[gcc r17-2664] c++: Diagnose return from [[noreturn]] function during constant evaluation [PR126354]

https://gcc.gnu.org/g:571aae1dc596ec58bca11534800a857109dce498

commit r17-2664-g571aae1dc596ec58bca11534800a857109dce498
Author: Jakub Jelinek <[email protected]>
Date:   Thu Jul 23 15:42:10 2026 +0200

    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  <[email protected]>
    
            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 <[email protected]>

Diff:
---
 gcc/cp/constexpr.cc                   |  7 +++++++
 gcc/testsuite/g++.dg/cpp1y/pr126354.C | 18 ++++++++++++++++++
 gcc/testsuite/g++.dg/cpp26/pr126354.C | 28 ++++++++++++++++++++++++++++
 3 files changed, 53 insertions(+)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index fe2c417bed7a..b4db9fd2bae4 100644
--- 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
index 000000000000..9ea9fb837663
--- /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
index 000000000000..3261df675628
--- /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" }
+}