On 12/1/23 17:42, Patrick Palka wrote:
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?
Or remove them in prune_lambda_captures if there's no real use?
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.
OK if you open a PR for the other cases and add a FIXME comment
referring to it.
Jason
