Add error message if test suite cannot be found

README.md

@@ -1728,6 +1728,22 @@ $ ctest --output-on-failure
 
 Note that during the `ctest` stage, several JSON test files are downloaded from an [external repository](https://github.com/nlohmann/json_test_data). If policies forbid downloading artifacts during testing, you can download the files yourself and pass the directory with the test files via `-DJSON_TestDataDirectory=path` to CMake. Then, no Internet connectivity is required. See [issue #2189](https://github.com/nlohmann/json/issues/2189) for more information.
 
+If the test suite is not found, several test suites will fail like this:
+
+```
+===============================================================================
+json/tests/src/make_test_data_available.hpp:21:
+TEST CASE:  check test suite is downloaded
+
+json/tests/src/make_test_data_available.hpp:23: FATAL ERROR: REQUIRE( utils::check_testsuite_downloaded() ) is NOT correct!
+  values: REQUIRE( false )
+  logged: Test data not found in 'json/cmake-build-debug/json_test_data'.
+          Please execute target 'download_test_data' before running this test suite.
+          See <https://github.com/nlohmann/json#execute-unit-tests> for more information.
+
+===============================================================================
+```
+
 In case you have downloaded the library rather than checked out the code via Git, test `cmake_fetch_content_configure` will fail. Please execute `ctest -LE git_required` to skip these tests. See [issue #2189](https://github.com/nlohmann/json/issues/2189) for more information.
 
 Some tests change the installed files and hence make the whole process not reproducible. Please execute `ctest -LE not_reproducible` to skip these tests. See [issue #2324](https://github.com/nlohmann/json/issues/2324) for more information.

tests/src/make_test_data_available.hpp

@@ -0,0 +1,22 @@
+#pragma once
+
+#include <cstdio>   // fopen, fclose, FILE
+#include <memory> // unique_ptr
+#include <test_data.hpp>
+#include <doctest.h>
+
+namespace utils
+{
+
+inline bool check_testsuite_downloaded()
+{
+    std::unique_ptr<std::FILE, decltype(&std::fclose)> file(std::fopen(TEST_DATA_DIRECTORY "/README.md", "r"), &std::fclose);
+    return file.operator bool();
+}
+
+TEST_CASE("check test suite is downloaded")
+{
+    REQUIRE_MESSAGE(utils::check_testsuite_downloaded(), "Test data not found in '" TEST_DATA_DIRECTORY "'. Please execute target 'download_test_data' before running this test suite. See <https://github.com/nlohmann/json#execute-unit-tests> for more information.");
+}
+
+}  // namespace utils

tests/src/unit-binary_formats.cpp

@@ -33,7 +33,7 @@ SOFTWARE.
 using nlohmann::json;
 
 #include <fstream>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 
 TEST_CASE("Binary Formats" * doctest::skip())
 {

tests/src/unit-bjdata.cpp

@@ -37,7 +37,7 @@ using nlohmann::json;
 #include <iostream>
 #include <fstream>
 #include <set>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 #include "test_utils.hpp"
 
 namespace

tests/src/unit-bson.cpp

@@ -34,7 +34,7 @@ using nlohmann::json;
 
 #include <fstream>
 #include <sstream>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 #include "test_utils.hpp"
 
 TEST_CASE("BSON")

tests/src/unit-cbor.cpp

@@ -37,7 +37,7 @@ using nlohmann::json;
 #include <iomanip>
 #include <iostream>
 #include <set>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 #include "test_utils.hpp"
 
 namespace

tests/src/unit-inspection.cpp

@@ -34,7 +34,7 @@ using nlohmann::json;
 
 #include <fstream>
 #include <sstream>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 
 TEST_CASE("object inspection")
 {

tests/src/unit-json_patch.cpp

@@ -33,7 +33,7 @@ SOFTWARE.
 using nlohmann::json;
 
 #include <fstream>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 
 TEST_CASE("JSON patch")
 {

tests/src/unit-msgpack.cpp

@@ -36,7 +36,7 @@ using nlohmann::json;
 #include <sstream>
 #include <iomanip>
 #include <set>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 #include "test_utils.hpp"
 
 namespace

tests/src/unit-regression1.cpp

@@ -40,7 +40,7 @@ using nlohmann::json;
 #include <sstream>
 #include <list>
 #include <cstdio>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 
 #ifdef JSON_HAS_CPP_17
     #include <variant>

tests/src/unit-testsuites.cpp

@@ -33,7 +33,7 @@ SOFTWARE.
 using nlohmann::json;
 
 #include <fstream>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 
 TEST_CASE("compliance tests from json.org")
 {

tests/src/unit-ubjson.cpp

@@ -35,7 +35,7 @@ using nlohmann::json;
 #include <iostream>
 #include <fstream>
 #include <set>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 #include "test_utils.hpp"
 
 namespace

tests/src/unit-unicode1.cpp

@@ -37,7 +37,7 @@ using nlohmann::json;
 #include <fstream>
 #include <sstream>
 #include <iomanip>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 
 TEST_CASE("Unicode (1/5)" * doctest::skip())
 {

tests/src/unit-unicode2.cpp

@@ -39,7 +39,7 @@ using nlohmann::json;
 #include <sstream>
 #include <iostream>
 #include <iomanip>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 
 // this test suite uses static variables with non-trivial destructors
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH

tests/src/unit-unicode3.cpp

@@ -39,7 +39,7 @@ using nlohmann::json;
 #include <sstream>
 #include <iostream>
 #include <iomanip>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 
 // this test suite uses static variables with non-trivial destructors
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH

tests/src/unit-unicode4.cpp

@@ -39,7 +39,7 @@ using nlohmann::json;
 #include <sstream>
 #include <iostream>
 #include <iomanip>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 
 // this test suite uses static variables with non-trivial destructors
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH

tests/src/unit-unicode5.cpp

@@ -39,7 +39,7 @@ using nlohmann::json;
 #include <sstream>
 #include <iostream>
 #include <iomanip>
-#include <test_data.hpp>
+#include "make_test_data_available.hpp"
 
 // this test suite uses static variables with non-trivial destructors
 DOCTEST_CLANG_SUPPRESS_WARNING_PUSH