On Sat, Dec 13, 2025 at 12:31:28PM -0500, Patrick Palka wrote:
> On Sat, 13 Dec 2025, Patrick Palka wrote:
>
> > On Sat, 13 Dec 2025, Jason Merrill wrote:
> >
> > > On 12/13/25 7:50 PM, Nathaniel Shead wrote:
> > > > Bootstrapped and regtested on x86_64-pc-linux-gnu (so far just dg.exp
> > > > and modules.exp), OK for trunk if full regtest succeeds?
> > > >
> > > > -- >8 --
> > > >
> > > > When evaluating a concept definition in a template, any lambdas in the
> > > > definition of the concept get instantiated in the context of where the
> > > > evaluation occurred.
> > > >
> > > > This causes two issues:
> > > >
> > > > - Any lambdas declared later in the body of the function get the wrong
> > > > discriminator, which causes ABI divergences with Clang.
> > > >
> > > > - Modules streaming gets confused, because the lambda is keyed to an
> > > > unrelated declaration. Keying the lambda to the concept also doesn't
> > > > work because we'd really want to key it to a concept instantiation
> > > > (that doesn't exist) so that merging works correctly.
> > > >
> > > > I think really we just want to throw away these lambdas declarations
> > > > after evaluating the concept. They can (and will) be recreated in
> > > > importers re-evaluating the concept with the given args regardless.
> > > >
> > > > This patch implements this by disabling scope recording for an
> > > > instantiation of a lambda keyed to a concept, and ensuring that the
> > > > lambda tag is added to an unrelated block that is then thrown away.
> > >
> > > Would it make sense to just push_to(/pop_from)_top_level in
> > > evaluate_concept_check? This seems like another instance of the recurring
> > > problem of not pushing out of a local scope sufficiently before handling a
> > > template.
> >
> > This is related to PR104111. Some downsides of going this route:
> >
> > template<class T> requires C<T> || D<T>
> > void f() {
> > if constexpr (C<T>) // potentially IFNDR if evaluation of C<T>
> > // depends on access context of f (though
> > // in practice we'll just reuse the cached
> > // value obtained earlier during satisfaction
> > // with the right access context)
> > ...
> > else
> > ...
> > }
> >
> > --
> >
> > template<class T> requires (!C<T>) // C<T> is not checked in
> > // access context of g
> > void g();
> >
> >
> > To me it seems that evaluating a concept-id in the access context
> > of where the concept-id appears is the better choice once we extend
> > the satisfaction cache to consider access context (which it currently
> > doesn't). Doing push_to_top_level would mean the above two testcases
> > could never work "as expected" even after we fix the satisfaction cache.
>
> Oops, perhaps you didn't mean to just do push_to_top_level. If
> we do push_to_top_level followed by push_access_scope to restore
> the previous access context perhaps this wouldn't have an effect
> on the PR104111 testcases.
>
Thanks for the comments. I think that makes sense to me; so something
like the following perhaps?
-- >8 --
When evaluating a concept definition in a template, any lambdas in the
definition of the concept get instantiated in the context of where the
evaluation occurred.
This causes two issues:
- Any lambdas declared later in the body of the function get the wrong
discriminator, which causes ABI divergences with Clang.
- Modules streaming gets confused, because the lambda is keyed to an
unrelated declaration. Keying the lambda to the concept also doesn't
work because we'd really want to key it to a concept instantiation
(that doesn't exist) so that merging works correctly.
I think really we just want to throw away these lambdas declarations
after evaluating the concept. They can (and will) be recreated in
importers re-evaluating the concept with the given args regardless.
This patch implements this by disabling scope recording for an
instantiation of a lambda keyed to a concept, and pushing into an
unrelated context so that the lambda's type is not mistakenly added
into the scope it was instantiated from.
PR c++/123075
gcc/cp/ChangeLog:
* constraint.cc (evaluate_concept_check): Push to an unrelated
scope, but keep the same access context.
* pt.cc (tsubst_lambda_expr): Don't record lambda scopes for
lambdas attached to a concept.
gcc/testsuite/ChangeLog:
* g++.dg/cpp2a/concepts-lambda25.C: New test.
* g++.dg/modules/lambda-13.h: New test.
* g++.dg/modules/lambda-13_a.H: New test.
* g++.dg/modules/lambda-13_b.C: New test.
