[CSSimplify] Move invalid pack expansion check to enable closure result conversion

Checking for pack expansion type mismatch impedes non-Void-to-Void
conversion allowed for single-expression closures.

Resolves: rdar://108904190

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -7282,22 +7282,6 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
     }
   }
 
-  // Matching types where one side is a pack expansion and the other is not
-  // means a pack expansion was used where it isn't supported.
-  if (type1->is<PackExpansionType>() != type2->is<PackExpansionType>()) {
-    if (!shouldAttemptFixes())
-      return getTypeMatchFailure(locator);
-
-    if (type1->isPlaceholder() || type2->isPlaceholder())
-      return getTypeMatchSuccess();
-
-    auto *loc = getConstraintLocator(locator);
-    if (recordFix(AllowInvalidPackExpansion::create(*this, loc)))
-      return getTypeMatchFailure(locator);
-
-    return getTypeMatchSuccess();
-  }
-
   if (kind >= ConstraintKind::Conversion) {
     // An lvalue of type T1 can be converted to a value of type T2 so long as
     // T1 is convertible to T2 (by loading the value).  Note that we cannot get
@@ -7720,6 +7704,22 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
     }
   }
 
+  // Matching types where one side is a pack expansion and the other is not
+  // means a pack expansion was used where it isn't supported.
+  if (type1->is<PackExpansionType>() != type2->is<PackExpansionType>()) {
+    if (!shouldAttemptFixes())
+      return getTypeMatchFailure(locator);
+
+    if (type1->isPlaceholder() || type2->isPlaceholder())
+      return getTypeMatchSuccess();
+
+    auto *loc = getConstraintLocator(locator);
+    if (recordFix(AllowInvalidPackExpansion::create(*this, loc)))
+      return getTypeMatchFailure(locator);
+
+    return getTypeMatchSuccess();
+  }
+
   // Attempt fixes iff it's allowed, both types are concrete and
   // we are not in the middle of attempting one already.
   if (shouldAttemptFixes() && !flags.contains(TMF_ApplyingFix)) {

test/Constraints/pack-expansion-expressions.swift

@@ -495,3 +495,8 @@ do {
     _ = (repeat overloaded(42, each v)) // Ok
   }
 }
+
+// rdar://108904190 - top-level 'repeat' not allowed in single-expression closures
+func test_pack_expansion_to_void_conv_for_closure_result<each T>(x: repeat each T) -> () -> () {
+  return { repeat print(each x) } // Ok
+}