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

esphome/build_gen/espidf.py

@@ -8,6 +8,17 @@ import esphome.config_validation as cv
 from esphome.core import CORE
 from esphome.helpers import mkdir_p, write_file_if_changed
 
+# Replaces the IDF default C++ standard (-std=gnu++2b appended to
+# CXX_COMPILE_OPTIONS by project.cmake's __build_init) with the one set via
+# cg.set_cpp_standard(). Emitted between include(project.cmake) and project(),
+# i.e. after IDF appends its default and before the options are consumed, and
+# applies project-wide like PlatformIO build_unflags.
+CPP_STANDARD_TEMPLATE = """\
+idf_build_get_property(esphome_cxx_compile_options CXX_COMPILE_OPTIONS)
+list(FILTER esphome_cxx_compile_options EXCLUDE REGEX "^-std=")
+list(APPEND esphome_cxx_compile_options "-std={standard}")
+idf_build_set_property(CXX_COMPILE_OPTIONS "${{esphome_cxx_compile_options}}")"""
+
 
 def get_available_components() -> list[str] | None:
     """Get list of built-in ESP-IDF components from project_description.json.
@@ -84,6 +95,12 @@ def get_project_cmakelists(minimal: bool = False) -> str:
         for flag in project_compile_opts
     )
 
+    cpp_standard_options = (
+        CPP_STANDARD_TEMPLATE.format(standard=CORE.cpp_standard)
+        if CORE.cpp_standard
+        else ""
+    )
+
     # Per-project list exposed as a CMake variable so converted PIO libs
     # can reference ${ESPHOME_PROJECT_MANAGED_COMPONENTS} without baking
     # project-specific names into their cached CMakeLists.
@@ -140,6 +157,8 @@ set(EXTRA_COMPONENT_DIRS ${{CMAKE_SOURCE_DIR}}/src)
 
 include($ENV{{IDF_PATH}}/tools/cmake/project.cmake)
 
+{cpp_standard_options}
+
 {extra_compile_options}
 
 {managed_components_property}
@@ -200,9 +219,6 @@ idf_component_register(
     REQUIRES ${{ESPHOME_PROJECT_BUILTIN_COMPONENTS}}
 )
 
-# Apply C++ standard
-target_compile_features(${{COMPONENT_LIB}} PUBLIC cxx_std_20)
-
 # ESPHome linker options
 target_link_options(${{COMPONENT_LIB}} PUBLIC
     {link_opts_str}

esphome/build_gen/platformio.py

@@ -33,12 +33,27 @@ def format_ini(data: dict[str, str | list[str]]) -> str:
     return content
 
 
+# All -std= variants a platform/framework may set by default, in both the GNU
+# and strict dialects; unflagged so the cg.set_cpp_standard() value is the
+# only standard left in the build.
+CPP_STD_VARIANTS = [
+    f"{prefix}{year}"
+    for year in ("11", "14", "17", "20", "23", "26", "2a", "2b", "2c")
+    for prefix in ("gnu++", "c++")
+]
+
+
 def get_ini_content():
     CORE.add_platformio_option(
         "lib_deps",
         [x.as_lib_dep for x in CORE.platformio_libraries.values()]
         + ["${common.lib_deps}"],
     )
+    if CORE.cpp_standard:
+        for variant in CPP_STD_VARIANTS:
+            if variant != CORE.cpp_standard:
+                CORE.add_build_unflag(f"-std={variant}")
+        CORE.add_build_flag(f"-std={CORE.cpp_standard}")
     # Sort to avoid changing build flags order
     CORE.add_platformio_option("build_flags", sorted(CORE.build_flags))
 

esphome/core/__init__.py

@@ -593,6 +593,8 @@ class EsphomeCore:
         self.build_flags: set[str] = set()
         # A set of build unflags to set in the platformio project
         self.build_unflags: set[str] = set()
+        # The C++ language standard for the build (e.g. "gnu++20"), set via cg.set_cpp_standard()
+        self.cpp_standard: str | None = None
         # A set of defines to set for the compile process in esphome/core/defines.h
         self.defines: set[Define] = set()
         # A map of all platformio options to apply
@@ -649,6 +651,7 @@ class EsphomeCore:
         self.platformio_libraries = {}
         self.build_flags = set()
         self.build_unflags = set()
+        self.cpp_standard = None
         self.defines = set()
         self.platformio_options = {}
         self.loaded_integrations = set()

esphome/cpp_generator.py

@@ -705,15 +705,8 @@ def add_build_unflag(build_unflag: str) -> None:
 
 
 def set_cpp_standard(standard: str) -> None:
-    """Set C++ standard with compiler flag `-std={standard}`."""
-    CORE.add_build_unflag("-std=gnu++11")
-    CORE.add_build_unflag("-std=gnu++14")
-    CORE.add_build_unflag("-std=gnu++17")
-    CORE.add_build_unflag("-std=gnu++23")
-    CORE.add_build_unflag("-std=gnu++2a")
-    CORE.add_build_unflag("-std=gnu++2b")
-    CORE.add_build_unflag("-std=gnu++2c")
-    CORE.add_build_flag(f"-std={standard}")
+    """Set the C++ language standard for the build (e.g. ``gnu++20``)."""
+    CORE.cpp_standard = standard
 
 
 def add_define(name: str, value: SafeExpType = None):