Sema: Respect TVO_CanBindToPack

Author: slavapestov

lib/Sema/CSSimplify.cpp

@@ -4204,6 +4204,14 @@ ConstraintSystem::matchTypesBindTypeVar(
     }
   }
 
+  if (type->is<PackType>() || type->is<PackArchetypeType>()) {
+    if (!typeVar->getImpl().canBindToPack())
+      return getTypeMatchFailure(locator);
+  } else if (!type->is<PlaceholderType>()) {
+    if (typeVar->getImpl().canBindToPack())
+      return getTypeMatchFailure(locator);
+  }
+
   // We do not allow keypaths to go through AnyObject. Let's create a fix
   // so this can be diagnosed later.
   if (auto loc = typeVar->getImpl().getLocator()) {
@@ -6288,6 +6296,12 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
         // trivially solved.
         return getTypeMatchSuccess();
       }
+
+      // If exactly one of the type variables can bind to an pack, we
+      // can't merge these two type variables.
+      if (rep1->getImpl().canBindToPack()
+            != rep2->getImpl().canBindToPack())
+        return getTypeMatchFailure(locator);
     }
 
     switch (kind) {

lib/Sema/ConstraintSystem.cpp

@@ -200,6 +200,12 @@ void ConstraintSystem::assignFixedType(TypeVariableType *typeVar, Type type,
     return;
 
   if (!type->isTypeVariableOrMember()) {
+    if (type->is<PackType>() || type->is<PackArchetypeType>()) {
+      assert(typeVar->getImpl().canBindToPack());
+    } else if (!type->is<PlaceholderType>()) {
+      assert(!typeVar->getImpl().canBindToPack());
+    }
+
     // If this type variable represents a literal, check whether we picked the
     // default literal type. First, find the corresponding protocol.
     //