add test for format string compile-time check

test/compile-error-test/CMakeLists.txt

@@ -8,7 +8,18 @@ set(test_names "")
 # Adds a name of test into `test_names` list
 # Sets `test_{}_error_expected` (where {} is test name) to 0 or 1 depending on test result
 function (expect_compile name code_fragment)
-  cmake_parse_arguments(EXPECT_COMPILE "ERROR;FORMAT;XCHAR" "" "" ${ARGN})
+  cmake_parse_arguments(EXPECT_COMPILE "ERROR;FORMAT;XCHAR;FORMAT_STRING_CHECK" "" "" ${ARGN})
+  if (EXPECT_COMPILE_FORMAT_STRING_CHECK)
+    if (CMAKE_CXX_STANDARD LESS 20)
+      return()
+    else ()
+      set(else_branch "")
+      if (EXPECT_COMPILE_ERROR)
+        set(else_branch "#error")
+      endif ()
+      set(code_fragment "#ifdef FMT_HAS_CONSTEVAL\n${code_fragment}\n#else\n${else_branch}\n#endif")
+    endif ()
+  endif ()
   set(test_names_copy "${test_names}")
   list(APPEND test_names_copy "${name}")
   set(test_names "${test_names_copy}" PARENT_SCOPE)
@@ -131,5 +142,13 @@ expect_compile(udl-check "
   #endif
 " FORMAT)
 
+# 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()