[TypeChecker] NFC: Add a test-case for rdar://106737972

Author: xedin

test/Constraints/variadic_generic_functions.swift

@@ -69,3 +69,13 @@ func contextualTyping() {
   let (_, _): ([Int], String?) = dependent([42], [""]) // expected-error {{cannot convert value of type '(Int?, String?)' to specified type '([Int], String?)'}}
   let (_, _, _): (String?, String?, Int) = dependent([42], [""]) // expected-error {{'(Int?, String?)' is not convertible to '(String?, String?, Int)', tuples have a different number of elements}}
 }
+
+// rdar://106737972 - crash-on-invalid with default argument
+do {
+  func foo<each T>(_: repeat each T = bar().element) {} // expected-note {{in call to function 'foo'}}
+  // expected-error@-1 {{variadic parameter cannot have a default value}}
+  // expected-error@-2 {{value pack expansion can only appear inside a function argument list or tuple element}}
+  // expected-error@-3 {{generic parameter 'each T' could not be inferred}}
+
+  func bar<each T>() -> (repeat each T) {}
+}