This is the problematic fmt version and code locations:
https://github.com/fmtlib/fmt/blob/11.1.4/test/format-impl-test.cc#L310
https://github.com/fmtlib/fmt/blob/11.1.4/test/format-impl-test.cc#L318
This is what clang 20.1.3 reports:
test/format-impl-test.cc:310:20: error: 'is_floating_point' cannot be specialized: Users are not allowed to specialize this standard library entity [-Winvalid-specialization]
310 | template <> struct is_floating_point<double_double> : std::true_type {};
| ^
/usr/bin/../include/c++/v1/__type_traits/is_floating_point.h:30:29: note: marked 'no_specializations' here
30 | struct _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS is_floating_point
| ^
/usr/bin/../include/c++/v1/__config:1177:9: note: expanded from macro '_LIBCPP_NO_SPECIALIZATIONS'
1177 | [[_Clang::__no_specializations__("Users are not allowed to specialize this standard library entity")]]
| ^
test/format-impl-test.cc:318:20: error: 'is_floating_point' cannot be specialized: Users are not allowed to specialize this standard library entity [-Winvalid-specialization]
318 | template <> struct is_floating_point<slow_float> : std::true_type {};
| ^
/usr/bin/../include/c++/v1/__type_traits/is_floating_point.h:30:29: note: marked 'no_specializations' here
30 | struct _LIBCPP_TEMPLATE_VIS _LIBCPP_NO_SPECIALIZATIONS is_floating_point
| ^
/usr/bin/../include/c++/v1/__config:1177:9: note: expanded from macro '_LIBCPP_NO_SPECIALIZATIONS'
1177 | [[_Clang::__no_specializations__("Users are not allowed to specialize this standard library entity")]]
| ^
2 errors generated.
This is how I triggered the build:
cmake -S ./fmtlib_src \
-B ./fmtlib_build \
-G Ninja \
-DCMAKE_BUILD_TYPE=Release \
-DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
-DFMT_TEST=ON
The compile_commands.json holds this command:
clang++ -DFMT_HEADER_ONLY=1 -DGTEST_HAS_STD_WSTRING=1 -D_SILENCE_TR1_NAMESPACE_DEPRECATION_WARNING=1 -Ifmtlib_src/include -isystem fmtlib_src/test/gtest/. -O3 -DNDEBUG -fvisibility=hidden -fvisibility-inlines-hidden -o test/CMakeFiles/format-impl-test.dir/format-impl-test.cc.o -c fmtlib_src/test/format-impl-test.cc
This is an other reference, saying that by specializing this is_floating_point standard library entity you actually create an UB:
https://en.cppreference.com/w/cpp/types/is_floating_point
If the program adds specializations for std::is_floating_point the behavior is undefined.
In the standard, C++20, for example it is mentioned in general:
https://isocpp.org/files/papers/N4860.pdf
chapter 16.5.4.2.1 Namespace std, page 480
1 Unless otherwise specified, the behavior of a C++ program is undefined if it adds declarations or definitions to namespace std or to a namespace within namespace std.
2 Unless explicitly prohibited, a program may add a template specialization for any standard library class template to namespace std provided that (a) the added declaration depends on at least one program-defined type and (b) the specialization meets the standard library requirements for the original template.
3 The behavior of a C++ program is undefined if it declares an explicit or partial specialization of any standard library variable template, except where explicitly permitted by the specification of that variable template.
4 The behavior of a C++ program is undefined if it declares
(4.1) — an explicit specialization of any member function of a standard library class template, or
(4.2) — an explicit specialization of any member function template of a standard library class or class template, or
(4.3) — an explicit or partial specialization of any member class template of a standard library class or class template, or
(4.4) — a deduction guide for any standard library class template.
5 A program may explicitly instantiate a class template defined in the standard library only if the declaration
(a) depends on the name of at least one program-defined type and (b) the instantiation meets the standard
library requirements for the original template.
Obviously, both double_double and slow_float are UDTs (=user-defined types = program-defined types) so paragraph 5) may exempt the rule:
A program may explicitly instantiate a class template defined in the standard library only if the declaration depends on the name of at least one program-defined type.
But such exemption are in direct contradiction with:
Also, pay attention to the Extending the namespace std section at:
https://en.cppreference.com/w/cpp/language/extending_std
Thanks in advance :)
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