On Fri, 1 Dec 2023, Jason Merrill wrote:
> On 12/1/23 12:32, Patrick Palka wrote:
> > On Tue, 14 Nov 2023, Jason Merrill wrote:
> >
> > > On 11/14/23 11:10, Patrick Palka wrote:
> > > > Bootstrapped and regtested on x86_64-pc-linux-gnu, does this look OK for
> > > > trunk?
> > > >
> > > > -- >8 --
> > > >
> > > > For decltype((x)) within a lambda where x is not captured, we dubiously
> > > > require that the lambda has a capture default, unlike for decltype(x).
> > > > This patch fixes this inconsistency; I couldn't find a justification for
> > > > it in the standard.
> > >
> > > The relevant passage seems to be
> > >
> > > https://eel.is/c++draft/expr.prim#id.unqual-3
> > >
> > > "If naming the entity from outside of an unevaluated operand within S
> > > would
> > > refer to an entity captured by copy in some intervening lambda-expression,
> > > then let E be the innermost such lambda-expression.
> > >
> > > If there is such a lambda-expression and if P is in E's function parameter
> > > scope but not its parameter-declaration-clause, then the type of the
> > > expression is the type of a class member access expression ([expr.ref])
> > > naming
> > > the non-static data member that would be declared for such a capture in
> > > the
> > > object parameter ([dcl.fct]) of the function call operator of E."
> > >
> > > In this case I guess there is no such lambda-expression because naming x
> > > won't
> > > refer to a capture by copy if the lambda doesn't capture anything, so we
> > > ignore the lambda.
> > >
> > > Maybe refer to that in a comment? OK with that change.
> > >
> > > I'm surprised that it refers specifically to capture by copy, but I guess
> > > a
> > > capture by reference should have the same decltype as the captured
> > > variable?
> >
> > Ah, seems like it. So maybe we should get rid of the redundant
> > by-reference capture-default handling, to more closely mirror the
> > standard?
> >
> > Also now that r14-6026-g73e2bdbf9bed48 made capture_decltype return
> > NULL_TREE to mean the capture is dependent, it seems we should just
> > inline capture_decltype into finish_decltype_type rather than
> > introducing another special return value to mean "fall back to ordinary
> > handling".
> >
> > How does the following look? Bootstrapped and regtested on
> > x86_64-pc-linux-gnu.
> >
> > -- >8 --
> >
> > PR c++/83167
> >
> > gcc/cp/ChangeLog:
> >
> > * semantics.cc (capture_decltype): Inline into its only caller ...
> > (finish_decltype_type): ... here. Update nearby comment to refer
> > to recent standard. Restrict uncaptured variable handling to just
> > lambdas with a by-copy capture-default.
> >
> > gcc/testsuite/ChangeLog:
> >
> > * g++.dg/cpp0x/lambda/lambda-decltype4.C: New test.
> > ---
> > gcc/cp/semantics.cc | 107 +++++++-----------
> > .../g++.dg/cpp0x/lambda/lambda-decltype4.C | 15 +++
> > 2 files changed, 55 insertions(+), 67 deletions(-)
> > create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C
> >
> > diff --git a/gcc/cp/semantics.cc b/gcc/cp/semantics.cc
> > index fbbc18336a0..fb4c3992e34 100644
> > --- a/gcc/cp/semantics.cc
> > +++ b/gcc/cp/semantics.cc
> > @@ -53,7 +53,6 @@ along with GCC; see the file COPYING3. If not see
> > static tree maybe_convert_cond (tree);
> > static tree finalize_nrv_r (tree *, int *, void *);
> > -static tree capture_decltype (tree);
> > /* Used for OpenMP non-static data member privatization. */
> > @@ -11856,21 +11855,48 @@ finish_decltype_type (tree expr, bool
> > id_expression_or_member_access_p,
> > }
> > else
> > {
> > - /* Within a lambda-expression:
> > -
> > - Every occurrence of decltype((x)) where x is a possibly
> > - parenthesized id-expression that names an entity of
> > - automatic storage duration is treated as if x were
> > - transformed into an access to a corresponding data member
> > - of the closure type that would have been declared if x
> > - were a use of the denoted entity. */
> > if (outer_automatic_var_p (STRIP_REFERENCE_REF (expr))
> > && current_function_decl
> > && LAMBDA_FUNCTION_P (current_function_decl))
> > {
> > - type = capture_decltype (STRIP_REFERENCE_REF (expr));
> > - if (!type)
> > - goto dependent;
> > + /* [expr.prim.id.unqual]/3: If naming the entity from outside of an
> > + unevaluated operand within S would refer to an entity captured by
> > + copy in some intervening lambda-expression, then let E be the
> > + innermost such lambda-expression.
