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

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.

2026-07-23  Jakub Jelinek  <jakub@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.

Reviewed-by: Jason Merrill <jason@redhat.com>

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index fe2c417bed7ae49506a97a047743af7dc04b0931..b4db9fd2bae42663ae1f13de742b2e1b459b29c1 100644 (file)
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -4572,6 +4572,13 @@ cxx_eval_call_expression (const constexpr_ctx *ctx, tree t,
              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

diff --git a/gcc/testsuite/g++.dg/cpp1y/pr126354.C b/gcc/testsuite/g++.dg/cpp1y/pr126354.C
new file mode 100644 (file)
index 0000000..9ea9fb8
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/pr126354.C
@@ -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 ();

diff --git a/gcc/testsuite/g++.dg/cpp26/pr126354.C b/gcc/testsuite/g++.dg/cpp26/pr126354.C
new file mode 100644 (file)
index 0000000..3261df6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp26/pr126354.C
@@ -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" }
+}