From 02bfd8a9a54e3ccc1e1e5a31c4782d2fb652d7c8 Mon Sep 17 00:00:00 2001
From: Dair Grant <dair@refnum.com>
Date: Mon, 2 Mar 2020 14:51:51 +0000
Subject: [PATCH] Add FMT_HAS_CPP14_ATTRIBUTE / FMT_HAS_CPP17_ATTRIBUTE to test
 for language-specific attributes.

FMT_DEPRECATED is now defined as FMT_HAS_CPP14_ATTRIBUTE(deprecated), as this attribute was introduced in C++14.

FMT_FALLTHROUGH is now defined as FMT_HAS_CPP17_ATTRIBUTE(fallthrough), as this attribute was introduced in C++17.

FMT_MAYBE_UNUSED is defined as FMT_HAS_CPP17_ATTRIBUTE(maybe_unused), as this attribute was introduced in C++17.

FMT_MAYBE_UNUSED has been applied to fix a couple of -Wunused-member-function warnings from clang.
---
 include/fmt/core.h       | 17 +++++++++++++++--
 include/fmt/format-inl.h |  3 +++
 include/fmt/format.h     |  4 ++--
 3 files changed, 20 insertions(+), 4 deletions(-)

diff --git a/include/fmt/core.h b/include/fmt/core.h
index 8bcc0d06..25647037 100644
--- a/include/fmt/core.h
+++ b/include/fmt/core.h
@@ -36,6 +36,12 @@
 #  define FMT_HAS_CPP_ATTRIBUTE(x) 0
 #endif
 
+#define FMT_HAS_CPP14_ATTRIBUTE(attribute) \
+  (__cplusplus >= 201402L && FMT_HAS_CPP_ATTRIBUTE(attribute))
+
+#define FMT_HAS_CPP17_ATTRIBUTE(attribute) \
+  (__cplusplus >= 201703L && FMT_HAS_CPP_ATTRIBUTE(attribute))
+
 #ifdef __clang__
 #  define FMT_CLANG_VERSION (__clang_major__ * 100 + __clang_minor__)
 #else
@@ -132,9 +138,16 @@
 #  define FMT_NORETURN
 #endif
 
+#ifndef FMT_MAYBE_UNUSED
+#  if FMT_HAS_CPP17_ATTRIBUTE(maybe_unused)
+#    define FMT_MAYBE_UNUSED [[maybe_unused]]
+#  else
+#    define FMT_MAYBE_UNUSED
+#  endif
+#endif
+
 #ifndef FMT_DEPRECATED
-#  if (FMT_HAS_CPP_ATTRIBUTE(deprecated) && __cplusplus >= 201402L) || \
-      FMT_MSC_VER >= 1900
+#  if FMT_HAS_CPP14_ATTRIBUTE(deprecated) || FMT_MSC_VER >= 1900
 #    define FMT_DEPRECATED [[deprecated]]
 #  else
 #    if defined(__GNUC__) || defined(__clang__)
diff --git a/include/fmt/format-inl.h b/include/fmt/format-inl.h
index c42f7280..5d294bc7 100644
--- a/include/fmt/format-inl.h
+++ b/include/fmt/format-inl.h
@@ -86,6 +86,7 @@ FMT_FUNC int safe_strerror(int error_code, char*& buffer,
     }
 
     // Handle the result of GNU-specific version of strerror_r.
+    FMT_MAYBE_UNUSED
     int handle(char* message) {
       // If the buffer is full then the message is probably truncated.
       if (message == buffer_ && strlen(buffer_) == buffer_size_ - 1)
@@ -95,11 +96,13 @@ FMT_FUNC int safe_strerror(int error_code, char*& buffer,
     }
 
     // Handle the case when strerror_r is not available.
+    FMT_MAYBE_UNUSED
     int handle(internal::null<>) {
       return fallback(strerror_s(buffer_, buffer_size_, error_code_));
     }
 
     // Fallback to strerror_s when strerror_r is not available.
+    FMT_MAYBE_UNUSED
     int fallback(int result) {
       // If the buffer is full then the message is probably truncated.
       return result == 0 && strlen(buffer_) == buffer_size_ - 1 ? ERANGE
diff --git a/include/fmt/format.h b/include/fmt/format.h
index 537a80ce..07d4f5f8 100644
--- a/include/fmt/format.h
+++ b/include/fmt/format.h
@@ -71,7 +71,7 @@
 #  else
 #    define FMT_FALLTHROUGH
 #  endif
-#elif (FMT_HAS_CPP_ATTRIBUTE(fallthrough) && (__cplusplus >= 201703)) || \
+#elif FMT_HAS_CPP17_ATTRIBUTE(fallthrough) || \
     (defined(_MSVC_LANG) && _MSVC_LANG >= 201703L)
 #  define FMT_FALLTHROUGH [[fallthrough]]
 #else
@@ -3549,7 +3549,7 @@ FMT_END_NAMESPACE
     /* Use a macro-like name to avoid shadowing warnings. */ \
     struct FMT_COMPILE_STRING : fmt::compile_string {        \
       using char_type = fmt::remove_cvref_t<decltype(*s)>;   \
-      __VA_ARGS__ FMT_CONSTEXPR                              \
+      FMT_MAYBE_UNUSED __VA_ARGS__ FMT_CONSTEXPR             \
       operator fmt::basic_string_view<char_type>() const {   \
         /* FMT_STRING only accepts string literals. */       \
         return fmt::internal::literal_to_view(s);            \