Sema: Open-code a usage of SubscriptDecl::getIndicesInterfaceType()

Also add a FIXME, since its wrong.

Author: slavapestov

lib/Sema/CSApply.cpp

@@ -4429,7 +4429,18 @@ namespace {
           cs.TC.requestMemberLayout(subscript);
 
           auto dc = subscript->getInnermostDeclContext();
-          auto indexType = subscript->getIndicesInterfaceType();
+
+          // FIXME: It's not correct to turn the subscript's parameter list
+          // into a single type here. Further down we pass it to coerceToType(),
+          // but the rules for argument list conversions are different than
+          // tuple conversions, and coerceToType() doesn't handle varargs or
+          // default arguments.
+          auto indexType = AnyFunctionType::composeInput(
+            cs.TC.Context,
+            subscript->getInterfaceType()
+              ->castTo<AnyFunctionType>()
+              ->getParams(),
+            /*canonicalVararg=*/false);
 
           SubstitutionMap subs;
           if (auto sig = dc->getGenericSignatureOfContext()) {