Re: [PATCH 2/3] c++: Clear lambda scope for unattached member template lambdas

[Jason Merrill]

On 1/31/25 8:45 AM, Nathaniel Shead wrote:

Bootstrapped and regtested on x86_64-pc-linux-gnu, OK for trunk?

OK.

-- >8 --

In r15-7202 we made lambdas between a template parameter scope and a
class/function/initializer be considered TU-local, in lieu of working
out how to mangle them to the succeeding declaration.

I neglected to clear any existing mangling on the template declaration
however; this means that such lambdas can occasionally get a lambda
scope, and will in general inherit the lambda scope of their
instantiation context (whatever that might be).

This patch ensures that the scope is cleared on the template declaration
as well.

gcc/cp/ChangeLog:

        * lambda.cc (record_lambda_scope): Clear mangling scope for
        otherwise unattached lambdas in class member templates.

gcc/testsuite/ChangeLog:

        * g++.dg/cpp2a/lambda-uneval22.C: Add check that the primary
        specialisation of the lambda is TU-local.

Signed-off-by: Nathaniel Shead <nathanielosh...@gmail.com>
---
  gcc/cp/lambda.cc                             | 11 +++++++++++
  gcc/testsuite/g++.dg/cpp2a/lambda-uneval22.C |  3 ++-
  2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/lambda.cc b/gcc/cp/lambda.cc
index 5593636eaf8..73cf816b6e1 100644
--- a/gcc/cp/lambda.cc
+++ b/gcc/cp/lambda.cc
@@ -1575,6 +1575,17 @@ record_lambda_scope (tree lambda)
        }
      }

+ /* An otherwise unattached class-scope lambda in a member template

+     should not have a mangling scope, as the mangling scope will not
+     correctly inherit on instantiation.  */
+  tree ctx = TYPE_CONTEXT (closure);
+  if (scope
+      && ctx
+      && CLASS_TYPE_P (ctx)
+      && ctx == TREE_TYPE (scope)
+      && current_template_depth > template_class_depth (ctx))
+    scope = NULL_TREE;
+
    LAMBDA_EXPR_EXTRA_SCOPE (lambda) = scope;
    if (scope)
      maybe_key_decl (scope, TYPE_NAME (closure));
diff --git a/gcc/testsuite/g++.dg/cpp2a/lambda-uneval22.C 
b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval22.C
index 9c0e8128f10..1a25a0255fc 100644
--- a/gcc/testsuite/g++.dg/cpp2a/lambda-uneval22.C
+++ b/gcc/testsuite/g++.dg/cpp2a/lambda-uneval22.C
@@ -5,7 +5,7 @@ struct S {
    using T = decltype([]{ return I; });

template <int I>

-  decltype([]{ return I; }) f() { return {}; }
+  decltype([]{ return I; }) f();  // { dg-error "declared using local type" }
  };

void a(S::T<0>*); // { dg-error "declared using local type" }

@@ -18,4 +18,5 @@ int main() {
    b(nullptr);
    c(nullptr);
    d(nullptr);
+  S{}.f<2>()();
  }