[PATCH] c++: Diagnose return from [[noreturn]] function during constant evaluation [PR126354]

[Jakub Jelinek]

Hi!

This PR complains about g++ not diagnosing return from [[noreturn]]
function (which is UB) during constant evaluation.

This patch diagnoses that.
I haven't included the [[noreturn]] function name in the diagnostics
as it is printed in the context:
/home/jakub/src/gcc/gcc/testsuite/g++.dg/cpp1y/pr126354.C:18:24:   in 
'constexpr' expansion of 'bar()'
/home/jakub/src/gcc/gcc/testsuite/g++.dg/cpp1y/pr126354.C:15:7:   in 
'constexpr' expansion of 'foo(false)'
/home/jakub/src/gcc/gcc/testsuite/g++.dg/cpp1y/pr126354.C:15:7: error: 
'[[noreturn]]' call returns
But if you think it is better to emit
error ("%<[[noreturn]]%> %qD call returns", fun);
instead, I can certainly do that.

Bootstrapped/regtested on x86_64-linux and i686-linux, ok for trunk?
Or with the above mentioned change?

2026-07-23  Jakub Jelinek  <ja...@redhat.com>

        PR c++/126354
        * constexpr.cc (cxx_eval_call_expression): Diagnose return from
        [[noreturn]] function.

        * g++.dg/cpp1y/pr126354.C: New test.
        * g++.dg/cpp26/pr126354.C: New test.

--- gcc/cp/constexpr.cc.jj      2026-07-22 17:07:06.558244058 +0200
+++ gcc/cp/constexpr.cc 2026-07-22 18:17:43.577203062 +0200
@@ -4606,6 +4606,13 @@ cxx_eval_call_expression (const constexp
              cacheable = false;
              *jump_target = jmp_target;
            }
+         else if (!*non_constant_p && TREE_THIS_VOLATILE (fun))
+           {
+             /* Return from a [[noreturn]] function.  */
+             if (!ctx->quiet)
+               error ("%<[[noreturn]]%> call returns");
+             *non_constant_p = true;
+           }
          else if (DECL_CONSTRUCTOR_P (fun))
            /* This can be null for a subobject constructor call, in
               which case what we care about is the initialization
--- gcc/testsuite/g++.dg/cpp1y/pr126354.C.jj    2026-07-22 18:25:26.663075467 
+0200
+++ gcc/testsuite/g++.dg/cpp1y/pr126354.C       2026-07-22 18:25:01.211412160 
+0200
@@ -0,0 +1,18 @@
+// PR c++/126354
+// { dg-do compile { target c++14 } }
+
+[[noreturn]] constexpr void
+foo (bool x)
+{
+  if (x)
+    for (;;)
+      ;
+}
+
+constexpr bool
+bar ()
+{
+  foo (false);         // { dg-error "'\\\[\\\[noreturn\\\]\\\]' call returns" 
}
+}
+
+constexpr bool a = bar ();
--- gcc/testsuite/g++.dg/cpp26/pr126354.C.jj    2026-07-22 18:23:09.457890533 
+0200
+++ gcc/testsuite/g++.dg/cpp26/pr126354.C       2026-07-22 18:22:41.383261931 
+0200
@@ -0,0 +1,28 @@
+// PR c++/126354
+// { dg-do compile { target c++26 } }
+
+[[noreturn]] constexpr void
+foo (bool x)
+{
+  if (x)
+    throw 42;
+}
+
+consteval {
+  try
+    {
+      foo (true);
+      throw nullptr;
+    }
+  catch (int x)
+    {
+      if (x != 42)
+       throw nullptr;
+      return;
+    }
+  throw nullptr;
+}
+
+consteval {
+  foo (false);         // { dg-error "'\\\[\\\[noreturn\\\]\\\]' call returns" 
}
+}

        Jakub