Remove named concept iterable

src/test/jtx/TestHelpers.h

@@ -33,17 +33,17 @@ namespace ripple {
 namespace test {
 namespace jtx {
 
-// Helper to make vector from iterable
-template <typename T>
-concept iterable = requires(T& v)
-{
-    std::begin(v);
-    std::end(v);
-};
-
+// TODO We only need this long "requires" clause as polyfill, for C++20
+// implementations which are missing <ranges> header. Replace with
+// `std::ranges::range<Input>`, and accordingly use std::ranges::begin/end
+// when we have moved to better compilers.
 template <typename Input>
 auto
-make_vector(Input const& input) requires iterable<Input>
+make_vector(Input const& input)
+    requires requires(Input& v) {
+                 std::begin(v);
+                 std::end(v);
+             }
 {
     return std::vector(std::begin(input), std::end(input));
 }