[ConstraintSystem] Only attempt to infer a type from a default argument

if there is a non-null callee.

Otherwise, getParameterList(callee) will crash if the callee is a closure.
Closures can't have default arguments anyway, so there's no need to
attempt this new inference.

Author: hborla

lib/Sema/CSSimplify.cpp

@@ -1748,7 +1748,7 @@ static ConstraintSystem::TypeMatchResult matchCallArguments(
     // If type inference from default arguments is enabled, let's
     // add a constraint from the parameter if necessary, otherwise
     // there is nothing to do but move to the next parameter.
-    if (parameterBindings[paramIdx].empty()) {
+    if (parameterBindings[paramIdx].empty() && callee) {
       auto &ctx = cs.getASTContext();
 
       if (ctx.TypeCheckerOpts.EnableTypeInferenceFromDefaultArguments) {

test/decl/func/vararg.swift

@@ -1,4 +1,5 @@
 // RUN: %target-typecheck-verify-swift
+// RUN: %target-typecheck-verify-swift -enable-experimental-type-inference-from-defaults
 
 var t1a: (Int...) = (1)
 // expected-error@-1 {{cannot create a variadic tuple}}