Sema: SE-0110 tuple splatting should not implode argument list with varargs

Fixes <https://bugs.swift.org/browse/SR-9470>.

Author: slavapestov

lib/Sema/CSSimplify.cpp

@@ -1173,6 +1173,17 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
             !params[0].isVariadic());
   };
 
+  auto canImplodeParams = [&](ArrayRef<AnyFunctionType::Param> params) {
+    if (params.size() == 1)
+      return false;
+
+    for (auto param : params)
+      if (param.isVariadic() || param.isInOut())
+        return false;
+
+    return true;
+  };
+
   auto implodeParams = [&](SmallVectorImpl<AnyFunctionType::Param> &params) {
     auto input = AnyFunctionType::composeInput(getASTContext(), params,
                                                /*canonicalVararg=*/false);
@@ -1193,21 +1204,18 @@ ConstraintSystem::matchFunctionTypes(FunctionType *func1, FunctionType *func2,
 
     if (last != path.rend()) {
       if (last->getKind() == ConstraintLocator::ApplyArgToParam) {
-        if (isSingleParam(func2Params)) {
-          if (!isSingleParam(func1Params)) {
-            implodeParams(func1Params);
-          }
-        } else if (getASTContext().isSwiftVersionAtLeast(4)
-                   && !getASTContext().isSwiftVersionAtLeast(5)
-                   && !isSingleParam(func2Params)) {
+        if (isSingleParam(func2Params) &&
+            canImplodeParams(func1Params)) {
+          implodeParams(func1Params);
+        } else if (!getASTContext().isSwiftVersionAtLeast(5) &&
+                   isSingleParam(func1Params) &&
+                   canImplodeParams(func2Params)) {
           auto *simplified = locator.trySimplifyToExpr();
           // We somehow let tuple unsplatting function conversions
           // through in some cases in Swift 4, so let's let that
           // continue to work, but only for Swift 4.
           if (simplified && isa<DeclRefExpr>(simplified)) {
-            if (isSingleParam(func1Params)) {
-              implodeParams(func2Params);
-            }
+            implodeParams(func2Params);
           }
         }
       }

test/Compatibility/tuple_arguments_4.swift

@@ -1677,3 +1677,9 @@ class Mappable<T> {
 
 let x = Mappable(())
 _ = x.map { (_: Void) in return () }
+
+// https://bugs.swift.org/browse/SR-9470
+do {
+  func f(_: Int...) {}
+  let _ = [(1, 2, 3)].map(f) // expected-error {{cannot invoke 'map' with an argument list of type '((Int...) -> ())'}}
+}

test/Constraints/tuple_arguments.swift

@@ -1679,4 +1679,10 @@ class Mappable<T> {
 }
 
 let x = Mappable(())
-_ = x.map { (_: Void) in return () }
+_ = x.map { (_: Void) in return () }
+
+// https://bugs.swift.org/browse/SR-9470
+do {
+  func f(_: Int...) {}
+  let _ = [(1, 2, 3)].map(f) // expected-error {{cannot invoke 'map' with an argument list of type '((Int...) -> ())'}}
+}