GCC 9.4.0 compilation error

[sergiud]

Compiling fmt 11.1.2 on Ubuntu 20.04 with GCC 9.4.0 fails as follows:

2.949 In file included from /build/fmt/test/ostream-test.cc:10:
2.949 /build/fmt/include/fmt/format.h: In instantiation of 'constexpr decltype (ctx.out()) fmt::v11::formatter<fmt::v11::detail::std_string_view<Char>, Char>::format(fmt::v11::detail::std_string_view<Char>, FormatContext&) const [with FormatContext = fmt::v11::context; Char = char; decltype (ctx.out()) = fmt::v11::basic_appender<char>]':
2.949 /build/fmt/include/fmt/base.h:1126:54:   required by substitution of 'template<class Char, class T, class U> decltype ((fmt::v11::formatter<U, Char>().format((* p), (* ctx)), std::true_type())) fmt::v11::detail::has_formatter_impl(T*, fmt::v11::buffered_context<Char>*) [with Char = char; T = fmt::v11::detail::std_string_view<char>; U = fmt::v11::detail::std_string_view<char>]'
2.949 /build/fmt/include/fmt/base.h:1131:43:   required from 'constexpr bool fmt::v11::detail::has_formatter() [with T = fmt::v11::detail::std_string_view<char>; Char = char]'
2.949 /build/fmt/include/fmt/base.h:1187:62:   required by substitution of 'template<bool B, class T, class F> using conditional_t = typename std::conditional::type [with bool B = fmt::v11::detail::has_formatter<fmt::v11::detail::std_string_view<char>, char>(); T = fmt::v11::detail::std_string_view<char>&; F = void]'
2.949 /build/fmt/include/fmt/base.h:1187:15:   required by substitution of 'template<class T, typename std::enable_if<fmt::v11::detail::use_formatter<T, typename std::remove_const<_Tp>::type>::value, int>::type <anonymous> > static fmt::v11::conditional_t<has_formatter<T, char>(), T&, void> fmt::v11::detail::type_mapper<char>::map<T, <enumerator> >(T&) [with T = fmt::v11::detail::std_string_view<char>; typename std::enable_if<fmt::v11::detail::use_formatter<T, typename std::remove_const<_Tp>::type>::value, int>::type <anonymous> = 0]'
2.949 /build/fmt/include/fmt/base.h:1195:57:   required by substitution of 'template<class T, class Char> using is_formattable = fmt::v11::bool_constant<(! std::is_same<decltype (fmt::v11::detail::type_mapper<Char>::map(declval<typename std::conditional<std::is_void<_Tp>::value, int*, T>::type&>())), void>::value)> [with T = fmt::v11::detail::std_string_view<char>; Char = char]'
2.949 /build/fmt/test/ostream-test.cc:273:3:   required from here
2.949 /build/fmt/include/fmt/format.h:3764:43: error: no matching function for call to 'fmt::v11::formatter<fmt::v11::detail::std_string_view<char>, char, void>::format(fmt::v11::detail::std_string_view<char>&, fmt::v11::context&) const'
2.949  3764 |       return formatter<Base, Char>::format(value, ctx);             \
2.949       |              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
2.949 /build/fmt/include/fmt/format.h:3775:1: note: in expansion of macro 'FMT_FORMAT_AS'
2.949  3775 | FMT_FORMAT_AS(detail::std_string_view<Char>, basic_string_view<Char>);
2.949       | ^~~~~~~~~~~~~
2.949 /build/fmt/include/fmt/format.h:3632:20: note: candidate: 'template<class FormatContext> constexpr decltype (ctx.out()) fmt::v11::detail::native_formatter<T, Char, TYPE>::format(const T&, FormatContext&) const [with FormatContext = FormatContext; T = fmt::v11::basic_string_view<char>; Char = char; fmt::v11::detail::type TYPE = fmt::v11::detail::type::string_type]'
2.949  3632 | FMT_CONSTEXPR auto native_formatter<T, Char, TYPE>::format(
2.949       |                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
2.949 /build/fmt/include/fmt/format.h:3632:20: note:   template argument deduction/substitution failed:
2.949 /build/fmt/include/fmt/format.h:3764:44: note:   cannot convert 'value' (type 'fmt::v11::detail::std_string_view<char>') to type 'const fmt::v11::basic_string_view<char>&'
2.949  3764 |       return formatter<Base, Char>::format(value, ctx);             \
2.949       |                                            ^~~~~
2.949 /build/fmt/include/fmt/format.h:3775:1: note: in expansion of macro 'FMT_FORMAT_AS'
2.949  3775 | FMT_FORMAT_AS(detail::std_string_view<Char>, basic_string_view<Char>);
2.949       | ^~~~~~~~~~~~~

I could bisect the issue to c59ee96.

Repro using Docker:

FROM ubuntu:focal

RUN --mount=type=cache,target=/var/cache/apt \
apt-get update && DEBIAN_FRONTEND=noninteractive apt-get install -y \
--no-install-recommends --no-install-suggests \
ca-certificates \
ccache \
cmake \
g++ \
git \
ninja-build

ENV CMAKE_CXX_COMPILER_LAUNCHER=/usr/bin/ccache

WORKDIR /build/

RUN git clone https://github.com/fmtlib/fmt.git -b 11.1.2 --depth=1

RUN --mount=type=cache,target=/root/.ccache,sharing=shared \
cmake -S fmt/ -B build-fmt/ \
-G Ninja \
&& cmake --build build-fmt/

[vitaut]

Same as #4313 (already fixed).