Sema: Horrific simulation of Swift 3 bug with argument labels for Swift 3 mode

In Swift 3.0.1, argument labels are ignored when calling a function
having a single parameter of 'Any' type. That is, if we have:

func foo(_: Any) {}

Both of the following were accepted in a no-assert build (an assert
build would crash, but the GM builds of Xcode ship with asserts off):

foo(123)
foo(data: 123)

This behavior was fixed by 578e36a7e1,
but unfortunately we have to revert to the old behavior *and* defeat
the assertion when in Swift 3 mode.

Swift 4 mode still has the correct behavior, where the second call
'foo(data: 123)' produces a diagnostic.

Now, I have to pour myself a strong drink to forget this ever happened.

Fixes <rdar://problem/28952837>.

lib/Sema/CSApply.cpp

@@ -4702,13 +4702,34 @@ Expr *ExprRewriter::coerceCallArguments(
     bool hasTrailingClosure,
     ConstraintLocatorBuilder locator) {
 
-  // Total hack: In Swift 3 mode, we can end up with an arity mismatch due to
-  // loss of ParenType sugar.
+  // Local function to produce a locator to refer to the ith element of the
+  // argument tuple.
+  auto getArgLocator = [&](unsigned argIdx, unsigned paramIdx)
+                         -> ConstraintLocatorBuilder {
+    return locator.withPathElement(
+             LocatorPathElt::getApplyArgToParam(argIdx, paramIdx));
+  };
+
+  bool matchCanFail = false;
+
+  // If you value your sanity, ignore the body of this 'if' statement.
   if (cs.getASTContext().isSwiftVersion3()) {
+    // Total hack: In Swift 3 mode, we can end up with an arity mismatch due to
+    // loss of ParenType sugar.
     if (isa<TupleExpr>(arg))
       if (auto *parenType = dyn_cast<ParenType>(paramType.getPointer()))
         if (isa<TupleType>(parenType->getUnderlyingType().getPointer()))
           paramType = parenType->getUnderlyingType();
+
+    // Total hack: In Swift 3 mode, argument labels are ignored when calling
+    // function type with a single Any parameter.
+    if (paramType->isAny()) {
+      if (auto tupleArgType = dyn_cast<TupleType>(arg->getType().getPointer())) {
+        if (tupleArgType->getNumElements() == 1) {
+          matchCanFail = true;
+        }
+      }
+    }
   }
 
   bool allParamsMatch = cs.getType(arg)->isEqual(paramType);
@@ -4778,7 +4799,8 @@ Expr *ExprRewriter::coerceCallArguments(
                                                 hasTrailingClosure,
                                                 /*allowFixes=*/false, listener,
                                                 parameterBindings);
-  assert(!failed && "Call arguments did not match up?");
+
+  assert((matchCanFail || !failed) && "Call arguments did not match up?");
   (void)failed;
 
   // We should either have parentheses or a tuple.
@@ -4810,14 +4832,6 @@ Expr *ExprRewriter::coerceCallArguments(
     return Identifier();
   };
 
-  // Local function to produce a locator to refer to the ith element of the
-  // argument tuple.
-  auto getArgLocator = [&](unsigned argIdx, unsigned paramIdx)
-                         -> ConstraintLocatorBuilder {
-    return locator.withPathElement(
-             LocatorPathElt::getApplyArgToParam(argIdx, paramIdx));
-  };
-
   auto &tc = getConstraintSystem().getTypeChecker();
   SmallVector<TupleTypeElt, 4> toSugarFields;
   SmallVector<TupleTypeElt, 4> fromTupleExprFields(

lib/Sema/CSSimplify.cpp

@@ -623,6 +623,26 @@ static ConstraintSystem::SolutionKind
 matchCallArguments(ConstraintSystem &cs, ConstraintKind kind,
                    Type argType, Type paramType,
                    ConstraintLocatorBuilder locator) {
+
+  if (paramType->isAny()) {
+    if (argType->is<InOutType>())
+      return ConstraintSystem::SolutionKind::Error;
+
+    // If the param type is Any, the function can only have one argument.
+    // Check if exactly one argument was passed to this function, otherwise
+    // we obviously have a mismatch.
+    if (auto tupleArgType = dyn_cast<TupleType>(argType.getPointer())) {
+      // Total hack: In Swift 3 mode, argument labels are ignored when calling
+      // function type with a single Any parameter.
+      if (tupleArgType->getNumElements() != 1 ||
+          (!cs.getASTContext().isSwiftVersion3() &&
+           tupleArgType->getElement(0).hasName())) {
+        return ConstraintSystem::SolutionKind::Error;
+      }
+    }
+    return ConstraintSystem::SolutionKind::Solved;
+  }
+
   // Extract the parameters.
   ValueDecl *callee;
   unsigned calleeLevel;
@@ -644,16 +664,6 @@ matchCallArguments(ConstraintSystem &cs, ConstraintKind kind,
                                       parameterBindings))
     return ConstraintSystem::SolutionKind::Error;
 
