[GNATS 29059] Lifetime extension #90

Description

​From: 'Momin Arafa [C]' via support support@edg.com
​To: EDG support
​Cc: Evgeny Abramovich, Ayman Hanna, Mariam Flayyan [C]
​Subject: RE: Possible issue in EDG: Temporary lifetime not extended when reference is bound to a subobject via member access and subscript [EDGcpfe/29032]
​Date: Thursday, September 17, 2026 at 7:41:40 AM
​
​Hi EDG Support,
​
​Following up on EDGcpfe/29032 � we are still seeing a lifetime extension issue with a simpler case not covered by that patch.
​
​Consider:
​int d;
​struct A { int b; ~A() { ++d; } };
​int main() {
​ const int &r = A().b;
​ assert(d == 0); // fails � destructor already called
​}
​
​GCC and Clang correctly extend the lifetime of A() through r. EDG instead copies the value of A().b into a fresh int temporary and binds r to that, then immediately destroys A() at the end of the full-expression.
​
​Best regards,
​Momin Arafa

Notes

​daveed, 2026-09-22:
​This is a sequel to EDGcpfe/29032, which I'm moving to a new entry because we want to make a release.
​I had the fix below, but it introduces regressions, including in Clang's test suite (SemaCXX/constant-expression-cxx2a.cpp). The cmd_line.c change is correct (i.e., it's pretty clear that those versions of Clang and GCC added support for CWG 616), but there is something off with the rest of the change.
​
​
​lin2:~/Dev/cpfe/src$ git diff e7d793320efe920c57c7d1821e1846944f36bf45
​diff --git a/src/cmd_line.c b/src/cmd_line.c
​index 2e8b276058..fa48a6e010 100644
​--- a/src/cmd_line.c
​+++ b/src/cmd_line.c
​@@ -4319,9 +4319,7 @@ default mode (e.g., exception handling).
​ if (!option_kind_used[(int)optk_digit_separators]) {
​ digit_separators_enabled = TRUE;
​ } /* if /
​- if (rvalue_references_enabled &&
​- !gpp_version_is(<90000) && !clangcpp_version_is(<40000)) {
​- / A field selection on a class prvalue is an xvalue (CWG 616). /
​+ if (rvalue_references_enabled && !gpp_mode && !clang_mode) {
​ selection_from_prvalue_is_xvalue = TRUE;
​ } / if /
​ relaxed_constexpr_enabled = TRUE;
​diff --git a/src/exprutil.c b/src/exprutil.c
​index 83453da56f..9b2fa13a58 100644
​--- a/src/exprutil.c
​+++ b/src/exprutil.c
​@@ -25273,10 +25273,6 @@ is_temp can be NULL if that information is not needed.
​ clear_expr_or_stmt_traversal_block(&tblock);
​ tblock.process_expr = examine_expr_for_auto_object;
​ tblock.follow_addressing_path = TRUE;
​- / A prvalue.field xvalue (CWG 616) names a subobject of the class
​- prvalue, so the class object must be considered as well. */
​- tblock.follow_class_rvalue_addressing_path =
​- selection_from_prvalue_is_xvalue;
​ traverse_expr(expr, &tblock);
​ is_auto_object = tblock.result;
​ if (is_temp != NULL) *is_temp = tblock.is_temp;
​@@ -25327,10 +25323,6 @@ underlying entity is a temporary, return is_temp set to TRUE.
​ clear_expr_or_stmt_traversal_block(&tblock);
​ tblock.process_expr = examine_expr_for_auto_object;
​ tblock.follow_addressing_path = TRUE;
​- / A pointer into a prvalue.field xvalue (CWG 616) addresses a
​- subobject of the class prvalue. */
​- tblock.follow_class_rvalue_addressing_path =
​- selection_from_prvalue_is_xvalue;
​ traverse_expr(expr, &tblock);
​ is_addr_of_auto = tblock.result;
​ *is_temp = tblock.is_temp;