cmake: add C++ integration test

Also:
- include code to verify a C++-specific public header regression
  reported in 8.19.0-rc2.
- curl/curl.h: mention C++ global namespace in comment.
- GHA/dist: add CI job for C++. Runtime: 15 seconds.

Follow-up to ee9b000438 #20686
Ref: #20682

Closes #20687

.github/workflows/distcheck.yml

@@ -316,6 +316,9 @@ jobs:
         run: ./tests/cmake/test.sh add_subdirectory ${TESTOPTS} -DCURL_USE_OPENSSL=ON
       - name: 'via find_package'
         run: ./tests/cmake/test.sh find_package ${TESTOPTS} -DCURL_USE_OPENSSL=ON
+      - name: 'via find_package (C++)'
+        if: ${{ contains(matrix.image, 'ubuntu') }}
+        run: TEST_CMAKE_FLAGS=-DTEST_CPP=ON ./tests/cmake/test.sh find_package ${TESTOPTS} -DCURL_USE_OPENSSL=ON
 
       - name: 'via ExternalProject (old cmake)'
         if: ${{ contains(matrix.image, 'ubuntu') }}

Makefile.am

@@ -62,6 +62,7 @@ CMAKE_DIST =                                     \
   CMakeLists.txt                                 \
   tests/cmake/CMakeLists.txt                     \
   tests/cmake/test.c                             \
+  tests/cmake/test.cpp                           \
   tests/cmake/test.sh
 
 EXTRA_DIST = CHANGES.md COPYING RELEASE-NOTES Dockerfile .clang-tidy.yml .editorconfig $(CMAKE_DIST)

include/curl/curl.h

@@ -3329,8 +3329,8 @@ CURL_EXTERN CURLcode curl_easy_ssls_export(CURL *handle,
 #if defined(__STDC__) && (__STDC__ >= 1)
 /* This preprocessor magic that replaces a call with the exact same call is
    only done to make sure application authors pass exactly three arguments
-   to these functions. Use recursive macros to allow reusing these symbols
+   to these functions. Use recursive macros to allow using these symbols via
-   as C++ method names. */
+   the C++ global namespace '::' or reuse them as method names. */
 #define curl_easy_setopt(handle, opt, param) \
   curl_easy_setopt(handle, opt, param)
 #define curl_easy_getinfo(handle, info, arg) \

tests/cmake/CMakeLists.txt

@@ -25,7 +25,15 @@
 cmake_minimum_required(VERSION 3.7...3.16 FATAL_ERROR)
 message(STATUS "Using CMake version ${CMAKE_VERSION}")
 
-project(test-consumer C)
+option(TEST_CPP "Test C++" OFF)
+
+if(TEST_CPP)
+  project(test-CPP-consumer CXX)
+  set(_source "test.cpp")
+else()
+  project(test-C-consumer C)
+  set(_source "test.c")
+endif()
 
 option(TEST_INTEGRATION_MODE "Integration mode" "find_package")
 
@@ -93,7 +101,7 @@ elseif(TEST_INTEGRATION_MODE STREQUAL "ExternalProject")
       ${CURL_TEST_OPTS}
     BUILD_BYPRODUCTS "${_curl_static_lib}")
 
-  add_executable(test-consumer-static-fetch "test.c")
+  add_executable(test-consumer-static-fetch "${_source}")
   add_dependencies(test-consumer-static-fetch curl-external)
   if(WIN32)
     target_compile_definitions(test-consumer-static-fetch PRIVATE "CURL_STATICLIB")
@@ -107,26 +115,26 @@ if(TEST_INTEGRATION_MODE STREQUAL "find_package" OR
    TEST_INTEGRATION_MODE STREQUAL "add_subdirectory" OR
    TEST_INTEGRATION_MODE STREQUAL "FetchContent")
 
-  add_executable(test-consumer-static-ns "test.c")
+  add_executable(test-consumer-static-ns "${_source}")
   target_link_libraries(test-consumer-static-ns PRIVATE "CURL::libcurl_static")
 
-  add_executable(test-consumer-shared-ns "test.c")
+  add_executable(test-consumer-shared-ns "${_source}")
   target_link_libraries(test-consumer-shared-ns PRIVATE "CURL::libcurl_shared")
 
   # Alias for either shared or static library
-  add_executable(test-consumer-selected-ns "test.c")
+  add_executable(test-consumer-selected-ns "${_source}")
   target_link_libraries(test-consumer-selected-ns PRIVATE "CURL::libcurl")
 endif()
 
 if(TEST_INTEGRATION_MODE STREQUAL "add_subdirectory" OR
    TEST_INTEGRATION_MODE STREQUAL "FetchContent")
 
-  add_executable(test-consumer-static-bare "test.c")
+  add_executable(test-consumer-static-bare "${_source}")
   target_link_libraries(test-consumer-static-bare PRIVATE "libcurl_static")
 
-  add_executable(test-consumer-shared-bare "test.c")
+  add_executable(test-consumer-shared-bare "${_source}")
   target_link_libraries(test-consumer-shared-bare PRIVATE "libcurl_shared")
 
-  add_executable(test-consumer-selected-bare "test.c")
+  add_executable(test-consumer-selected-bare "${_source}")
   target_link_libraries(test-consumer-selected-bare PRIVATE "libcurl")
 endif()

tests/cmake/test.c

@@ -27,7 +27,7 @@
 int main(int argc, const char **argv)
 {
   (void)argc;
-  puts("libcurl test:");
+  puts("libcurl C test:");
   puts(argv[0]);
   puts(curl_version());
   puts("---");

tests/cmake/test.cpp

@@ -0,0 +1,44 @@
+/***************************************************************************
+ *                                  _   _ ____  _
+ *  Project                     ___| | | |  _ \| |
+ *                             / __| | | | |_) | |
+ *                            | (__| |_| |  _ <| |___
+ *                             \___|\___/|_| \_\_____|
+ *
+ * Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
+ *
+ * This software is licensed as described in the file COPYING, which
+ * you should have received as part of this distribution. The terms
+ * are also available at https://curl.se/docs/copyright.html.
+ *
+ * You may opt to use, copy, modify, merge, publish, distribute and/or sell
+ * copies of the Software, and permit persons to whom the Software is
+ * furnished to do so, under the terms of the COPYING file.
+ *
+ * This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
+ * KIND, either express or implied.
+ *
+ * SPDX-License-Identifier: curl
+ *
+ ***************************************************************************/
+#include <curl/curl.h>
+#include <iostream>
+
+class CurlClass {
+public:
+  void curl_multi_setopt(void *a, int b, long c) {
+    std::cout << curl_version() << std::endl;
+  }
+};
+
+int main(int argc, const char **argv)
+{
+  (void)argc;
+  std::cout << "libcurl C++ test:" << std::endl;
+  std::cout << argv[0] << std::endl;
+  CurlClass mycurl;
+  mycurl.curl_multi_setopt(nullptr, 0, 0);
+  ::curl_easy_setopt(nullptr, CURLOPT_URL, "https://example.com/");
+  std::cout << "---" << std::endl;
+  return 0;
+}