[CSSimplify] VariadicGenerics: Don't attempt to wrap optional into one-element tuple

If we have a tuple with unresolved pack expansion on one side
and an optional type on the other, prevent `matchTypes` from
wrapping optional into a one-element tuple because the matching
should be handled as part of the optional injection.

Resolves: rdar://152940244
(cherry picked from commit 6cfbafd)

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -7345,16 +7345,20 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
   // match `$T3` and propagate `Pack{Int}` to `$T2`.
   //
   // This is also important for situations like: `$T2 conv (Int, $T_exp)`
-  // becuase expansion could be defaulted to an empty pack which means
+  // because expansion could be defaulted to an empty pack which means
   // that under substitution that element would disappear and the type
   // would be just `(Int)`.
   //
-  // Notable exception here is `Any` which doesn't require wrapping and
-  // would be handled by existental promotion in cases where it's allowed.
+  // Notable exceptions here are: `Any` which doesn't require wrapping and
+  // would be handled by an existential promotion in cases where it's allowed,
+  // and `Optional<T>` which would be handled by optional injection.
   if (isTupleWithUnresolvedPackExpansion(origType1) ||
       isTupleWithUnresolvedPackExpansion(origType2)) {
-    if (desugar1->is<TupleType>() != desugar2->is<TupleType>() &&
-        (!desugar1->isAny() && !desugar2->isAny())) {
+    if (isa<TupleType>(desugar1) != isa<TupleType>(desugar2) &&
+        !desugar1->getOptionalObjectType() &&
+        !desugar2->getOptionalObjectType() &&
+        !desugar1->isAny() &&
+        !desugar2->isAny()) {
       return matchTypes(
           desugar1->is<TupleType>() ? type1
                                     : TupleType::get({type1}, getASTContext()),

test/Constraints/variadic_generic_constraints.swift

@@ -76,3 +76,17 @@ func badCallToZip<each T, each U>(t: repeat each T, u: repeat each U) {
   // expected-error@-1 {{global function 'zip(t:u:)' requires the type packs 'each T' and 'each U' have the same shape}}
   // expected-error@-2 {{pack expansion requires that 'each U' and 'each T' have the same shape}}
 }
+
+do {
+  func test<A, B, each C>(
+    _: A,
+    _: B,
+    _: repeat each C
+  ) throws -> (A, B, repeat each C) {
+    fatalError()
+  }
+
+  func test() {
+    guard let _ = try? test(1, 2, 3) else { return } // Ok
+  }
+}