[gcc r16-9536] c++: constexpr class access in generic lambda [PR126754]

https://gcc.gnu.org/g:1ef50505862d6702e95e76b3ce2c5ed3a93610cb

commit r16-9536-g1ef50505862d6702e95e76b3ce2c5ed3a93610cb
Author: Jason Merrill <[email protected]>
Date:   Fri Aug 14 09:37:39 2026 -0400

    c++: constexpr class access in generic lambda [PR126754]
    
    We don't want to see TARGET_EXPR in template trees because it's an
    implementation detail, not a representation of the source.  But constant
    evaluation normally represents a constant class value with a TARGET_EXPR.
    For GCC 16 let's duplicate the default case handling.
    
            PR c++/126754
    
    gcc/cp/ChangeLog:
    
            * pt.cc (tsubst_expr) <case TARGET_EXPR>: Only unreachable
            when checking.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1y/constexpr-lambda2.C: New test.

Diff:
---
 gcc/cp/pt.cc                                   |  4 +++-
 gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C | 11 +++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 9fba230846af..0f52a84fb036 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -23187,7 +23187,9 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
     case TARGET_EXPR:
       /* TARGET_EXPR represents temporary objects and should not appear in
 	 templated trees.  */
-      gcc_unreachable ();
+      if (flag_checking)
+	gcc_unreachable ();
+      RETURN (t);
 
     case OFFSET_REF:
       {
diff --git a/gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C b/gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C
new file mode 100644
index 000000000000..1ad31f83fc66
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C
@@ -0,0 +1,11 @@
+// PR c++/126754
+// { dg-do compile { target c++14 } }
+
+struct S { int i; };
+int main() {
+  constexpr S a{1};
+  const auto f = [](auto b) {
+    return 1 - a.i;  // this expression must be independent of b in order to trigger the crash
+  };
+  return f(0);
+}