Signed-off-by: Nathaniel Shead <[email protected]>
Reviewed-by: Jason Merrill <[email protected]>
Reviewed-by: Patrick Palka <[email protected]>
---
gcc/cp/constraint.cc | 20 ++++++++++++-
gcc/cp/pt.cc | 7 ++++-
.../g++.dg/cpp2a/concepts-lambda25.C | 28 +++++++++++++++++++
gcc/testsuite/g++.dg/modules/lambda-13.h | 22 +++++++++++++++
gcc/testsuite/g++.dg/modules/lambda-13_a.H | 6 ++++
gcc/testsuite/g++.dg/modules/lambda-13_b.C | 6 ++++
6 files changed, 87 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp2a/concepts-lambda25.C
create mode 100644 gcc/testsuite/g++.dg/modules/lambda-13.h
create mode 100644 gcc/testsuite/g++.dg/modules/lambda-13_a.H
create mode 100644 gcc/testsuite/g++.dg/modules/lambda-13_b.C
diff --git a/gcc/cp/constraint.cc b/gcc/cp/constraint.cc
index 6abd0966fcd..92a3a780008 100644
--- a/gcc/cp/constraint.cc
+++ b/gcc/cp/constraint.cc
@@ -2860,9 +2860,27 @@ evaluate_concept_check (tree check)
gcc_assert (concept_check_p (check));
+ /* We don't want any declarations instantiated from a concept evaluation
+ to enter the binding table for the current scope, such as lambdas, so
+ leave that scope. But maintain the access context (PR104111). */
+ tree scope = current_scope ();
+ if (CLASS_TYPE_P (scope))
+ scope = TYPE_MAIN_DECL (scope);
+ else if (TREE_CODE (scope) != FUNCTION_DECL)
+ scope = NULL_TREE;
+
+ push_to_top_level ();
+ if (scope)
+ push_access_scope (scope);
+
/* Check for satisfaction without diagnostics. */
sat_info quiet (tf_none, NULL_TREE);
- return constraint_satisfaction_value (check, /*args=*/NULL_TREE, quiet);
+ tree r = constraint_satisfaction_value (check, /*args=*/NULL_TREE, quiet);
+
+ if (scope)
+ pop_access_scope (scope);
+ pop_from_top_level ();
+ return r;
}
/* Evaluate the requires-expression T, returning either boolean_true_node
diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc
index 341e5ab8808..b24e646cc29 100644
--- a/gcc/cp/pt.cc
+++ b/gcc/cp/pt.cc
@@ -20589,7 +20589,12 @@ tsubst_lambda_expr (tree t, tree args, tsubst_flags_t
complain, tree in_decl)
return error_mark_node;
}
- if (LAMBDA_EXPR_EXTRA_SCOPE (t))
+ if (LAMBDA_EXPR_EXTRA_SCOPE (t)
+ /* When evaluating a concept we instantiate any lambda bodies
+ in the context of the evaluation. For ABI reasons don't
+ record a scope for this instantiated lambda so we don't
+ throw off the scope counter. */
+ && TREE_CODE (LAMBDA_EXPR_EXTRA_SCOPE (t)) != CONCEPT_DECL)
record_lambda_scope (r);
if (TYPE_NAMESPACE_SCOPE_P (TREE_TYPE (t)))
/* If we're pushed into another scope (PR105652), fix it. */
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-lambda25.C
b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda25.C
new file mode 100644
index 00000000000..188a52c7fd9
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda25.C
@@ -0,0 +1,28 @@
+// PR c++/123075
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fkeep-inline-functions" }
+
+template <typename T>
+concept r = []{ return true; }();
+
+template <typename T, typename U>
+inline auto foo() {
+ static_assert(r<T>);
+ r<U>;
+ return []{ return false; };
+}
+
+template <typename T>
+struct S {
+ static_assert(r<T>);
+ decltype([]{ return true; }) l;
+};
+S<char> s;
+
+bool use = (foo<int, double>()() || s.l());
+
+// There should only be one lambda keyed to 'foo()' and 'S::l'
+// { dg-final { scan-assembler {_ZZ3fooIidEDavENKUlvE_clEv:} } }
+// { dg-final { scan-assembler {_ZNK1SIcEUlvE_clEv:} } }
+// { dg-final { scan-assembler-not {_ZZ3fooIidEDavENKUlvE0_clEv:} } }
+// { dg-final { scan-assembler-not {_ZNK1SIcEUlvE0_clEv:} } }
diff --git a/gcc/testsuite/g++.dg/modules/lambda-13.h
b/gcc/testsuite/g++.dg/modules/lambda-13.h
new file mode 100644
index 00000000000..275e6d2269a
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/lambda-13.h
@@ -0,0 +1,22 @@
+// PR c++/123075
+
+template <typename T>
+concept r = []{ return true; }();
+
+template <typename T>
+inline void foo() {
+ static_assert(r<T>);
+}
+
+template void foo<int>();
+
+template <typename T>
+struct S {
+ static_assert(r<T>);
+};
+
+template struct S<double>;
+
+enum E {
+ X = r<E>,
+};
diff --git a/gcc/testsuite/g++.dg/modules/lambda-13_a.H
b/gcc/testsuite/g++.dg/modules/lambda-13_a.H
new file mode 100644
index 00000000000..2a748fef88f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/lambda-13_a.H
@@ -0,0 +1,6 @@
+// PR c++/123075
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fmodule-header" }
+// { dg-module-cmi {} }
+
+#include "lambda-13.h"
diff --git a/gcc/testsuite/g++.dg/modules/lambda-13_b.C
b/gcc/testsuite/g++.dg/modules/lambda-13_b.C
new file mode 100644
index 00000000000..fac66bc5c23
--- /dev/null
+++ b/gcc/testsuite/g++.dg/modules/lambda-13_b.C
@@ -0,0 +1,6 @@
+// PR c++/123075
+// { dg-do compile { target c++20 } }
+// { dg-additional-options "-fmodules -fno-module-lazy" }
+
+#include "lambda-13.h"
+import "lambda-13_a.H";
--
2.51.0
