[typechecker] allow @autoclosure closure to be a typealias

Author: Suyash Srijan

lib/Sema/TypeCheckType.cpp

@@ -1996,7 +1996,27 @@ Type TypeResolver::resolveAttributedType(TypeAttributes &attrs,
       hasFunctionAttr = true;
       break;
     }
-
+  
+  // If we have an @autoclosure then try resolving the top level type repr
+  // first as it may be pointing to a typealias
+  if (attrs.has(TAK_autoclosure)) {
+    if (auto CITR = dyn_cast<ComponentIdentTypeRepr>(repr)) {
+      auto typeAliasResolver = TypeResolverContext::TypeAliasDecl;
+      if (auto type = resolveTopLevelIdentTypeComponent(resolution, CITR,
+                                                        typeAliasResolver)) {
+        if (auto TAT = dyn_cast<TypeAliasType>(type.getPointer())) {
+          if (auto underlyingRepr = TAT->getDecl()
+                                       ->getUnderlyingTypeLoc()
+                                        .getTypeRepr()) {
+            if (!underlyingRepr->isInvalid()) {
+              repr = underlyingRepr;
+            }
+          }
+        }
+      }
+    }
+  }
+  
   // Function attributes require a syntactic function type.
   auto *fnRepr = dyn_cast<FunctionTypeRepr>(repr);
 

test/attr/attr_autoclosure.swift

@@ -234,3 +234,14 @@ func rdar_30906031(in arr: [Int], fn: @autoclosure () -> Int) -> Bool {
     arr.lazy.filter { $0 >= escapableF() }.isEmpty
   }
 }
+
+// SR-2688
+class Foo {
+  typealias FooClosure = () -> String
+  func fooFunction(closure: @autoclosure FooClosure) {} // ok
+}
+
+class Bar {
+  typealias BarClosure = (String) -> String
+  func barFunction(closure: @autoclosure BarClosure) {} // expected-error {{argument type of @autoclosure parameter must be '()'}}
+}