[flow] Check interface variance

Summary: Changelog: [errors] flow will now validate variance in interface declarations

Reviewed By: marcoww6, gkz

Differential Revision: D85993064

fbshipit-source-id: 1a324bbd6ab65161b3ac264ca01ea173ff983362

Author: SamChou19815

src/typing/type_annotation.ml

@@ -3060,7 +3060,7 @@ module Make (Statement : Statement_sig.S) : Type_annotation_sig.S = struct
     let (iface_sig, properties) =
       add_interface_properties env properties ~this:(implicit_mixed_this reason) iface_sig
     in
-    let (_, t) = Class_type_sig.classtype ~check_polarity:false cx iface_sig in
+    let (_, t) = Class_type_sig.classtype ~check_polarity:true cx iface_sig in
     ( t,
       iface_sig,
       {

tests/polarity/call.js

@@ -1,2 +1,2 @@
 type O<+T> = { (x: T): void }; // Error: +T in a negative position
-interface I<+T> { (x: T): void }; // Error: +T in a negative position (TODO)
+interface I<+T> { (x: T): void }; // Error: +T in a negative position

tests/polarity/polarity.exp

@@ -13,6 +13,21 @@ References:
               ^ [1]
 
 
+Error ----------------------------------------------------------------------------------------------------- call.js:2:23
+
+Cannot use `T` [1] in an input position because `T` [1] is expected to occur only in output positions.
+[incompatible-variance]
+
+   call.js:2:23
+   2| interface I<+T> { (x: T): void }; // Error: +T in a negative position
+                            ^
+
+References:
+   call.js:2:14
+   2| interface I<+T> { (x: T): void }; // Error: +T in a negative position
+                   ^ [1]
+
+
 Error ------------------------------------------------------------------------------------------------------ fun.js:3:19
 
 Cannot use `T` [1] in an input position because `T` [1] is expected to occur only in output positions.
@@ -294,4 +309,4 @@ References:
 
 
 
-Found 22 errors
+Found 23 errors