--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval2.C	Thu Feb 13 11:34:05 2020 +0900
@@ -0,0 +1,54 @@
+// { dg-do compile { target c++2a } }
+
+// ill-formed, no diagnostic required: the two expressions are
+// functionally equivalent but not equivalent
+template <int N> void foo(const char (&s)[([]{}, N)]);
+template <int N> void foo(const char (&s)[([]{}, N)]);
+
+// two different declarations because the non-dependent portions are not
+// considered equivalent
+template <class T> void spam(decltype([]{}) (*s)[sizeof(T)]);
+template <class T> void spam(decltype([]{}) (*s)[sizeof(T)]);
+
+template <class T>
+using A = decltype([] { });
+// A<int> and A<char> refer to different closure types
+
+template <class T>
+auto f(T) -> decltype([]() { T::invalid; } ()); // { dg-error "invalid" }
+void f(...);
+
+template <class T, unsigned = sizeof([]() { T::invalid; })> // { dg-error "invalid" }
+void g(T);
+void g(...);
+
+template <class T>
+auto h(T) -> decltype([x = T::invalid]() { });
+void h(...);
+
+template <class T>
+auto i(T) -> decltype([]() -> typename T::invalid { });
+void i(...);
+
+template <class T>
+auto j(T t) -> decltype([](auto x) -> decltype(x.invalid) { } (t));
+void j(...);
+
+template <class,class> struct different {};
+template <class T> struct different<T,T> { typename T::invalid t; };
+
+template <class,class> struct same;
+template <class T> struct same<T,T> {};
+
+int main()
+{
+  foo<1>("");	 // { dg-error "ambiguous" }
+  spam<char>(nullptr);		// { dg-error "ambiguous" }
+  different<A<int>,A<char>>();
+  same<A<int>,A<int>>();
+  f(0); // error: invalid expression not part of the immediate context
+  g(0); // error: invalid expression not part of the immediate context
+  h(0); // error: invalid expression not part of the immediate context
+  i(0); // error: invalid expression not part of the immediate context
+  j(0); // deduction fails on #1, calls #2
+}