c++: Suppress -Wreturn-type warnings for functions with failed assertions [PR91388]

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.

2025-12-19  Jakub Jelinek  <jakub@redhat.com>

	PR c++/91388
	* semantics.cc (finish_static_assert): Suppress -Wreturn-type warnings
	in functions with failed assertions.

	* g++.dg/cpp1z/static_assert1.C: New test.

diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
index e420fd4ebafb9a24eac289d9a92dbc87a4e1f3db..a2d655a60c1deb948fd07b9542ba6916640fe155 100644 (file)
--- a/gcc/cp/semantics.cc
+++ b/gcc/cp/semantics.cc
@@ -12927,6 +12927,16 @@ finish_static_assert (tree condition, tree message, location_t location,
            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)
        {

diff --git a/gcc/testsuite/g++.dg/cpp1z/static_assert1.C b/gcc/testsuite/g++.dg/cpp1z/static_assert1.C
new file mode 100644 (file)
index 0000000..3c94eb8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/static_assert1.C
@@ -0,0 +1,17 @@
+// PR c++/91388
+// 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> ();