"missing return statement" warning when a template ends in a dependent call to a [[noreturn]] function #255

Severity: warning noise (no build failure).
Component: check_void_return_okay in src/statements.c
(ec_implicit_return_from_non_void_function).

Reproducer

//options_all:--gnu_version=150000 --diag_error=implicit_return_from_non_void_function
[[noreturn]] void fail(int);
template <class T> struct R {
  T v; bool ok;
  T &get() { if (ok) return v; else fail(v); }
};
int &use(R<int> &r) { return r.get(); }

Actual (without --diag_error):

"v2.cpp", line 5: warning: missing return statement at end of non-void function "R<T>::get"

Expected: no diagnostic. g++ 15 (-Wall -Wextra) is silent: GCC checks
-Wreturn-type on the instantiation, where fail(v) is known to be
[[noreturn]].

Analysis

The warning is issued when the template definition is parsed. At that point
fail(v) is a dependent call whose target isn't known, so the end of the
function looks reachable. It is not reconsidered when R<int>::get is
instantiated, where the call resolves to the [[noreturn]] fail(int).

Variants tried (all with --gnu_version=150301 --c++17):

A possible fix: when the function is a template (prototype instantiation)
and the trailing statement on the reachable path is a call that is still
dependent, defer the implicit-return check to instantiation (or suppress it),
much as the constexpr case above it already defers to instantiation.

Impact on Boost

boost/system/result.hpp, result<T, E>::value() (and its const&, &&
overloads):

BOOST_CXX14_CONSTEXPR T& value( boost::source_location const& loc = BOOST_CURRENT_LOCATION ) &
{
    if( has_value() )
    {
        return variant2::unsafe_get<0>( v_ );
    }
    else
    {
        throw_exception_from_error( variant2::unsafe_get<1>( v_ ), loc );
    }
}

throw_exception_from_error is overloaded and BOOST_NORETURN (lines 38-68).
Every translation unit that includes boost/system/result.hpp gets these
warnings. That covers all of Boost.URL and Boost.JSON, plus anything else that
uses boost::system::result. See the README for counts from the full build.