[esp32] Suppress -Wvolatile in the direct ESP-IDF build (#17404)

tests/unit_tests/build_gen/test_espidf.py

@@ -243,6 +243,7 @@ def test_get_project_cmakelists_no_cpp_standard(tmp_path: Path) -> None:
         patch("esphome.build_gen.espidf.get_esp32_variant", return_value="ESP32"),
         patch.object(CORE, "name", "test"),
         patch.object(CORE, "cpp_standard", None),
+        patch.object(CORE, "cxx_build_flags", set()),
     ):
         from esphome.build_gen.espidf import get_project_cmakelists
 
@@ -251,6 +252,28 @@ def test_get_project_cmakelists_no_cpp_standard(tmp_path: Path) -> None:
     assert "CXX_COMPILE_OPTIONS" not in content
 
 
+def test_get_project_cmakelists_cxx_build_flags(tmp_path: Path) -> None:
+    """Flags registered via cg.add_cxx_build_flag() are appended to
+    CXX_COMPILE_OPTIONS (C++-only, GCC warns if they reach C compiles)
+    between include(project.cmake) and project()."""
+    with (
+        patch("esphome.build_gen.espidf.get_esp32_variant", return_value="ESP32"),
+        patch.object(CORE, "name", "test"),
+        patch.object(CORE, "cpp_standard", None),
+        patch.object(CORE, "cxx_build_flags", {"-Wno-volatile"}),
+    ):
+        from esphome.build_gen.espidf import get_project_cmakelists
+
+        content = get_project_cmakelists(minimal=True)
+
+    flag_line = 'idf_build_set_property(CXX_COMPILE_OPTIONS "-Wno-volatile" APPEND)'
+    assert flag_line in content
+    include_pos = content.index("tools/cmake/project.cmake")
+    flag_pos = content.index(flag_line)
+    project_pos = content.index("project(test)")
+    assert include_pos < flag_pos < project_pos
+
+
 def test_get_component_cmakelists_no_compile_features() -> None:
     """The C++ standard is pinned project-wide via CXX_COMPILE_OPTIONS in the
     top-level CMakeLists; the src component must not set its own."""

tests/unit_tests/build_gen/test_platformio.py

@@ -200,3 +200,32 @@ def test_get_ini_content_no_cpp_standard(
     content = platformio.get_ini_content()
 
     assert "-std=" not in content
+
+
+def test_write_cxx_flags_script_emits_registered_flags(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """Flags registered via cg.add_cxx_build_flag() are emitted as CXXFLAGS,
+    sorted, so they apply to C++ compiles only."""
+    CORE.build_path = str(tmp_path)
+    monkeypatch.setattr(CORE, "cxx_build_flags", {"-Wno-volatile", "-Wno-deprecated"})
+
+    platformio.write_cxx_flags_script()
+
+    content = (tmp_path / platformio.CXX_FLAGS_FILE_NAME).read_text()
+    assert (
+        'env.Append(CXXFLAGS=["-Wno-deprecated"])\n'
+        'env.Append(CXXFLAGS=["-Wno-volatile"])\n'
+    ) in content
+
+
+def test_write_cxx_flags_script_no_flags(
+    tmp_path: Path, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    CORE.build_path = str(tmp_path)
+    monkeypatch.setattr(CORE, "cxx_build_flags", set())
+
+    platformio.write_cxx_flags_script()
+
+    content = (tmp_path / platformio.CXX_FLAGS_FILE_NAME).read_text()
+    assert "CXXFLAGS" not in content

tests/unit_tests/test_framework_helpers.py

@@ -26,6 +26,7 @@ from esphome.framework_helpers import (
     create_venv,
     download_from_mirrors,
     get_project_compile_flags,
+    get_project_cxx_compile_flags,
     get_project_link_flags,
     get_python_env_executable_path,
     get_system_python_path,
@@ -1048,3 +1049,25 @@ class TestGetProjectLinkFlags:
         ):
             result = get_project_link_flags()
             assert result == sorted(result)
+
+
+def _make_core_cxx(flags: set[str]) -> MagicMock:
+    core = MagicMock()
+    core.cxx_build_flags = flags
+    return core
+
+
+class TestGetProjectCxxCompileFlags:
+    def test_returns_sorted_flags(self) -> None:
+        with patch(
+            "esphome.core.CORE",
+            _make_core_cxx({"-Wno-volatile", "-Wno-deprecated"}),
+        ):
+            assert get_project_cxx_compile_flags() == [
+                "-Wno-deprecated",
+                "-Wno-volatile",
+            ]
+
+    def test_empty_flags(self) -> None:
+        with patch("esphome.core.CORE", _make_core_cxx(set())):
+            assert get_project_cxx_compile_flags() == []