[gcc r16-4113] c++: implicit 'this' in generic lambda [PR122048]

[Jason Merrill]

https://gcc.gnu.org/g:90ad957406104f9cc04365f6b071521a7c013dda

commit r16-4113-g90ad957406104f9cc04365f6b071521a7c013dda
Author: Jason Merrill <jason@redhat.com>
Date:   Wed Sep 24 22:31:37 2025 +0100

    c++: implicit 'this' in generic lambda [PR122048]
    
    Here template substitution was replacing a reference to the 'this' of X::f
    with the implicit closure parameter of the operator(), which is wrong.  The
    closure parameter is never a suitable replacement for a 'this' parameter.
    
            PR c++/122048
    
    gcc/cp/ChangeLog:
    
            * pt.cc (tsubst_expr): Don't use a lambda current_class_ptr.
    
    gcc/testsuite/ChangeLog:
    
            * g++.dg/cpp1y/lambda-generic-this6.C: New test.

Diff:
---
 gcc/cp/pt.cc                                      | 3 ++-
 gcc/testsuite/g++.dg/cpp1y/lambda-generic-this6.C | 8 ++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 06287f0b0452..96ead4f1b554 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -22330,7 +22330,8 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)
 	  if (r == NULL_TREE && TREE_CODE (t) == PARM_DECL)
 	    {
 	      /* We get here for a use of 'this' in an NSDMI.  */
-	      if (DECL_NAME (t) == this_identifier && current_class_ptr)
+	      if (DECL_NAME (t) == this_identifier && current_class_ptr
+		  && !LAMBDA_TYPE_P (TREE_TYPE (TREE_TYPE (current_class_ptr))))
 		RETURN (current_class_ptr);
 
 	      /* Parameters of non-templates map to themselves (e.g. in
diff --git a/gcc/testsuite/g++.dg/cpp1y/lambda-generic-this6.C b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-this6.C
new file mode 100644
index 000000000000..8bbff1d9fd25
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/lambda-generic-this6.C
@@ -0,0 +1,8 @@
+// PR c++/122048
+// { dg-do compile { target c++14 } }
+
+class X {
+  void f();
+  int i;
+};
+void X::f() {[&](auto) {sizeof i;}(1);}