change(build): add -Werror=return-type to default C and C++ flags (#10216)

Author: igrr

Diff for: platform.txt

@@ -46,10 +46,14 @@ compiler.warning_flags.default=
 compiler.warning_flags.more=-Wall
 compiler.warning_flags.all=-Wall -Wextra
 
+# Additional flags specific to Arduino (not based on IDF flags).
+# Update tools/platformio-build.py when changing these flags.
+compiler.common_werror_flags=-Werror=return-type
+
 # Compile Flags
 compiler.cpreprocessor.flags="@{compiler.sdk.path}/flags/defines" "-I{build.source.path}" -iprefix "{compiler.sdk.path}/include/" "@{compiler.sdk.path}/flags/includes" "-I{compiler.sdk.path}/{build.memory_type}/include"
-compiler.c.flags="@{compiler.sdk.path}/flags/c_flags" {compiler.warning_flags} {compiler.optimization_flags}
-compiler.cpp.flags="@{compiler.sdk.path}/flags/cpp_flags" {compiler.warning_flags} {compiler.optimization_flags}
+compiler.c.flags="@{compiler.sdk.path}/flags/c_flags" {compiler.warning_flags} {compiler.optimization_flags} {compiler.common_werror_flags}
+compiler.cpp.flags="@{compiler.sdk.path}/flags/cpp_flags" {compiler.warning_flags} {compiler.optimization_flags} {compiler.common_werror_flags}
 compiler.S.flags="@{compiler.sdk.path}/flags/S_flags" {compiler.warning_flags} {compiler.optimization_flags}
 compiler.c.elf.flags="@{compiler.sdk.path}/flags/ld_flags" "@{compiler.sdk.path}/flags/ld_scripts"
 compiler.c.elf.libs="@{compiler.sdk.path}/flags/ld_libs"

Diff for: tools/platformio-build.py

@@ -164,6 +164,15 @@ def add_tinyuf2_extra_image():
     )
 )
 
+#
+# Additional flags specific to Arduino core (not based on IDF)
+#
+
+env.Append(
+    CFLAGS=["-Werror=return-type"],
+    CXXFLAGS=["-Werror=return-type"],
+)
+
 #
 # Target: Build Core Library
 #