add test for format string compile-time check

test/compile-error-test/CMakeLists.txt

@@ -17,7 +17,23 @@ set(non_error_test_content "")
 # For non-error tests (we expect them to compile successfully):
 #  * adds a code segment as separate function to `non_error_test_content`
 function (expect_compile name code_fragment)
-  cmake_parse_arguments(EXPECT_COMPILE "ERROR" "" "" ${ARGN})
+  cmake_parse_arguments(EXPECT_COMPILE "ERROR;FORMAT_STRING_CHECK" "" "" ${ARGN})
+  if (EXPECT_COMPILE_FORMAT_STRING_CHECK)
+    if (CMAKE_CXX_STANDARD LESS 20)
+      return()
+    else ()
+      set(else_branch "")
+      if (EXPECT_COMPILE_ERROR)
+        set(else_branch "#error")
+      endif ()
+      set(code_fragment "
+        #ifdef FMT_HAS_CONSTEVAL
+          ${code_fragment}
+        #else
+          ${else_branch}
+        #endif")
+    endif ()
+  endif ()
 
   string(MAKE_C_IDENTIFIER "${name}" test_name)
 
@@ -169,5 +185,13 @@ expect_compile(udl-check "
   #endif
 ")
 
+# Compile-time argument type check
+expect_compile(format-string-number-spec "
+  fmt::format(\"{:d}\", 42);
+" FORMAT_STRING_CHECK)
+expect_compile(format-string-number-spec-error "
+  fmt::format(\"{:d}\", \"I am not a number\");
+" FORMAT_STRING_CHECK ERROR)
+
 # Run all tests
 run_tests()