[core] Make set_cpp_standard work on the native IDF toolchain (#16907)

tests/unit_tests/build_gen/test_espidf.py

@@ -162,3 +162,53 @@ def test_get_project_cmakelists_emits_managed_components_property(
                 "idf_build_set_property(ESPHOME_PROJECT_MANAGED_COMPONENTS"
                 " espressif__esp-dsp APPEND)"
             ) in content
+
+
+def test_get_project_cmakelists_replaces_cpp_standard(tmp_path: Path) -> None:
+    """cg.set_cpp_standard() replaces the IDF default -std in
+    CXX_COMPILE_OPTIONS 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", "gnu++20"),
+    ):
+        from esphome.build_gen.espidf import get_project_cmakelists
+
+        content = get_project_cmakelists(minimal=True)
+
+    assert (
+        "idf_build_get_property(esphome_cxx_compile_options CXX_COMPILE_OPTIONS)"
+        in content
+    )
+    assert 'list(FILTER esphome_cxx_compile_options EXCLUDE REGEX "^-std=")' in content
+    assert 'list(APPEND esphome_cxx_compile_options "-std=gnu++20")' in content
+    # The replacement must come after project.cmake (which appends the IDF
+    # default) and before project() (which consumes the options).
+    include_pos = content.index("tools/cmake/project.cmake")
+    replace_pos = content.index("CXX_COMPILE_OPTIONS")
+    project_pos = content.index("project(test)")
+    assert include_pos < replace_pos < project_pos
+
+
+def test_get_project_cmakelists_no_cpp_standard(tmp_path: Path) -> None:
+    with (
+        patch("esphome.build_gen.espidf.get_esp32_variant", return_value="ESP32"),
+        patch.object(CORE, "name", "test"),
+        patch.object(CORE, "cpp_standard", None),
+    ):
+        from esphome.build_gen.espidf import get_project_cmakelists
+
+        content = get_project_cmakelists(minimal=True)
+
+    assert "CXX_COMPILE_OPTIONS" not in content
+
+
+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."""
+    with patch.object(CORE, "build_flags", set()):
+        from esphome.build_gen.espidf import get_component_cmakelists
+
+        content = get_component_cmakelists()
+
+    assert "target_compile_features" not in content

tests/unit_tests/build_gen/test_platformio.py

@@ -160,3 +160,43 @@ def test_write_ini_no_change_when_content_same(
     call_args = mock_write_file_if_changed.call_args[0]
     assert call_args[0] == ini_file
     assert content in call_args[1]
+
+
+@pytest.fixture
+def clean_core(monkeypatch: pytest.MonkeyPatch) -> None:
+    monkeypatch.setattr(CORE, "name", "test")
+    monkeypatch.setattr(CORE, "platformio_options", {})
+    monkeypatch.setattr(CORE, "platformio_libraries", {})
+    monkeypatch.setattr(CORE, "build_flags", set())
+    monkeypatch.setattr(CORE, "build_unflags", set())
+
+
+def test_get_ini_content_pins_cpp_standard(
+    clean_core: None, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    """cg.set_cpp_standard() pins -std via build_flags and unflags every other
+    known standard so the platform/framework default is stripped."""
+    monkeypatch.setattr(CORE, "cpp_standard", "gnu++20")
+
+    content = platformio.get_ini_content()
+
+    flags_section = content.split("build_flags =")[1].split("build_unflags =")[0]
+    unflags_section = content.split("build_unflags =")[1].split("extra_scripts")[0]
+    assert "-std=gnu++20\n" in flags_section
+    # Both the GNU and strict dialects of every other standard are stripped.
+    for year in ("11", "14", "17", "23", "26", "2a", "2b", "2c"):
+        assert f"-std=gnu++{year}\n" in unflags_section
+        assert f"-std=c++{year}\n" in unflags_section
+    assert "-std=c++20\n" in unflags_section
+    # The selected standard must not unflag itself.
+    assert "-std=gnu++20\n" not in unflags_section
+
+
+def test_get_ini_content_no_cpp_standard(
+    clean_core: None, monkeypatch: pytest.MonkeyPatch
+) -> None:
+    monkeypatch.setattr(CORE, "cpp_standard", None)
+
+    content = platformio.get_ini_content()
+
+    assert "-std=" not in content