test: test_package now use libfmt when needed

test_package/CMakeLists.txt

@@ -27,3 +27,4 @@ find_package(mp-units CONFIG REQUIRED)
 
 add_executable(test_package test_package.cpp)
 target_link_libraries(test_package PRIVATE mp-units::mp-units)
+target_compile_definitions(test_package PRIVATE MP_UNITS_USE_LIBFMT=$<BOOL:${MP_UNITS_USE_LIBFMT}>)

test_package/conanfile.py

@@ -24,12 +24,20 @@ import os
 
 from conan import ConanFile
 from conan.tools.build import can_run
-from conan.tools.cmake import CMake, cmake_layout
+from conan.tools.cmake import CMake, CMakeToolchain, cmake_layout
+from conan.tools.scm import Version
 
 
 class TestPackageConan(ConanFile):
     settings = "os", "arch", "compiler", "build_type"
-    generators = "CMakeDeps", "CMakeToolchain", "VirtualRunEnv"
+    generators = "CMakeDeps"
+
+    @property
+    def _use_libfmt(self):
+        compiler = self.settings.compiler
+        version = Version(self.settings.compiler.version)
+        std_support = compiler == "msvc" and version >= 193 and compiler.cppstd == 23
+        return not std_support
 
     def requirements(self):
         self.requires(self.tested_reference_str)
@@ -37,6 +45,11 @@ class TestPackageConan(ConanFile):
     def layout(self):
         cmake_layout(self)
 
+    def generate(self):
+        tc = CMakeToolchain(self)
+        tc.variables["MP_UNITS_USE_LIBFMT"] = self._use_libfmt
+        tc.generate()
+
     def build(self):
         cmake = CMake(self)
         cmake.configure()

test_package/test_package.cpp

@@ -20,6 +20,7 @@
 // OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 // SOFTWARE.
 
+#include <mp-units/format.h>
 #include <mp-units/ostream.h>
 #include <mp-units/systems/isq/space_and_time.h>
 #include <mp-units/systems/si/unit_symbols.h>
@@ -35,5 +36,5 @@ constexpr QuantityOf<isq::speed> auto avg_speed(QuantityOf<isq::distance> auto d
 int main()
 {
   using namespace mp_units::si::unit_symbols;
-  std::cout << "Average speed = " << avg_speed(240 * km, 2 * h) << '\n';
+  std::cout << MP_UNITS_STD_FMT::format("Average speed = {}\n", avg_speed(240 * km, 2 * h));
 }