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.
+ 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
