C++20 toolchain

- Define the pw_strict_host_clang_size_optimized_cpp20 toolchain that
  compiles with C++20.
- Create the cpp20_compatibility group and add it to the
  gn_full_build_check presubmit check.

Bug: 683
Change-Id: I732fb65b0588b2a84899e817015c438edf68d72e
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/96442
Pigweed-Auto-Submit: Wyatt Hepler <hepler@google.com>
Reviewed-by: Ted Pudlik <tpudlik@google.com>
Commit-Queue: Wyatt Hepler <hepler@google.com>

BUILD.gn

@@ -319,6 +319,17 @@ group("cpp14_compatibility") {
   ]
 }
 
+# Build Pigweed with -std=c++20 to ensure compatibility. Compile with
+# optimizations since the compiler tends to catch more errors with optimizations
+# enabled than without.
+group("cpp20_compatibility") {
+  _cpp20_tc = "$_internal_toolchains:pw_strict_host_clang_size_optimized_cpp20"
+  deps = [
+    ":apps($_cpp20_tc)",
+    ":pw_module_tests.run($_cpp20_tc)",
+  ]
+}
+
 # The default toolchain is not used for compiling C/C++ code.
 if (current_toolchain != default_toolchain) {
   group("apps") {
@@ -335,7 +346,7 @@ if (current_toolchain != default_toolchain) {
     }
   }
 
-  # All Pigweed modules that can be built using gn. This is not built by default.
+  # All Pigweed modules that can be built using gn. Not built by default.
   group("pw_modules") {
     deps = pw_modules
   }

pw_build/BUILD.gn

@@ -127,6 +127,13 @@ config("cpp17") {
   ]
 }
 
+config("cpp20") {
+  cflags_cc = [
+    "-std=c++20",
+    "-Wno-register",
+  ]
+}
+
 # Removes system-dependent prefixes from macros like __FILE__ and debug symbols.
 config("relative_paths") {
   _transformations = [

pw_presubmit/py/pw_presubmit/pigweed_presubmit.py

@@ -110,9 +110,10 @@ def gn_full_build_check(ctx: PresubmitContext) -> None:
         'pw_env_setup:build_pigweed_python_source_tree',
     ]
 
-    # TODO(b/234645359): Re-enable on Windows when cpp14_compatibility builds.
+    # TODO(b/234645359): Re-enable on Windows when compatibility tests build.
     if sys.platform != 'win32':
         build_targets.append('cpp14_compatibility')
+        build_targets.append('cpp20_compatibility')
 
     build.gn_gen(ctx.root, ctx.output_dir)
     build.ninja(ctx.output_dir, *build_targets)

targets/host/target_docs.rst

@@ -49,11 +49,15 @@ downstream projects.
 
 Toolchains for other C++ standards
 ==================================
-Pigweed code requires C++17. A few modules, such as ``pw_tokenizer``, work with
-C++14. To facilitate testing these modules with C++14, Pigweed uses the
-``pw_strict_host_clang_debug_cpp14`` toolchain. This toolchain is only permitted
-for use in upstream pigweed, but downstream users may create a similar toolchain
-if needed.
+Most Pigweed code requires C++17, but a few modules, such as ``pw_tokenizer``,
+work with C++14. All Pigweed code is compatible with C++20. Pigweed defines
+toolchains for testing with C++14 and C++20.
+
+* ``pw_strict_host_clang_debug_cpp14`` -- Builds with ``-std=c++14``.
+* ``pw_strict_host_clang_size_optimized_cpp20`` -- Builds with ``-std=c++20``.
+
+These toolchains are only permitted for use in upstream pigweed, but downstream
+users may create similar toolchains as needed.
 
 --------
 Building

targets/host/target_toolchains.gni

@@ -394,4 +394,19 @@ pw_internal_host_toolchains = [
       pw_unit_test_MAIN = "$dir_pw_unit_test:printf_main"
     }
   },
+  {
+    name = "pw_strict_host_clang_size_optimized_cpp20"
+    _toolchain_base = pw_toolchain_host_clang.size_optimized
+    forward_variables_from(_toolchain_base, "*", _excluded_members)
+    defaults = {
+      forward_variables_from(_toolchain_base.defaults, "*")
+      forward_variables_from(_host_common, "*")
+      forward_variables_from(_pigweed_internal, "*")
+      forward_variables_from(_os_specific_config, "*")
+
+      remove_default_configs = [ "$dir_pw_build:cpp17" ]
+      default_configs += _clang_default_configs
+      default_configs += [ "$dir_pw_build:cpp20" ]
+    }
+  },
 ]