On Thu, 14 May 2026, Marek Polacek wrote:
> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/16.2?
>
> -- >8 --
> Thanks to DR 696 (r253266), this works:
>
> int g;
> void fn ()
> {
> int &c = g;
> auto l = [] { c++; };
> l();
> }
>
> because `c` in the lambda body is not an odr-use because we can
> evaluate it to a constant and so there's no capture. But when
> fn is a template, we reject the code and crash. This patch fixes
> both.
>
> Outside a template, the call to maybe_constant_value in mark_use
> evaluates `c` to `(int&) &g` but in a template, it remains `c`.
> Then we emit an error, and crash on the error_mark_node from
> process_outer_var_ref. One of the reasons is
> else if (TYPE_REF_P (TREE_TYPE (expression)))
> /* FIXME cp_finish_decl doesn't fold reference initializers. */
> return true;
> in value_dependent_expression_p but even if that changed, we still
> wouldn't get the referent because decl_really_constant_value wouldn't
> give it to us; the DECL_INITIAL is not a TREE_CONSTANT yet.
>
> So I stopped trying to make this work in a template, and instead
> I'm delaying the processing to instantiating when we know that
> maybe_constant_value wouldn't even try to evaluate.
It seems with this patch we still incorrectly reject the case where the outer
local variable is initially type-dependent (not a regression):
int g;
template<class T>
void
fn1 ()
{
T c = g;
auto l = [] { c++; }; // bogus error: 'c' is not captured
l();
}
void
bar ()
{
fn1<int&> ();
}
via process_outer_var_ref, this time called from finish_id_expression_1.
Maybe we can make both testcases work if we instead give process_outer_var_ref
an early exit for when the outer variable is from a template?
>
> The is_capture_proxy change is due to lambda-scope10.C: we are
> checking is_nondependent_constant_expression before the enclosing
> function has been built up.
>
> PR c++/123536
>
> gcc/cp/ChangeLog:
>
> * expr.cc (mark_use) <case VAR_DECL>: Check
> is_nondependent_constant_expression before
> process_outer_var_ref.
> * lambda.cc (is_capture_proxy): Check DECL_CONTEXT.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/lambda/lambda-const12.C: New test.
> ---
> gcc/cp/expr.cc | 3 +-
> gcc/cp/lambda.cc | 1 +
> .../g++.dg/cpp0x/lambda/lambda-const12.C | 48 +++++++++++++++++++
> 3 files changed, 51 insertions(+), 1 deletion(-)
> create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const12.C
>
> diff --git a/gcc/cp/expr.cc b/gcc/cp/expr.cc
> index 4d017d530ef..15036cf7cad 100644
> --- a/gcc/cp/expr.cc
> +++ b/gcc/cp/expr.cc
> @@ -131,7 +131,8 @@ mark_use (tree expr, bool rvalue_p, bool read_p,
> }
> }
> if (outer_automatic_var_p (expr)
> - && decl_constant_var_p (expr))
> + && decl_constant_var_p (expr)
> + && is_nondependent_constant_expression (expr))
> {
> if (rvalue_p)
> {
> diff --git a/gcc/cp/lambda.cc b/gcc/cp/lambda.cc
> index af0b66afe81..0b35c18da13 100644
> --- a/gcc/cp/lambda.cc
> +++ b/gcc/cp/lambda.cc
> @@ -288,6 +288,7 @@ is_capture_proxy (tree decl)
> && !(DECL_ARTIFICIAL (decl)
> && DECL_LANG_SPECIFIC (decl)
> && DECL_OMP_PRIVATIZED_MEMBER (decl))
> + && DECL_CONTEXT (decl)
> && LAMBDA_FUNCTION_P (DECL_CONTEXT (decl)));
> }
>
> diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const12.C
> b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const12.C
> new file mode 100644
> index 00000000000..eca53461c69
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-const12.C
> @@ -0,0 +1,48 @@
> +// PR c++/123536
> +// { dg-do compile { target c++11 } }
> +
> +int g;
> +
> +template<int = 1>
> +void
> +fn1 ()
> +{
> + int &c = g;
> + auto l = [] { c++; };
> + l();
> +}
> +
> +template<int N = 1>
> +void
> +fn2 ()
> +{
> + const int &c = N;
> + auto l = [] { int i = c; (void) i; }; // { dg-error ".c. is not captured" }
> + l();
> +}
> +
> +void
> +fn3 ()
> +{
> + int &c = g;
> + auto l = [] { c++; };
> + l();
> +}
> +
> +void
> +fn4 ()
> +{
> + int n = 42;
> + const int &c = n;
> + auto l = [] { int i = c; (void) i; }; // { dg-error ".c. is not captured" }
> + l();
> +}
> +
> +void
> +bar ()
> +{
> + fn1 ();
> + fn2 ();
> + fn3 ();
> + fn4 ();
> +}
>
> base-commit: 8b40209e48c8212ad870724b83718ed244bdf6eb
> --
> 2.54.0
>
>
