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

Allow functions with type `(()) -> T` to be passed in places where we
expect `() -> T`, but only for -swift-version 4 (for -swift-version 3
this already works due to other horrible things in CSSimplify.cpp).

We need to look at how we can help migrate these cases to
-swift-version 5, but in the meantime, but that is something we can
consider separately.

Fixes rdar://problem/36670720 / https://bugs.swift.org/browse/SR-6796

(cherry picked from commit 6310aca)

Author: rudkx

lib/Sema/CSApply.cpp

@@ -6173,9 +6173,12 @@ 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

@@ -1516,3 +1516,14 @@ 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.
+}

test/Constraints/tuple_arguments.swift

@@ -1626,7 +1626,7 @@ func rdar33239714() {
 do {
   func foo(_: (() -> Void)?) {}
   func bar() -> ((()) -> Void)? { return nil }
-  foo(bar()) // expected-error {{cannot convert value of type '((()) -> Void)?' to expected argument type '(() -> Void)?'}}
+  foo(bar()) // Allow ((()) -> Void)? to be passed in place of (() -> Void)? for -swift-version 4 but not later.
 }
 
 // https://bugs.swift.org/browse/SR-6509
@@ -1643,7 +1643,6 @@ public extension Optional {
   }
 }
 
-
 // https://bugs.swift.org/browse/SR-6837
 do {
   func takeFn(fn: (_ i: Int, _ j: Int?) -> ()) {}
@@ -1654,3 +1653,14 @@ do {
   takeFn { (pair: (Int, Int?)) in } // Disallow for -swift-version 4 and later
   // expected-error@-1 {{contextual closure type '(Int, Int?) -> ()' expects 2 arguments, but 1 was used in closure body}}
 }
+
+// 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)? for -swift-version 4 but not later.
+
+  func logNoOptional<T>() -> (T) -> Void { }
+  f(a: logNoOptional() as ((()) -> Void)) // Also allow the optional-injected form.
+}