[4.1] Restore a very narrow function argument conversion for -swift-version 4.

lib/Sema/CSApply.cpp

@@ -6127,9 +6127,12 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
       // func bar() -> ((()) -> Void)? { return nil }
       // foo(bar) // This expression should compile in Swift 3 mode
       // ```
-      if (tc.getLangOpts().isSwiftVersion3()) {
-        auto obj1 = fromType->getAnyOptionalObjectType();
-        auto obj2 = toType->getAnyOptionalObjectType();
+      //
+      // See also: https://bugs.swift.org/browse/SR-6796
+      if (cs.getASTContext().isSwiftVersionAtLeast(3) &&
+          !cs.getASTContext().isSwiftVersionAtLeast(5)) {
+        auto obj1 = fromType->getOptionalObjectType();
+        auto obj2 = toType->getOptionalObjectType();
 
         if (obj1 && obj2) {
           auto *fn1 = obj1->getAs<AnyFunctionType>();

lib/Sema/CSSimplify.cpp

@@ -1190,6 +1190,35 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
     }
   }
 
+  // https://bugs.swift.org/browse/SR-6796
+  // Add a super-narrow hack to allow:
+  //   (()) -> T to be passed in place of () -> T
+  if (getASTContext().isSwiftVersionAtLeast(4) &&
+      !getASTContext().isSwiftVersionAtLeast(5)) {
+    SmallVector<LocatorPathElt, 4> path;
+    locator.getLocatorParts(path);
+
+    // Find the last path element, skipping GenericArgument elements
+    // so that we allow this exception in cases of optional types, and
+    // skipping OptionalPayload elements so that we allow this
+    // exception in cases of optional injection.
+    auto last = std::find_if(
+        path.rbegin(), path.rend(), [](LocatorPathElt &elt) -> bool {
+          return elt.getKind() != ConstraintLocator::GenericArgument &&
+                 elt.getKind() != ConstraintLocator::OptionalPayload;
+        });
+
+    if (last != path.rend()) {
+      if (last->getKind() == ConstraintLocator::ApplyArgToParam) {
+        if (auto *paren1 = dyn_cast<ParenType>(func1Input.getPointer())) {
+          auto innerTy = paren1->getUnderlyingType();
+          if (func2Input->isVoid() && innerTy->isVoid())
+            func1Input = innerTy;
+        }
+      }
+    }
+  }
+
   // Input types can be contravariant (or equal).
   SmallVector<AnyFunctionType::Param, 4> func1Params;
   SmallVector<AnyFunctionType::Param, 4> func2Params;

test/Compatibility/tuple_arguments.swift → test/Compatibility/tuple_arguments_3.swift

@@ -1,7 +1,8 @@
 // RUN: %target-typecheck-verify-swift -swift-version 3
 
 // Tests for tuple argument behavior in Swift 3, which was broken.
-// The Swift 4 test is in test/Constraints/tuple_arguments.swift.
+// The Swift 4 test is in test/Compatibility/tuple_arguments_4.swift.
+// The test for the most recent version is in test/Constraints/tuple_arguments.swift.
 
 // Key:
 // - "Crashes in actual Swift 3" -- snippets which crashed in Swift 3.0.1.
@@ -1502,3 +1503,20 @@ do {
   takeFn(fn: { (pair: (Int, Int?)) in } )
   takeFn { (pair: (Int, Int?)) in }
 }
+
+// https://bugs.swift.org/browse/SR-6796
+do {
+  func f(a: (() -> Void)? = nil) {}
+  func log<T>() -> ((T) -> Void)? { return nil }
+
+  f(a: log() as ((()) -> Void)?) // Allow ((()) -> Void)? to be passed in place of (() -> Void)?
+
+  func logNoOptional<T>() -> (T) -> Void { }
+  f(a: logNoOptional() as ((()) -> Void)) // Also allow the optional-injected form.
+
+  func g() {}
+  g(())
+
+  func h(_: ()) {}
+  h()
+}