test(cmakev2): verify IDF_TOOLCHAIN is propagated to sdkconfig

Add a buildv2 test that reconfigures the test app with both GCC and
clang toolchains and asserts that the selected IDF_TOOLCHAIN value
is propagated to sdkconfig. The clang case is skipped when a clang
compiler is not available in PATH.

tools/test_build_system/buildv2/test_toolchain.py

@@ -0,0 +1,58 @@
+# SPDX-FileCopyrightText: 2026 Espressif Systems (Shanghai) CO LTD
+# SPDX-License-Identifier: Apache-2.0
+import logging
+import shutil
+from pathlib import Path
+
+import pytest
+from test_build_system_helpers import EnvDict
+from test_build_system_helpers import file_contains
+from test_build_system_helpers import run_idf_py
+
+
+@pytest.mark.usefixtures('test_app_copy')
+def test_idf_toolchain_gcc_propagation(default_idf_env: EnvDict) -> None:
+    """IDF_TOOLCHAIN=gcc is propagated to the CMake cache and to sdkconfig."""
+    logging.info('Testing IDF_TOOLCHAIN=gcc propagation')
+
+    default_idf_env.pop('IDF_TOOLCHAIN', None)
+    run_idf_py('reconfigure', env=default_idf_env)
+
+    assert file_contains('build/CMakeCache.txt', 'IDF_TOOLCHAIN:STRING=gcc'), (
+        'IDF_TOOLCHAIN CMake cache variable must be "gcc" when building with the default toolchain'
+    )
+
+    sdkconfig_h = Path('build/config/sdkconfig.h').read_text()
+    assert '#define CONFIG_IDF_TOOLCHAIN "gcc"' in sdkconfig_h, (
+        'CONFIG_IDF_TOOLCHAIN must be "gcc" in sdkconfig.h for the default toolchain'
+    )
+    assert '#define CONFIG_IDF_TOOLCHAIN_GCC 1' in sdkconfig_h, (
+        'CONFIG_IDF_TOOLCHAIN_GCC must be set in sdkconfig.h for the default toolchain'
+    )
+
+
+@pytest.mark.usefixtures('test_app_copy')
+def test_idf_toolchain_clang_propagation(default_idf_env: EnvDict) -> None:
+    """IDF_TOOLCHAIN=clang is propagated to the CMake cache and to sdkconfig,
+    and selects a libc implementation that is compatible with clang."""
+    clang_path = shutil.which('clang')
+    if clang_path is None or 'esp-clang' not in str(Path(clang_path).resolve()):
+        pytest.skip('Espressif clang (esp-clang) not available in PATH')
+
+    logging.info('Testing IDF_TOOLCHAIN=clang propagation')
+
+    default_idf_env['IDF_TOOLCHAIN'] = 'clang'
+    run_idf_py('reconfigure', env=default_idf_env)
+
+    assert file_contains('build/CMakeCache.txt', 'IDF_TOOLCHAIN:STRING=clang'), (
+        'IDF_TOOLCHAIN CMake cache variable must be "clang" when IDF_TOOLCHAIN=clang is set'
+    )
+
+    sdkconfig_h = Path('build/config/sdkconfig.h').read_text()
+    assert '#define CONFIG_IDF_TOOLCHAIN "clang"' in sdkconfig_h, (
+        'CONFIG_IDF_TOOLCHAIN must be "clang" in sdkconfig.h when IDF_TOOLCHAIN=clang is set'
+    )
+    assert '#define CONFIG_IDF_TOOLCHAIN_CLANG 1' in sdkconfig_h, (
+        'CONFIG_IDF_TOOLCHAIN_CLANG must be set in sdkconfig.h when IDF_TOOLCHAIN=clang is set'
+    )
+    assert '#define CONFIG_LIBC_NEWLIB 1' in sdkconfig_h, 'CONFIG_LIBC_NEWLIB must be selected for clang builds'