Ensure that plugin class name is a valid C indentifier

We should prevent using the invalid plugin class names at CMake
configure stage already, so users receive the early error.

Fixes: QTBUG-135860
Change-Id: I259539f6cce70a035ccf458a62d9e5a02f238ef8
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

cmake/QtPublicCMakeHelpers.cmake

@@ -918,3 +918,26 @@ function(_qt_internal_configure_file mode)
 
     configure_file("${input_file}" "${arg_OUTPUT}" @ONLY)
 endfunction()
+
+# The function checks if `value` is a valid C indentifier.
+#
+# Synopsis
+#
+#  _qt_internal_is_c_identifier(<out_var> <value>)
+#
+# Arguments
+#
+#  `out_var`
+#    Variable name for the evaluation result.
+#
+#  `value`
+#    The string for the evaluation.
+function(_qt_internal_is_c_identifier out_var value)
+    string(MAKE_C_IDENTIFIER "${value}" value_valid)
+
+    if(value AND "${value}" STREQUAL "${value_valid}")
+        set(${out_var} "TRUE" PARENT_SCOPE)
+    else()
+        set(${out_var} "FALSE" PARENT_SCOPE)
+    endif()
+endfunction()

src/corelib/Qt6CoreMacros.cmake

@@ -2626,6 +2626,14 @@ function(qt6_add_plugin target)
         endif()
     endif()
 
+    _qt_internal_is_c_identifier(is_c_indentifier "${plugin_class_name}")
+    if(NOT is_c_indentifier)
+        message(FATAL_ERROR "The provided or calculated plugin CLASS_NAME '${plugin_class_name}' of"
+            " the '${target}' target is not a valid C++ class name. Please use only valid C++"
+            " identifiers."
+        )
+    endif()
+
     set_target_properties(${target} PROPERTIES QT_PLUGIN_CLASS_NAME "${plugin_class_name}")
 
     # Create a plugin initializer object library for static plugins.

tests/auto/cmake/CMakeLists.txt

@@ -482,5 +482,35 @@ if(NOT QNX AND NOT WASM AND NOT (WIN32 AND QT_BUILD_MINIMAL_STATIC_TESTS)
     _qt_internal_test_expect_pass(test_qt_add_ui_11)
 endif()
 
+# Valid plugin names
+set(valid_plugin_class_names
+    TestPluginNameUpper
+    Test_Plugin_Name
+    _Test_plugin_name
+    testpluginnamelower
+    Test0PluginName
+)
+foreach(plugin_class_name IN LISTS valid_plugin_class_names)
+    _qt_internal_test_expect_pass(test_plugin_class_name
+        TESTNAME test_plugin_class_name_${plugin_class_name}
+        BUILD_OPTIONS
+            -DPLUGIN_CLASS_NAME=${plugin_class_name}
+    )
+endforeach()
+
+# Invalid plugin names
+set(invalid_plugin_class_names
+    0TestPluginName
+    Test-Plugin-Name
+    Test.plugin.name
+)
+foreach(plugin_class_name IN LISTS invalid_plugin_class_names)
+    _qt_internal_test_expect_fail(test_plugin_class_name
+        TESTNAME test_plugin_class_name_${plugin_class_name}
+        BUILD_OPTIONS
+            -DPLUGIN_CLASS_NAME=${plugin_class_name}
+    )
+endforeach()
+
 # Add all tests using CMake's RunCMake test module
 add_subdirectory(RunCMake)

tests/auto/cmake/test_plugin_class_name/CMakeLists.txt

@@ -0,0 +1,13 @@
+# Copyright (C) 2025 The Qt Company Ltd.
+# SPDX-License-Identifier: BSD-3-Clause
+
+cmake_minimum_required(VERSION 3.20)
+
+project(test_plugin_class_name_${PLUGIN_CLASS_NAME} LANGUAGES CXX)
+
+find_package(Qt6Core REQUIRED)
+
+configure_file("${CMAKE_CURRENT_SOURCE_DIR}/plugin.cpp.in"
+    "${CMAKE_CURRENT_BINARY_DIR}/plugin.cpp" @ONLY)
+
+qt_add_plugin(test_plugin CLASS_NAME ${PLUGIN_CLASS_NAME} "${CMAKE_CURRENT_BINARY_DIR}/plugin.cpp")

tests/auto/cmake/test_plugin_class_name/plugin.cpp.in

@@ -0,0 +1,6 @@
+// Copyright (C) 2025 The Qt Company Ltd.
+// SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only
+
+class @PLUGIN_CLASS_NAME@ {
+    // noop
+};