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.
>
> > PR c++/123075
> >
> > gcc/cp/ChangeLog:
> >
> > * constraint.cc (evaluate_concept_check): Start a scope.
> > * name-lookup.cc (cp_binding_level_descriptor): Handle concept
> > scopes.
> > (begin_scope): Likewise.
> > * name-lookup.h (enum scope_kind): New sk_concept scope kind.
> > * 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]>
> > ---
> > gcc/cp/constraint.cc | 10 +++++++++-
> > gcc/cp/name-lookup.cc | 4 +++-
> > gcc/cp/name-lookup.h | 2 ++
> > gcc/cp/pt.cc | 7 ++++++-
> > gcc/testsuite/g++.dg/cpp2a/concepts-lambda25.C | 17 +++++++++++++++++
> > gcc/testsuite/g++.dg/modules/lambda-13.h | 11 +++++++++++
> > gcc/testsuite/g++.dg/modules/lambda-13_a.H | 6 ++++++
> > gcc/testsuite/g++.dg/modules/lambda-13_b.C | 6 ++++++
> > 8 files changed, 60 insertions(+), 3 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..93c68bc6e7a 100644
> > --- a/gcc/cp/constraint.cc
> > +++ b/gcc/cp/constraint.cc
> > @@ -2860,9 +2860,17 @@ 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
> > + enter a new scope that inhibits these declarations. */
> > + begin_scope (sk_concept, NULL_TREE);
> > +
> > /* 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);
> > +
> > + pop_bindings_and_leave_scope ();
> > + return r;
> > }
> > /* Evaluate the requires-expression T, returning either
> > boolean_true_node
> > diff --git a/gcc/cp/name-lookup.cc b/gcc/cp/name-lookup.cc
> > index 4c07fd40f64..e617105600d 100644
> > --- a/gcc/cp/name-lookup.cc
> > +++ b/gcc/cp/name-lookup.cc
> > @@ -4700,7 +4700,8 @@ cp_binding_level_descriptor (cp_binding_level *scope)
> > "template-explicit-spec-scope",
> > "transaction-scope",
> > "openmp-scope",
> > - "lambda-scope"
> > + "lambda-scope",
> > + "concept-scope",
> > };
> > static_assert (ARRAY_SIZE (scope_kind_names) == sk_count,
> > "must keep names aligned with scope_kind enum");
> > @@ -4793,6 +4794,7 @@ begin_scope (scope_kind kind, tree entity)
> > case sk_omp:
> > case sk_stmt_expr:
> > case sk_lambda:
> > + case sk_concept:
> > scope->keep = keep_next_level_flag;
> > break;
> > diff --git a/gcc/cp/name-lookup.h b/gcc/cp/name-lookup.h
> > index da277c49b1a..024ef738161 100644
> > --- a/gcc/cp/name-lookup.h
> > +++ b/gcc/cp/name-lookup.h
> > @@ -229,6 +229,8 @@ enum scope_kind {
> > sk_transaction, /* A synchronized or atomic statement. */
> > sk_omp, /* An OpenMP structured block. */
> > sk_lambda, /* A lambda scope. */
> > + sk_concept, /* The scope of a declaration in the definition
> > of a
> > + concept during evaluation. */
> > sk_count /* Number of scope_kind enumerations. */
> > };
> > 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..e064df67f42
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/cpp2a/concepts-lambda25.C
> > @@ -0,0 +1,17 @@
> > +// PR c++/123075
> > +// { dg-do compile { target c++20 } }
> > +
> > +template <typename T>
> > +concept r = []{ return true; }();
> > +
> > +template <typename T, typename U>
> > +inline auto foo() {
> > + static_assert(r<T>);
> > + r<U>;
> > + return []{ return false; };
> > +}
> > +
> > +bool x = foo<int, double>()();
> > +
> > +// There should only be one lambda keyed to 'foo()'
> > +// { dg-final { scan-assembler {_ZZ3fooIidEDavENKUlvE_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..dfe11954f8c
> > --- /dev/null
> > +++ b/gcc/testsuite/g++.dg/modules/lambda-13.h
> > @@ -0,0 +1,11 @@
> > +// PR c++/123075
> > +
> > +template <typename T>
> > +concept r = []{ return true; }();
> > +
> > +template <typename T>
> > +inline void foo() {
> > + static_assert(r<T>);
> > +}
> > +
> > +template void foo<int>();
> > 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";
>
>
