[flow] Do not break up MaybeT, OptionalT on non-distributive conditio…

…nal type

Summary:
MaybeT and OptionalT should not be destructed unless we explicitly decide that it should have the distributive behavior:

https://github.com/facebook/flow/blob/8e5ecac527cc76be12eae51a5219106dfbe8c69e/src/typing/flow_js.ml#L6554

https://github.com/facebook/flow/blob/8e5ecac527cc76be12eae51a5219106dfbe8c69e/src/typing/flow_js.ml#L6650-L6651

https://github.com/facebook/flow/blob/8e5ecac527cc76be12eae51a5219106dfbe8c69e/src/typing/flow_js.ml#L6659-L6660

In the main recursive flow_js, we did it correctly only for UnionT:

https://github.com/facebook/flow/blob/8e5ecac527cc76be12eae51a5219106dfbe8c69e/src/typing/flow_js.ml#L1715-L1722

but missed the case for `MaybeT` and `OptionalT`. This diff fixes that.

Changelog: [errors] Previously we incorrectly distribute-over-union for maybe or optional input types of conditional type. (e.g. `(?string) extends string ? true : false` is evaluated to `true | false` instead of `false`). This is now fixed, and code that depends on the bug might have new errors.

Reviewed By: panagosg7

Differential Revision: D67145501

fbshipit-source-id: 573933e56fef794ee29709645d48d4754aa874c5

src/typing/flow_js.ml

@@ -1152,7 +1152,11 @@ struct
           rec_flow cx trace (t, u)
         | (MaybeT _, ResolveUnionT { reason; resolved; unresolved; upper; id }) ->
           resolve_union cx trace reason id resolved unresolved l upper
-        | (MaybeT (reason, t), _) ->
+        | (MaybeT (reason, t), _)
+          when match u with
+               | ConditionalT { distributive_tparam_name; _ } ->
+                 Option.is_some distributive_tparam_name
+               | _ -> true ->
           let reason = replace_desc_reason RNullOrVoid reason in
           let t = push_type_alias_reason reason t in
           rec_flow cx trace (NullT.make reason, u);
@@ -1197,7 +1201,11 @@ struct
           resolve_union cx trace reason id resolved unresolved l upper
         | (OptionalT { reason = _; type_ = t; use_desc = _ }, ExtractReactRefT _) ->
           rec_flow cx trace (t, u)
-        | (OptionalT { reason = r; type_ = t; use_desc }, _) ->
+        | (OptionalT { reason = r; type_ = t; use_desc }, _)
+          when match u with
+               | ConditionalT { distributive_tparam_name; _ } ->
+                 Option.is_some distributive_tparam_name
+               | _ -> true ->
           let void = VoidT.why_with_use_desc ~use_desc r in
           rec_flow cx trace (void, u);
           rec_flow cx trace (t, u)

tests/conditional_type/conditional_type.exp

@@ -910,6 +910,81 @@ References:
              ^^^^^^ [2]
 
 
+Error -------------------------------------------------------------------------- non_generic_maybe_nullable_tests.js:2:1
+
+Cannot cast `true` to `MaybeStringExtendsString` because boolean [1] is incompatible with boolean literal `false` [2].
+[incompatible-cast]
+
+   non_generic_maybe_nullable_tests.js:2:1
+   2| true as MaybeStringExtendsString; // error
+      ^^^^ [1]
+
+References:
+   non_generic_maybe_nullable_tests.js:2:9
+   2| true as MaybeStringExtendsString; // error
+              ^^^^^^^^^^^^^^^^^^^^^^^^ [2]
+
+
+Error -------------------------------------------------------------------------- non_generic_maybe_nullable_tests.js:7:3
+
+Cannot cast `true` to `OptionalStringExtendsString` because boolean [1] is incompatible with boolean literal
+`false` [2]. [incompatible-cast]
+
+   non_generic_maybe_nullable_tests.js:7:3
+   7|   true as OptionalStringExtendsString; // error
+        ^^^^ [1]
+
+References:
+   non_generic_maybe_nullable_tests.js:7:11
+   7|   true as OptionalStringExtendsString; // error
+                ^^^^^^^^^^^^^^^^^^^^^^^^^^^ [2]
+
+
+Error ------------------------------------------------------------------------- non_generic_maybe_nullable_tests.js:13:1
+
+Cannot cast `false` to `StringExtendsMaybeString` because boolean [1] is incompatible with boolean literal `true` [2].
+[incompatible-cast]
+
+   non_generic_maybe_nullable_tests.js:13:1
+   13| false as StringExtendsMaybeString; // error
+       ^^^^^ [1]
+
+References:
+   non_generic_maybe_nullable_tests.js:13:10
+   13| false as StringExtendsMaybeString; // error
+                ^^^^^^^^^^^^^^^^^^^^^^^^ [2]
+
+
+Error ------------------------------------------------------------------------- non_generic_maybe_nullable_tests.js:16:1
+
+Cannot cast `true` to `StringOrNullOrVoidExtendsString` because boolean [1] is incompatible with boolean literal
+`false` [2]. [incompatible-cast]
+
+   non_generic_maybe_nullable_tests.js:16:1
+   16| true as StringOrNullOrVoidExtendsString; // error
+       ^^^^ [1]
+
+References:
+   non_generic_maybe_nullable_tests.js:16:9
+   16| true as StringOrNullOrVoidExtendsString; // error
+               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [2]
+
+
+Error ------------------------------------------------------------------------- non_generic_maybe_nullable_tests.js:21:1
+
+Cannot cast `false` to `StringExtendsStringOrNullOrVoid` because boolean [1] is incompatible with boolean literal
+`true` [2]. [incompatible-cast]
+
+   non_generic_maybe_nullable_tests.js:21:1
+   21| false as StringExtendsStringOrNullOrVoid; // error
+       ^^^^^ [1]
+
+References:
+   non_generic_maybe_nullable_tests.js:21:10
+   21| false as StringExtendsStringOrNullOrVoid; // error
+                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ [2]
+
+
 Error ---------------------------------------------------------------------------------------------- poly_t_input.js:5:1
 
 Cannot cast `v1` to empty because boolean literal `false` [1] is incompatible with empty [2]. [incompatible-cast]
@@ -1087,7 +1162,7 @@ References:
 
 
 
-Found 70 errors
+Found 75 errors
 
 Only showing the most relevant union/intersection branches.
 To see all branches, re-run Flow with --show-all-branches

tests/conditional_type/non_generic_maybe_nullable_tests.js

@@ -0,0 +1,21 @@
+type MaybeStringExtendsString = (?string) extends string ? true : false;
+true as MaybeStringExtendsString; // error
+false as MaybeStringExtendsString; // ok
+
+function optionalTypeTest(x?: string) {
+  type OptionalStringExtendsString = (typeof x) extends string ? true : false;
+  true as OptionalStringExtendsString; // error
+  false as OptionalStringExtendsString; // ok
+}
+
+type StringExtendsMaybeString = string extends (?string) ? true : false;
+true as StringExtendsMaybeString; // ok
+false as StringExtendsMaybeString; // error
+
+type StringOrNullOrVoidExtendsString = (string | null | void) extends string ? true : false;
+true as StringOrNullOrVoidExtendsString; // error
+false as StringOrNullOrVoidExtendsString; // ok
+
+type StringExtendsStringOrNullOrVoid = string extends (string | null | void) ? true : false;
+true as StringExtendsStringOrNullOrVoid; // ok
+false as StringExtendsStringOrNullOrVoid; // error