Add note about CMake standard version selection to unit tests

Document how CMake chooses which C++ standard version to use when
building tests.

tests/src/unit-comparison.cpp

@@ -27,16 +27,19 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE  OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 */
 
+// cmake/test.cmake selects the C++ standard versions with which to build a
+// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
+// When using macros that are only defined for particular versions of the standard
+// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
+// version macro in a comment close by, like this:
+// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+
 #include "doctest_compatibility.h"
 
 #define JSON_TESTS_PRIVATE
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
-
-// build this testcase in C++20-mode (CMake code detects macro use)
-// JSON_HAS_CPP_20
-
 #if JSON_HAS_THREE_WAY_COMPARISON
 // this can be replaced with the doctest stl extension header in version 2.5
 namespace doctest
@@ -144,6 +147,7 @@ TEST_CASE("lexicographical comparison operators")
                     CAPTURE(j)
                     // check precomputed values
 #if JSON_HAS_THREE_WAY_COMPARISON
+                    // JSON_HAS_CPP_20 (do not remove; see note at top of file)
                     CHECK((j_types[i] < j_types[j]) == expected_lt[i][j]);
 #else
                     CHECK(operator<(j_types[i], j_types[j]) == expected_lt[i][j]);
@@ -153,6 +157,7 @@ TEST_CASE("lexicographical comparison operators")
             }
         }
 #if JSON_HAS_THREE_WAY_COMPARISON
+        // JSON_HAS_CPP_20 (do not remove; see note at top of file)
         SECTION("comparison: 3-way")
         {
             std::vector<std::vector<std::partial_ordering>> expected =
@@ -508,6 +513,7 @@ TEST_CASE("lexicographical comparison operators")
         }
 
 #if JSON_HAS_THREE_WAY_COMPARISON
+        // JSON_HAS_CPP_20 (do not remove; see note at top of file)
         SECTION("comparison: 3-way")
         {
             std::vector<std::vector<std::partial_ordering>> expected =

tests/src/unit-conversions.cpp

@@ -27,6 +27,13 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE  OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 */
 
+// cmake/test.cmake selects the C++ standard versions with which to build a
+// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
+// When using macros that are only defined for particular versions of the standard
+// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
+// version macro in a comment close by, like this:
+// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+
 #include "doctest_compatibility.h"
 
 #define JSON_TESTS_PRIVATE
@@ -1582,12 +1589,4 @@ TEST_CASE("JSON to enum mapping")
     }
 }
 
-#ifdef JSON_HAS_CPP_17
-    #undef JSON_HAS_CPP_17
-#endif
-
-#ifdef JSON_HAS_CPP_14
-    #undef JSON_HAS_CPP_14
-#endif
-
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-items.cpp

@@ -1448,13 +1448,5 @@ TEST_CASE("items()")
     }
 }
 
-#ifdef JSON_HAS_CPP_17
-    #undef JSON_HAS_CPP_17
-#endif
-
-#ifdef JSON_HAS_CPP_14
-    #undef JSON_HAS_CPP_14
-#endif
-
 DOCTEST_GCC_SUPPRESS_WARNING_POP
 DOCTEST_CLANG_SUPPRESS_WARNING_POP

tests/src/unit-iterators2.cpp

@@ -27,13 +27,19 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE  OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 */
 
+// cmake/test.cmake selects the C++ standard versions with which to build a
+// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
+// When using macros that are only defined for particular versions of the standard
+// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
+// version macro in a comment close by, like this:
+// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+
 #include "doctest_compatibility.h"
 
 #include <nlohmann/json.hpp>
 using nlohmann::json;
 
 #if JSON_HAS_RANGES
-    // JSON_HAS_CPP_20 (magic keyword; do not remove)
     #include <algorithm>
     #include <ranges>
 #endif
@@ -890,6 +896,7 @@ TEST_CASE("iterators 2")
 
 
 #if JSON_HAS_RANGES
+    // JSON_HAS_CPP_20 (do not remove; see note at top of file)
     SECTION("ranges")
     {
         SECTION("concepts")

tests/src/unit-regression2.cpp

@@ -27,6 +27,13 @@ OUT OF OR IN CONNECTION WITH THE SOFTWARE  OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 */
 
+// cmake/test.cmake selects the C++ standard versions with which to build a
+// unit test based on the presence of JSON_HAS_CPP_<VERSION> macros.
+// When using macros that are only defined for particular versions of the standard
+// (e.g., JSON_HAS_FILESYSTEM for C++17 and up), please mention the corresponding
+// version macro in a comment close by, like this:
+// JSON_HAS_CPP_<VERSION> (do not remove; see note at top of file)
+
 #include "doctest_compatibility.h"
 
 // for some reason including this after the json header leads to linker errors with VS 2017...
@@ -48,7 +55,6 @@ using ordered_json = nlohmann::ordered_json;
 #endif
 
 #if JSON_HAS_EXPERIMENTAL_FILESYSTEM
-// JSON_HAS_CPP_17 (magic keyword; do not remove)
 #include <experimental/filesystem>
 namespace nlohmann::detail
 {
@@ -788,6 +794,7 @@ TEST_CASE("regression tests 2")
     }
 
 #if JSON_HAS_FILESYSTEM || JSON_HAS_EXPERIMENTAL_FILESYSTEM
+    // JSON_HAS_CPP_17 (do not remove; see note at top of file)
     SECTION("issue #3070 - Version 3.10.3 breaks backward-compatibility with 3.10.2 ")
     {
         nlohmann::detail::std_fs::path text_path("/tmp/text.txt");