[CSSimplify] Allow converting tuple with pack expansions to Void for closure result

Maintains existing rule where single-expression closure is allowed
to convert to contextual function type with `Void` result.

Author: xedin

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

test/Constraints/pack-expansion-expressions.swift

@@ -497,6 +497,9 @@ do {
 }
 
 // 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
+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}}
 }