Fix: enable FMT_NORETURN without exception support too (fmtlib#3917)

When building `format.cc` as such with GCC 13.2.1:

    $ g++ -c format.cc -DFMT_EXCEPTIONS=0 -Wmissing-noreturn -Werror

I get:

    In file included from format.cc:8:
    fmt/format-inl.h: In function ‘void fmt::v10::detail::assert_fail(const char*, int, const char*)’:
    fmt/format-inl.h:30:15: error: function might be candidate for attribute ‘noreturn’ [-Werror=suggest-attribute=noreturn]
       30 | FMT_FUNC void assert_fail(const char* file, int line, const char* message) {
          |               ^~~~~~~~~~~
    fmt/format-inl.h: In function ‘void fmt::v10::report_error(const char*)’:
    fmt/format-inl.h:129:15: error: function might be candidate for attribute ‘noreturn’ [-Werror=suggest-attribute=noreturn]
      129 | FMT_FUNC void report_error(const char* message) {
          |               ^~~~~~~~~~~~
    cc1plus: all warnings being treated as errors

Note that, with `FMT_EXCEPTIONS` defined to 0:

‣ report_error(const char *) uses FMT_THROW() which expands to calling
  assert_fail().

‣ assert_fail() calls std::terminate() which has the `[[noreturn]]`
  attribute since C++11 [1].

Therefore, with `FMT_EXCEPTIONS` defined to 0, both assert_fail() and
report_error() need to have the `[[noreturn]]` attribute too (if
available). In other words, `FMT_NORETURN` doesn't depend on
`FMT_EXCEPTIONS`.

Also adding `FMT_NORETURN` to two on_error() functions which call
report_error(const char *).

Other report_error() overloads eventually return, therefore they don't
need `FMT_NORETURN`.

[1]: https://en.cppreference.com/w/cpp/error/terminate

Signed-off-by: Philippe Proulx <[email protected]>

Author: eepp

Diff for: include/fmt/base.h

@@ -175,8 +175,7 @@
 #endif
 
 // Disable [[noreturn]] on MSVC/NVCC because of bogus unreachable code warnings.
-#if FMT_HAS_CPP_ATTRIBUTE(noreturn) && FMT_EXCEPTIONS && !FMT_MSC_VERSION && \
-    !defined(__NVCC__)
+#if FMT_HAS_CPP_ATTRIBUTE(noreturn) && !FMT_MSC_VERSION && !defined(__NVCC__)
 #  define FMT_NORETURN [[noreturn]]
 #else
 #  define FMT_NORETURN
@@ -2754,7 +2753,9 @@ template <typename Char, typename... Args> class format_string_checker {
     return id >= 0 && id < num_args ? parse_funcs_[id](context_) : begin;
   }
 
-  FMT_CONSTEXPR void on_error(const char* message) { report_error(message); }
+  FMT_NORETURN FMT_CONSTEXPR void on_error(const char* message) {
+    report_error(message);
+  }
 };
 
 // A base class for compile-time strings.

Diff for: include/fmt/format.h

@@ -4302,7 +4302,7 @@ void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
       return begin;
     }
 
-    void on_error(const char* message) { report_error(message); }
+    FMT_NORETURN void on_error(const char* message) { report_error(message); }
   };
   detail::parse_format_string<false>(fmt, format_handler(out, fmt, args, loc));
 }