From 98204b17a6b431ba660a6f1d9f21faf65a390d09 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Kalle=20Kiet=C3=A4v=C3=A4inen?=
 <1026741+kietavainen@users.noreply.github.com>
Date: Sun, 17 Nov 2024 22:18:28 +0200
Subject: [PATCH] Set CMake C++ standard (#3688)

Instead of setting `-std` compiler flag directly, set the
`CMAKE_CXX_STANDARD` variable. This lets CMake know the required C++
standard and evaluate the final compiler flag correctly, taking into
account compile features set by `target_compile_features()`.

This change preserves the existing behavior, where compiler extensions
are disabled for other targets than Cygwin.
---
 cmake/CXXSniffer.cmake | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/cmake/CXXSniffer.cmake b/cmake/CXXSniffer.cmake
index 07c06cdae..ef1ec6511 100644
--- a/cmake/CXXSniffer.cmake
+++ b/cmake/CXXSniffer.cmake
@@ -6,7 +6,8 @@ include (CheckCXXCompilerFlag)
 CHECK_CXX_COMPILER_FLAG("-std=c++17"   _HAS_CXX17)
 
 if (_HAS_CXX17)
-  set (_CXX14_FLAGS "-std=c++17")
+  set (CMAKE_CXX_STANDARD 17)
+  set (CMAKE_CXX_EXTENSIONS OFF)
 else (_HAS_CXX17)
  message (FATAL_ERROR "C++17 support missing. Try upgrading your C++ compiler. If you have a good reason for using an outdated compiler, please let us know at support@gothenburgbitfactory.org.")
 endif (_HAS_CXX17)
@@ -32,7 +33,7 @@ elseif (${CMAKE_SYSTEM_NAME} STREQUAL "GNU")
   set (GNUHURD true)
 elseif (${CMAKE_SYSTEM_NAME} STREQUAL "CYGWIN")
   set (CYGWIN true)
-  set (_CXX14_FLAGS "-std=gnu++17")
+  set (CMAKE_CXX_EXTENSIONS ON)
 else (${CMAKE_SYSTEM_NAME} MATCHES "Linux")
   set (UNKNOWN true)
 endif (${CMAKE_SYSTEM_NAME} MATCHES "Linux")