[CSSimplify] Delay matching if one of the types is a tuple with unresolved pack expansions

This handles situations like `Int <conv> (Int, (_: $T_exp)` or
`String arg conv (_: $T_exp)`. The matching has to be delayed
because the structure of the tuple type is not known until `$T_exp`
(which represents a pack expansion type) is resolved.

(cherry picked from commit 2ebade1)

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -6518,6 +6518,17 @@ bool ConstraintSystem::repairFailures(
   return !conversionsOrFixes.empty();
 }
 
+static bool isTupleWithUnresolvedPackExpansion(Type type) {
+  if (auto *tuple = type->getAs<TupleType>()) {
+    return llvm::any_of(tuple->getElements(), [&](const TupleTypeElt &elt) {
+      if (auto typeVar = elt.getType()->getAs<TypeVariableType>())
+        return typeVar->getImpl().isPackExpansion();
+      return false;
+    });
+  }
+  return false;
+}
+
 ConstraintSystem::TypeMatchResult
 ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
                              TypeMatchOptions flags,
@@ -6785,6 +6796,12 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
     return formUnsolvedResult();
   }
 
+  // If either side is a tuple that has unresolved pack expansions,
+  // delay matching until more is inferred about type structure.
+  if (isTupleWithUnresolvedPackExpansion(desugar1) ||
+      isTupleWithUnresolvedPackExpansion(desugar2))
+    return formUnsolvedResult();
+
   llvm::SmallVector<RestrictionOrFix, 4> conversionsOrFixes;
 
   // Decompose parallel structure.