Oops, accidentally deleted config test.

config-tests/concepts.cxx

@@ -0,0 +1,20 @@
+// Test for concepts support.  Not just the language feature; also headers.
+#include <iostream>
+#include <ranges>
+#include <vector>
+
+
+template<typename T> concept Foo = std::ranges::input_range<T>;
+
+
+template<Foo F> auto foo(F const &r)
+{
+  return std::distance(std::begin(r), std::end(r));
+}
+
+
+int main()
+{
+  std::vector<int> const v{1, 2, 3};
+  std::cout << foo(v) << '\n';
+}