Hi!
This is something Jonathan has asked for recently. E.g. in the recent
libstdc++ r16-6177 random.tcc changes, there was
if constexpr (__d <= 32)
return __generate_canonical_any<_RealT, uint64_t, __d>(__urng);
else
{
#if defined(__SIZEOF_INT128__)
static_assert(__d <= 64,
"irregular RNG with float precision >64 is not supported");
return __generate_canonical_any<
_RealT, unsigned __int128, __d>(__urng);
#else
static_assert(false, "irregular RNG with float precision"
" >32 requires __int128 support");
#endif
}
and when we hit there the static_assert, we don't get just an error about
that, but also a -Wreturn-type warning in the same function because that
path falls through to the end of function without returning a value.
But a function with a failed static_assert is erroneous and will never
fall through to the end. We could treat failed static_assert in functions
as __builtin_unreachable (), but I think it doesn't matter where exactly
in a function static_assert(false); appears, so this patch just suppresses
-Wreturn-type warning in that function instead.
Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
2025-12-19 Jakub Jelinek <[email protected]>
* semantics.cc (finish_static_assert): Suppress -Wreturn-type warnings
in functions with failed assertions.
* g++.dg/cpp1z/static_assert1.C: New test.
--- gcc/cp/semantics.cc.jj 2025-12-13 09:54:13.142393769 +0100
+++ gcc/cp/semantics.cc 2025-12-18 17:59:15.961112063 +0100
@@ -12927,6 +12927,16 @@ finish_static_assert (tree condition, tr
error_at (cloc, "static assertion failed: %.*s", len, msg);
diagnose_failing_condition (bad, cloc, show_expr_p);
+
+ /* Suppress -Wreturn-type for functions with failed static_asserts.
+ Otherwise templates like:
+ if constexpr (whatever)
+ return something (args);
+ else
+ static_assert (false, "explanation");
+ get a useless extra -Wreturn-type warning. */
+ if (current_function_decl)
+ suppress_warning (current_function_decl, OPT_Wreturn_type);
}
else if (condition && condition != error_mark_node)
{
--- gcc/testsuite/g++.dg/cpp1z/static_assert1.C.jj 2025-12-18
18:03:17.867763452 +0100
+++ gcc/testsuite/g++.dg/cpp1z/static_assert1.C 2025-12-18 18:03:00.642073110
+0100
@@ -0,0 +1,16 @@
+// Make sure we don't emit -Wreturn-type in functions with failed
static_asserts.
+// { dg-do compile { target c++17 } }
+
+template <int N>
+int
+foo ()
+{
+ if constexpr (N <= 42)
+ return N;
+ else
+ static_assert (false, "too high N"); // { dg-error "too high N" }
+} // { dg-bogus "no return
statement in function returning non-void" }
+
+int a = foo <0> ();
+int b = foo <42> ();
+int c = foo <100> ();
Jakub
