[CSSimplify] Prevent merging of type expansion type variables

Even if both sides are pack expansion variables they cannot be
merged because merging of them means merging of pattern and
shape types as well, which is easier to do when one of both
sides are bound.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -6577,6 +6577,12 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
         auto rep1 = getRepresentative(typeVar1);
         auto rep2 = getRepresentative(typeVar2);
 
+        // Pack expansion variables cannot be merged because
+        // they involve other type variables.
+        if (rep1->getImpl().isPackExpansion() ||
+            rep2->getImpl().isPackExpansion())
+          return formUnsolvedResult();
+
         // If exactly one of the type variables can bind to an lvalue, we
         // can't merge these two type variables.
         if (kind == ConstraintKind::Equal &&
@@ -6634,6 +6640,12 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
         auto rep1 = getRepresentative(typeVar1);
         auto rep2 = getRepresentative(typeVar2);
 
+        // Pack expansion variables cannot be merged because
+        // they involve other type variables.
+        if (rep1->getImpl().isPackExpansion() ||
+            rep2->getImpl().isPackExpansion())
+          return formUnsolvedResult();
+
         if (!rep1->getImpl().canBindToInOut() ||
             !rep2->getImpl().canBindToLValue()) {
           // Merge the equivalence classes corresponding to these two variables.