[gcc r17-3291] c++: constexpr class access in generic lambda [PR126754]

https://gcc.gnu.org/g:654eb1f70643a2c57b3c6a31c250c1213a23aa4f

commit r17-3291-g654eb1f70643a2c57b3c6a31c250c1213a23aa4f
Author: Jason Merrill <[email protected]>
Date:   Tue Aug 11 16:12:10 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.
    Let's not do the latter in a template.
    
            PR c++/126754
    
    gcc/cp/ChangeLog:
    
            * constexpr.cc (cxx_eval_outermost_constant_expr): Don't
            wrap in TARGET_EXPR when processing_template_decl.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1y/constexpr-lambda2.C: New test.

Diff:
---
 gcc/cp/constexpr.cc                            |  3 +++
 gcc/testsuite/g++.dg/cpp1y/constexpr-lambda2.C | 11 +++++++++++
 2 files changed, 14 insertions(+)

diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index e16c878d7258..c9ebc0027b7d 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -11288,6 +11288,9 @@ cxx_eval_outermost_constant_expr (tree t, bool allow_non_constant,
 	/* Don't add a TARGET_EXPR if our argument didn't have one.  */;
       else if (TREE_CODE (t) == TARGET_EXPR && TARGET_EXPR_CLEANUP (t))
 	r = get_target_expr (r);
+      else if (processing_template_decl)
+	/* Don't insert TARGET_EXPR in template trees.  */
+	return r;
       else
 	{
 	  r = get_target_expr (r, tf_warning_or_error | tf_no_cleanup);
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);
+}