Basic support for typeof x === 'symbol'

src/common/reason.ml

@@ -60,7 +60,7 @@ end
 
 type reason_desc =
   | RAnyExplicit | RAnyImplicit
-  | RNumber | RString | RBoolean | RMixed | REmpty | RVoid | RNull
+  | RNumber | RString | RBoolean | RMixed | REmpty | RVoid | RNull | RSymbol
   | RNullOrVoid
   | RLongStringLit of int (* Max length *)
   | RStringLit of string
@@ -398,6 +398,7 @@ let rec string_of_desc = function
   | RVoid -> "undefined"
   | RNull -> "null"
   | RNullOrVoid -> "null or undefined"
+  | RSymbol -> "symbol"
   | RStringLit "" -> "empty string"
   | RStringLit x -> spf "string literal `%s`" x
   | RNumberLit x -> spf "number literal `%s`" x
@@ -1071,6 +1072,7 @@ let inferred_union_elem_array_desc = RCustom
 let classification_of_reason r = match desc_of_reason ~unwrap:true r with
 | RNumber
 | RString
+| RSymbol
 | RBoolean
 | RLongStringLit _
 | RStringLit _

src/common/reason.mli

@@ -9,7 +9,7 @@ val mk_id: unit -> int
 
 type reason_desc =
   | RAnyExplicit | RAnyImplicit
-  | RNumber | RString | RBoolean | RMixed | REmpty | RVoid | RNull
+  | RNumber | RString | RBoolean | RMixed | REmpty | RVoid | RNull | RSymbol
   | RNullOrVoid
   | RLongStringLit of int (* Max length *)
   | RStringLit of string

src/typing/debug_js.ml

@@ -25,6 +25,7 @@ let string_of_pred_ctor = function
   | FunP -> "FunP"
   | ObjP -> "ObjP"
   | ArrP -> "ArrP"
+  | SymbolP -> "SymbolP"
   | SingletonBoolP _ -> "SingletonBoolP"
   | SingletonStrP _ -> "SingletonStrP"
   | SingletonNumP _ -> "SingletonNumP"
@@ -1335,6 +1336,7 @@ and json_of_pred_impl json_cx p = Hh_json.(
   | MaybeP
   | BoolP
   | StrP
+  | SymbolP
   | NumP
   | FunP
   | ObjP
@@ -1677,6 +1679,7 @@ let rec dump_t_ (depth, tvars) cx t =
     | Mixed_non_maybe -> "Mixed_non_maybe"
     | Mixed_non_null -> "Mixed_non_null"
     | Mixed_non_void -> "Mixed_non_void"
+    | Mixed_symbol -> "Mixed_symbol"
     | Empty_intersection -> "Empty_intersection"
   in
 

src/typing/flow_js.ml

@@ -8727,6 +8727,16 @@ and predicate cx trace t l p = match p with
   | NotP StrP ->
     rec_flow_t cx trace (Type_filter.not_string l, t)
 
+  (***********************)
+  (* typeof _ ~ "symbol" *)
+  (***********************)
+
+  | SymbolP ->
+    rec_flow_t cx trace (Type_filter.symbol l, t)
+
+  | NotP SymbolP ->
+    rec_flow_t cx trace (Type_filter.not_symbol l, t)
+
   (*********************)
   (* _ ~ "some string" *)
   (*********************)

src/typing/statement.ml

@@ -5465,6 +5465,7 @@ and predicates_of_condition cx e = Ast.(Expression.(
         | "number" -> Some NumP
         | "object" -> Some ObjP
         | "string" -> Some StrP
+        | "symbol" -> Some SymbolP
         | "undefined" -> Some VoidP
         | _ -> None
         in

src/typing/type.ml

@@ -720,6 +720,7 @@ module rec TypeTerm : sig
     | NumP (* number *)
     | ObjP (* object *)
     | StrP (* string *)
+    | SymbolP (* symbol *)
     | VoidP (* undefined *)
 
     | ArrP (* Array.isArray *)
@@ -753,6 +754,7 @@ module rec TypeTerm : sig
     | Mixed_non_null
     | Mixed_non_void
     | Mixed_function
+    | Mixed_symbol
     | Empty_intersection
 
   and any_source =
@@ -3194,6 +3196,7 @@ let rec string_of_predicate = function
   | NumP -> "number"
   | FunP -> "function"
   | ObjP -> "object"
+  | SymbolP -> "symbol"
 
   (* Array.isArray *)
   | ArrP -> "array"

src/typing/type_filter.ml

@@ -281,6 +281,19 @@ let not_string t =
   | DefT (_, (AnyT _ | StrT _)) -> DefT (reason_of_t t, EmptyT)
   | _ -> t
 
+let symbol t =
+  match t with
+  | DefT (r, MixedT _) ->
+      DefT (replace_reason_const RSymbol r, MixedT Mixed_symbol)
+  | _ ->
+    (* TODO: since symbols aren't supported, `t` is never a symbol so always empty *)
+    let reason = reason_of_t t in
+    DefT (replace_reason_const RSymbol reason, EmptyT)
+
+let not_symbol t =
+  (* TODO: since symbols aren't supported, `t` is never a symbol so always pass it through *)
+  t
+
 let number t =
   match t with
   | DefT (r, MixedT Mixed_truthy) -> DefT (replace_reason_const NumT.desc r, NumT Truthy)
@@ -307,6 +320,7 @@ let object_ cx t =
     let proto = ObjProtoT reason in
     let obj = Obj_type.mk_with_proto cx reason ?dict proto in
     begin match flavor with
+    | Mixed_symbol
     | Mixed_truthy
     | Mixed_non_maybe
     | Mixed_non_null -> obj

src/typing/type_mapper.ml

@@ -522,6 +522,7 @@ class virtual ['a] t = object(self)
     | NumP
     | ObjP
     | StrP
+    | SymbolP
     | VoidP
     | ArrP
     | PropExistsP _ -> p

src/typing/type_visitor.ml

@@ -223,6 +223,7 @@ class ['a] t = object(self)
   | NumP -> acc
   | ObjP -> acc
   | StrP -> acc
+  | SymbolP -> acc
   | VoidP -> acc
   | ArrP -> acc
   | PropExistsP _ -> acc

tests/refinements/refinements.exp

@@ -3081,6 +3081,23 @@ References:
                ^^^^^ [2]
 
 
+Error --------------------------------------------------------------------------------------------------- typeof.js:83:6
+
+Cannot cast `x` to string because symbol [1] is incompatible with string [2].
+
+   typeof.js:83:6
+   83|     (x: string); // error
+            ^
+
+References:
+   typeof.js:81:24
+   81| function testSymbol(x: mixed) {
+                              ^^^^^ [1]
+   typeof.js:83:9
+   83|     (x: string); // error
+               ^^^^^^ [2]
+
+
 Error --------------------------------------------------------------------------------------------------- undef.js:31:13
 
 Cannot perform arithmetic operation because null or undefined [1] is not a number.
@@ -3350,4 +3367,4 @@ Cannot perform arithmetic operation because undefined [1] is not a number.
 
 
 
-Found 242 errors
+Found 243 errors

tests/refinements/typeof.js

@@ -77,3 +77,9 @@ function testInstanceIsObject() {
     (x: empty); // error
   }
 }
+
+function testSymbol(x: mixed) {
+  if (typeof x === "symbol") { // ok
+    (x: string); // error
+  }
+}