Merge pull request #37941 from LucianoPAlmeida/SR-14784-diag

[SR-14784][Sema] Make sure we look through optionals on anchor fnType for MissingCallFailure

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -3252,7 +3252,8 @@ bool MissingCallFailure::diagnoseAsError() {
     switch (last.getKind()) {
     case ConstraintLocator::ContextualType:
     case ConstraintLocator::ApplyArgToParam: {
-      auto fnType = getType(anchor)->castTo<FunctionType>();
+      auto type = getType(anchor)->lookThroughAllOptionalTypes();
+      auto fnType = type->castTo<FunctionType>();
       emitDiagnostic(diag::missing_nullary_call, fnType->getResult())
           .fixItInsertAfter(insertLoc, "()");
       return true;

test/Constraints/function.swift

@@ -245,3 +245,8 @@ func test_passing_noescape_function_ref_to_generic_parameter() {
     }
   }
 }
+
+// SR-14784
+func SR14784<T>(_ fs: () -> T..., a _ : Int) -> T {
+  fs.first! // expected-error{{function produces expected type 'T'; did you mean to call it with '()'?}} {{11-11=()}}
+}