std::bitset formatting (nested_formatter) does not work with wchar_t

[Boddlnagg]

I tried to test the formatting of std::bitset using fmt/std.h, but with wchar_t (and char16_t) instead of char, using this test:

TEST(std_test, format_bitset_Wide) {
  auto bs = std::bitset<6>(42);
  EXPECT_EQ(fmt::format(L"{}", bs), L"101010");
  EXPECT_EQ(fmt::format(L"{:0>8}", bs), L"00101010");
  EXPECT_EQ(fmt::format(L"{:-^12}", bs), L"---101010---");
}

It didn't compile until I applied the following changes to std.h:

--- a/include/fmt/std.h
+++ b/include/fmt/std.h
@@ -184,7 +184,7 @@ FMT_END_NAMESPACE
 FMT_BEGIN_NAMESPACE
 FMT_EXPORT
 template <std::size_t N, typename Char>
-struct formatter<std::bitset<N>, Char> : nested_formatter<string_view> {
+struct formatter<std::bitset<N>, Char> : nested_formatter<basic_string_view<Char>, Char> {
  private:
   // Functor because C++11 doesn't support generic lambdas.
   struct writer {
@@ -204,7 +204,7 @@ struct formatter<std::bitset<N>, Char> : nested_formatter<string_view> {
   template <typename FormatContext>
   auto format(const std::bitset<N>& bs, FormatContext& ctx) const
       -> decltype(ctx.out()) {
-    return write_padded(ctx, writer{bs});
+    return this->write_padded(ctx, writer{bs});
   }
 };

Now it runs into a segfault, which appears to be caused by the set_fill call in nested_formatter::write_padded:

    specs.set_fill(
        basic_string_view<Char>(specs_.fill<Char>(), specs_.fill_size()));

in combination with the implementation of fill() for non-char types returning a nullpointer:

  template <typename Char, FMT_ENABLE_IF(!std::is_same<Char, char>::value)>
  constexpr auto fill() const -> const Char* {
    return nullptr;
  }

How is this supposed to work (or rather, can it be made to work)?

[phprus]

@Boddlnagg
Please test PR #4286

[Boddlnagg]

Yes, this seems to work 👍