[CSSimplify] Allow converting pack expansion types and tuples with pack expansions to Void for closure result

lib/Sema/CSSimplify.cpp

@@ -6985,6 +6985,14 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
         return formUnsolvedResult();
       }
 
+      // Closure result is allowed to convert to Void in certain circumstances,
+      // let's forego tuple matching because it is guaranteed to fail and jump
+      // to `() -> T` to `() -> Void` rule.
+      if (locator.endsWith<LocatorPathElt::ClosureBody>()) {
+        if (containsPackExpansionType(tuple1) && tuple2->isVoid())
+          break;
+      }
+
       // Add each tuple type to the locator before matching the element types.
       // This is useful for diagnostics, because the error message can use the
       // full tuple type for several element mismatches. Use the original types
@@ -7282,22 +7290,6 @@ 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 +7712,22 @@ 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 +495,11 @@ 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) {
+  let _: () -> Void = { repeat print(each x) } // Ok
+  let _: () -> Void = { (repeat print(each x)) } // Ok
+  let _: (Int) -> Void = { repeat ($0, print(each x)) } // expected-warning {{'repeat (Int, ())' is unused}}
+  let _: (Int, String) -> Void = { ($0, repeat ($1, print(each x))) } // expected-warning {{'(Int, repeat (String, ()))' is unused}}
+}