[CSSimplify] Don't propagate contextual parameter type if it's a pack expansion

Pack expansion types are handled by matching logic, optimization
that attempts to propagate types into the body of the closure should
consider them unsuitable even if the pack expansion matches a single
parameter.

Author: xedin

lib/Sema/CSSimplify.cpp

@@ -10987,6 +10987,11 @@ bool ConstraintSystem::resolveClosure(TypeVariableType *typeVar,
     if (contextualTy->isTypeVariableOrMember())
       return false;
 
+    // Cannot propagate pack expansion type from context,
+    // it has to be handled by type matching logic.
+    if (contextualTy->is<PackExpansionType>())
+      return false;
+
     // If contextual type has an error, let's wait for inference,
     // otherwise contextual would interfere with diagnostics.
     if (contextualTy->hasError())

test/Constraints/pack-expansion-expressions.swift

@@ -240,3 +240,13 @@ func packOutsideExpansion<each T>(_ t: repeat each T) {
   _ = each tuple.element
   // expected-error@-1{{pack reference 'T' can only appear in pack expansion}}
 }
+
+func test_pack_expansions_with_closures() {
+  func takesVariadicFunction<each T>(function: (repeat each T) -> Int) {}
+
+  func test(fn: (Int, String) -> Int, x: Int) {
+    takesVariadicFunction { fn(x, "") } // Ok
+    takesVariadicFunction { y in fn(x, y) } // Ok
+    takesVariadicFunction { y, z in fn(y, z) } // Ok
+  }
+}