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b22c76c625
compiler-opts.m4's check for `MOZ_CXX_SUPPORTS_WARNING(-W, unreachable-code-return, ac_cxx_has_wunreachable_code_return)` is broken. The C/C++ code that configure emits for MOZ_CXX_SUPPORTS_WARNING() actually contains a -Wunreachable-code-return warning and, thus, doesn't actually detect that clang supports the -Wunreachable-code-return flag.
This configure code in MOZ_CXX_SUPPORTS_WARNING():
AC_TRY_COMPILE([],
[return(0);],
$3="yes",
$3="no")
generates something like:
int main() {
return(0);
; return 0; }
where the second return, automatically emitted by configure, is unreachable and causes a -Wunreachable-code-return warning.
The fix is to remove the redundant return(0) from MOZ_CXX_SUPPORTS_WARNING(). This allows clang's -Wunreachable-code-return flag to be detected, but then -Wunreachable-code-return breaks other configure checks, including third-party libraries' configure checks (in particular jemalloc) that also have redundant `return(0)`. So all the third-party libraries' configure checks would need to be fixed upstream, which seems like more hassle than the value of the -Wunreachable-code-return warnings.