-  // In the empty existential parameter case,
-  // it's sufficient to simply match call arguments.
-  if (paramType->isAny()) {
-    // Argument of the existential type can't be inout.
-    if (argType->is<InOutType>())
-      return ConstraintSystem::SolutionKind::Error;
-
-    return ConstraintSystem::SolutionKind::Solved;
-  }
-
   // Check the argument types for each of the parameters.
   ConstraintSystem::TypeMatchOptions subflags =
     ConstraintSystem::TMF_GenerateConstraints;

test/Compatibility/tuple_arguments.swift

@@ -1269,3 +1269,17 @@ do {
   let _: (Int, Int) -> () = { _ = ($0, $1) }
   let _: (Int, Int) -> () = { t, u in _ = (t, u) }
 }
+
+// rdar://problem/28952837 - argument labels ignored when calling function
+// with single 'Any' parameter
+func takesAny(_: Any) {}
+
+do {
+  let fn: (Any) -> () = { _ in }
+
+  fn(123)
+  fn(data: 123)
+
+  takesAny(123)
+  takesAny(data: 123)
+}

test/Constraints/closures.swift

@@ -349,6 +349,9 @@ class C_SR_2505 : P_SR_2505 {
   }
 
   func call(_ c: C_SR_2505) -> Bool {
+    // Note: no diagnostic about capturing 'self', because this is a
+    // non-escaping closure -- that's how we know we have selected
+    // test(it:) and not test(_)
     return c.test { o in test(o) }
   }
 }

test/Constraints/diagnostics.swift

@@ -829,9 +829,13 @@ func foo1255_2() -> Int {
 
 // SR-2505: "Call arguments did not match up" assertion
 
+// Here we're simulating the busted Swift 3 behavior -- see
+// test/Constraints/diagnostics_swift4.swift for the correct
+// behavior.
+
 func sr_2505(_ a: Any) {} // expected-note {{}}
 sr_2505()          // expected-error {{missing argument for parameter #1 in call}}
-sr_2505(a: 1)      // expected-error {{extraneous argument label 'a:' in call}}
+sr_2505(a: 1)      // FIXME: emit a warning saying this becomes an error in Swift 4
 sr_2505(1, 2)      // expected-error {{extra argument in call}}
 sr_2505(a: 1, 2)   // expected-error {{extra argument in call}}
 
@@ -851,7 +855,8 @@ extension C_2505 {
 class C2_2505: P_2505 {
 }
 
-let c_2505 = C_2505(arg: [C2_2505()]) // expected-error {{argument labels '(arg:)' do not match any available overloads}} expected-note {{overloads for 'C_2505' exist}}
+// FIXME: emit a warning saying this becomes an error in Swift 4
+let c_2505 = C_2505(arg: [C2_2505()])
 
 // Diagnostic message for initialization with binary operations as right side
 let foo1255_3: String = 1 + 2 + 3 // expected-error {{cannot convert value of type 'Int' to specified type 'String'}}

test/Constraints/tuple_arguments.swift

@@ -1254,3 +1254,17 @@ do {
   let _: (Int, Int) -> () = { _ = ($0, $1) }
   let _: (Int, Int) -> () = { t, u in _ = (t, u) }
 }
+
+// rdar://problem/28952837 - argument labels ignored when calling function
+// with single 'Any' parameter
+func takesAny(_: Any) {}
+
+do {
+  let fn: (Any) -> () = { _ in }
+
+  fn(123)
+  fn(data: 123) // expected-error {{extraneous argument label 'data:' in call}}
+
+  takesAny(123)
+  takesAny(data: 123) // expected-error {{extraneous argument label 'data:' in call}}
+}