[TypeChecker] Support _openExistential with async functions

Author: xedin

lib/Sema/ConstraintSystem.cpp

@@ -2278,6 +2278,7 @@ static std::pair<Type, Type> getTypeOfReferenceWithSpecialTypeCheckingSemantics(
                                          FunctionType::ExtInfoBuilder()
                                              .withNoEscape(true)
                                              .withThrows(true)
+                                             .withAsync(true)
                                              .build());
     FunctionType::Param args[] = {
       FunctionType::Param(existentialTy),
@@ -2287,6 +2288,7 @@ static std::pair<Type, Type> getTypeOfReferenceWithSpecialTypeCheckingSemantics(
                                      FunctionType::ExtInfoBuilder()
                                          .withNoEscape(false)
                                          .withThrows(true)
+                                         .withAsync(true)
                                          .build());
     return {refType, refType};
   }

test/decl/func/async.swift

@@ -48,3 +48,17 @@ func thereIsNoEscape(_ body: () async -> Void) async {
     await takeEscaping(escapingBody)
   }
 }
+
+func testAsyncExistentialOpen(_ v: P1) async {
+  func syncUnderlyingType<T>(u: T) {}
+  func syncThrowsUnderlyingType<T>(u: T) throws {}
+
+  func asyncUnderlyingType<T>(u: T) async {}
+  func asyncThrowsUnderlyingType<T>(u: T) async throws {}
+
+  _openExistential(v, do: syncUnderlyingType)
+  await _openExistential(v, do: asyncUnderlyingType)
+
+  try! _openExistential(v, do: syncThrowsUnderlyingType)
+  try! await _openExistential(v, do: asyncThrowsUnderlyingType)
+}