> > +
> > + If there is such a lambda-expression and if P is in E's function
> > + parameter scope but not its parameter-declaration-clause, then
> > the
> > + type of the expression is the type of a class member access
> > + expression naming the non-static data member that would be
> > declared
> > + for such a capture in the object parameter of the function call
> > + operator of E." */
>
> Hmm, looks like this code is only checking the innermost lambda, it needs to
> check all containing lambdas for one that would capture it by copy.
Unfortunately this seems to be a can of worms, since IIUC we also have
to check that there's no non-default-capture lambda in the stack as
well, e.g.
int main() {
int x;
[] {
[=] {
using ty1 = decltype((x)); // refers to local variable despite
// innermost by-copy capture-default
using ty1 = int&;
};
};
[=] {
[] {
using ty1 = decltype((x)); // same
using ty1 = int&;
};
};
[=] {
[&] {
using ty1 = decltype((x)); // refers to hypothetical capture proxy
using ty1 = const int&;
};
};
[&] {
[=] {
using ty1 = decltype((x)); // same
using ty1 = const int&;
};
};
}
And we have to refine the logic for whether to perform the HIDDEN_LAMBDA
name lookup (which we currently unconditionally do):
int main() {
int x;
[x] {
[x] {
using ty1 = decltype((x)); // refers to actual capture proxy,
// found by HIDDEN_LAMBDA name lookup
using ty1 = const int&;
};
};
[x] {
[] {
using ty1 = decltype((x)); // refers to local variable,
// HIDDEN_LAMBDA name lookup not performed
using ty1 = int&;
};
};
}
These could probably be fixed locally within finish_decltype_type,
but then there's PR86697 which basically extends all of these
capture-related issues to 'decltype(f(x))' instead of 'decltype((x))',
which suggests a proper fix should probably be in process_outer_var_ref
instead of in finish_decltype_type? Perhaps when in an unevaluated
context, process_outer_var_ref should still rewrite uses into capture
proxies but not actually add them to the closure or something like that?
I don't think I have the cycles to work on these issues this stage..
Would the latest patch be OK at least? It seems to be a strict
improvement.
>
> > + tree decl = STRIP_REFERENCE_REF (expr);
> > + tree lam = CLASSTYPE_LAMBDA_EXPR (DECL_CONTEXT
> > (current_function_decl));
> > + tree cap = lookup_name (DECL_NAME (decl), LOOK_where::BLOCK,
> > + LOOK_want::HIDDEN_LAMBDA);
> > +
> > + if (cap && is_capture_proxy (cap))
> > + type = TREE_TYPE (cap);
> > + else if (LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lam) == CPLD_COPY)
> > + {
> > + type = TREE_TYPE (decl);
> > + if (TYPE_REF_P (type)
> > + && TREE_CODE (TREE_TYPE (type)) != FUNCTION_TYPE)
> > + type = TREE_TYPE (type);
> > + }
> > +
> > + if (type && !TYPE_REF_P (type))
> > + {
> > + tree obtype = TREE_TYPE (DECL_ARGUMENTS
> > (current_function_decl));
> > + if (WILDCARD_TYPE_P (non_reference (obtype)))
> > + /* We don't know what the eventual obtype quals will be. */
> > + goto dependent;
> > + int quals = cp_type_quals (type);
> > + if (INDIRECT_TYPE_P (obtype))
> > + quals |= cp_type_quals (TREE_TYPE (obtype));
> >
> > Shouldn't we propagate cv-quals of a by-value object parameter as well?
>
> Ah, I think you're right.
>
> Jason
>
>
