Compilation issue of fmt::format's example

[laper32]

Compiler: MSVC 17.0.0 (Visual Studio 2022)
C++ Version: 20
fmt version: latest git clone

The exactly line of error has been marked at the source code below.
You can check the comment to find it.

Source code:

This comes from https://fmt.dev/latest/api.html#formatting-user-defined-types

#include <fmt/core.h>
#include <fmt/format.h>

struct point { double x, y; };

template <> struct fmt::formatter<point> {
	// Presentation format: 'f' - fixed, 'e' - exponential.
	char presentation = 'f';

	// Parses format specifications of the form ['f' | 'e'].
	constexpr auto parse(format_parse_context& ctx) -> decltype(ctx.begin()) {
		// [ctx.begin(), ctx.end()) is a character range that contains a part of
		// the format string starting from the format specifications to be parsed,
		// e.g. in
		//
		//   fmt::format("{:f} - point of interest", point{1, 2});
		//
		// the range will contain "f} - point of interest". The formatter should
		// parse specifiers until '}' or the end of the range. In this example
		// the formatter should parse the 'f' specifier and return an iterator
		// pointing to '}'.

		// Parse the presentation format and store it in the formatter:
		auto it = ctx.begin(), end = ctx.end();
		if (it != end && (*it == 'f' || *it == 'e')) presentation = *it++;

		// Check if reached the end of the range:
		if (it != end && *it != '}')
			throw format_error("invalid format");

		// Return an iterator past the end of the parsed range:
		return it;
	}


	// Formats the point p using the parsed format specification (presentation)
	// stored in this formatter.
	template <typename FormatContext>
	auto format(const point& p, FormatContext& ctx) -> decltype(ctx.out()) {
		// ctx.out() is an output iterator to write to.
		return format_to(
			ctx.out(),
			presentation == 'f' ? "({:.1f}, {:.1f})" : "({:.1e}, {:.1e})", // All errors are come from here.
			p.x, p.y);
	}
};
int main() {
	point p = { 1, 2 };
	std::string s = fmt::format("{:f}", p);
}

Error message:

D:\workshop\cxx\CommonLab\main.cpp(345): error C7595: 'fmt::v8::basic_format_string<char,const double &,const double &>::basic_format_string': call to immediate function is not a constant expression
  D:\workshop\cxx\CommonLab\main.cpp(345): note: failure was caused by a read of a variable outside its lifetime
  D:\workshop\cxx\CommonLab\main.cpp(345): note: see usage of 'this'
  D:\workshop\cxx\CommonLab\fmt\include\fmt/core.h(1242): note: see reference to function template instantiation 'OutputIt fmt::v8::formatter<point,char,void>::format<Context>(const point &,FormatContext &)' being compiled
          with
          [
              OutputIt=fmt::v8::appender,
              Context=context,
              FormatContext=context
          ]
  D:\workshop\cxx\CommonLab\fmt\include\fmt/core.h(1225): note: see reference to function template instantiation 'void fmt::v8::detail::value<Context>::format_custom_arg<value_type,fmt::v8::formatter<point,char,void>>(void *,fmt::v8::basic_format_parse_context<char,fmt::v8::detail::error_handler> &,Context &)' being compiled
          with
          [
              Context=fmt::v8::format_context
          ]
  D:\workshop\cxx\CommonLab\fmt\include\fmt/core.h(1221): note: see reference to function template instantiation 'void fmt::v8::detail::value<Context>::format_custom_arg<value_type,fmt::v8::formatter<point,char,void>>(void *,fmt::v8::basic_format_parse_context<char,fmt::v8::detail::error_handler> &,Context &)' being compiled
          with
          [
              Context=fmt::v8::format_context
          ]
  D:\workshop\cxx\CommonLab\fmt\include\fmt/core.h(1686): note: see reference to function template instantiation 'fmt::v8::detail::value<Context>::value<const point>(T &)' being compiled
          with
          [
              Context=fmt::v8::format_context,
              T=const point
          ]
  D:\workshop\cxx\CommonLab\fmt\include\fmt/core.h(1686): note: see reference to function template instantiation 'fmt::v8::detail::value<Context>::value<const point>(T &)' being compiled
          with
          [
              Context=fmt::v8::format_context,
              T=const point
          ]
  D:\workshop\cxx\CommonLab\fmt\include\fmt/core.h(1804): note: see reference to function template instantiation 'fmt::v8::detail::value<Context> fmt::v8::detail::make_arg<true,Context,fmt::v8::detail::type::custom_type,point&,0>(T)' being compiled
          with
          [
              Context=fmt::v8::format_context,
              T=point &
          ]
  D:\workshop\cxx\CommonLab\fmt\include\fmt/core.h(1827): note: see reference to function template instantiation 'fmt::v8::format_arg_store<fmt::v8::format_context,point>::format_arg_store<point&>(point &)' being compiled
  D:\workshop\cxx\CommonLab\fmt\include\fmt/core.h(1827): note: see reference to function template instantiation 'fmt::v8::format_arg_store<fmt::v8::format_context,point>::format_arg_store<point&>(point &)' being compiled
  D:\workshop\cxx\CommonLab\fmt\include\fmt/core.h(3071): note: see reference to function template instantiation 'fmt::v8::format_arg_store<fmt::v8::format_context,point> fmt::v8::make_format_args<fmt::v8::format_context,point&>(point &)' being compiled
  D:\workshop\cxx\CommonLab\main.cpp(352): note: see reference to function template instantiation 'std::string fmt::v8::format<point&>(fmt::v8::basic_format_string<char,point &>,point &)' being compiled

Note: Looks like this example marked as problem many times, perhaps this example need also write into test cases.

[laper32]

Similar issues:
#2574
#2522

[vitaut]

Those are not similar issues but exactly the same (fixed) issue. Please see the dev docs: https://fmt.dev/dev/api.html#formatting-user-defined-types.