":1,"column":5},"end":{"line":1,"column":11}},
+                      "range":[5,11]
+                    },
+                    "optional":false
+                  }
+                ],
+                "returnType":{
+                  "type":"VoidTypeAnnotation",
+                  "loc":{"source":null,"start":{"line":1,"column":15},"end":{"line":1,"column":19}},
+                  "range":[15,19]
+                },
+                "rest":null,
+                "typeParameters":null
+              }
+            ]
+          }
+        },
+        "typeParameters":null
+      },
+      "directive":null
+    }
+  ],
+  "comments":[]
+}

src/parser/type_parser.ml

@@ -229,7 +229,7 @@ module Type (Parse: Parser_common.PARSER) : TYPE = struct
     in fun env ->
       with_loc (fun env ->
         Expect.token env T_LBRACKET;
-        let tl = types env [] in
+        let tl = types (with_no_anon_function_type false env) [] in
         Expect.token env T_RBRACKET;
         Type.Tuple tl
       ) env