[pushed 16] c++: constexpr class access in generic lambda [PR126754]

Tested x86_64-pc-linux-gnu, applying to 16 branch.

-- 8< --

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.
---
 gcc/cp/pt.cc                                   |  4 +++-
 gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C | 11 +++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 9fba230846a..0f52a84fb03 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 00000000000..1ad31f83fc6
--- /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);
+}

base-commit: 3cdb74f43678a97b01f1de0cfe9eb218d73460f7
-- 
2.55.0