c++: Add test for PR c++/93211

The fix for PR c++/90711 also fixed this PR.

gcc/testsuite/ChangeLog:

	PR c++/93211
	PR c++/90711
	* g++.dg/template/koenig11.C: New test.

gcc/testsuite/ChangeLog

+2020-04-03  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/93211
+	PR c++/90711
+	* g++.dg/template/koenig11.C: New test.
+
 2020-04-03  Andre Vieira  <andre.simoesdiasvieira@arm.com>
 
 	* gcc.target/arm/mve/intrinsics/mve_vector_float.c: Put -mfpu=auto back.

gcc/testsuite/g++.dg/template/koenig11.C

+// PR c++/93211
+// { dg-do compile { target c++11 } }
+
+template<typename T>
+void g(T);
+
+template<typename T, decltype(g(T{})) = 0>
+void f() {}
+
+template<typename T, decltype(::g(T{})) = 0>
+void f() {} // { dg-bogus "redefinition" }