c++: decltype of (non-captured variable) [PR83167]
Checks
Commit Message
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.
PR c++/83167
gcc/cp/ChangeLog:
* semantics.cc (finish_decltype_type): If capture_decltype
returns NULL_TREE, fall back to the ordinary code path.
(capture_decltype): Return NULL_TREE if the lambda has no
capture-default.
gcc/testsuite/ChangeLog:
* g++.dg/cpp0x/lambda/lambda-decltype4.C: New test.
---
gcc/cp/semantics.cc | 6 +++---
.../g++.dg/cpp0x/lambda/lambda-decltype4.C | 15 +++++++++++++++
2 files changed, 18 insertions(+), 3 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C
Comments
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?
Jason
> PR c++/83167
>
> gcc/cp/ChangeLog:
>
> * semantics.cc (finish_decltype_type): If capture_decltype
> returns NULL_TREE, fall back to the ordinary code path.
> (capture_decltype): Return NULL_TREE if the lambda has no
> capture-default.
>
> gcc/testsuite/ChangeLog:
>
> * g++.dg/cpp0x/lambda/lambda-decltype4.C: New test.
> ---
> gcc/cp/semantics.cc | 6 +++---
> .../g++.dg/cpp0x/lambda/lambda-decltype4.C | 15 +++++++++++++++
> 2 files changed, 18 insertions(+), 3 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 8090c71809f..6fdd6c45972 100644
> --- a/gcc/cp/semantics.cc
> +++ b/gcc/cp/semantics.cc
> @@ -11732,7 +11732,8 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
> /* If the expression is just "this", we want the
> cv-unqualified pointer for the "this" type. */
> type = TYPE_MAIN_VARIANT (TREE_TYPE (expr));
> - else
> +
> + if (!type)
> {
> /* Otherwise, where T is the type of e, if e is an lvalue,
> decltype(e) is defined as T&; if an xvalue, T&&; otherwise, T. */
> @@ -12639,8 +12640,7 @@ capture_decltype (tree decl)
> switch (LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lam))
> {
> case CPLD_NONE:
> - error ("%qD is not captured", decl);
> - return error_mark_node;
> + return NULL_TREE;
>
> case CPLD_COPY:
> type = TREE_TYPE (decl);
> diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C
> new file mode 100644
> index 00000000000..0062d7b8672
> --- /dev/null
> +++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C
> @@ -0,0 +1,15 @@
> +// PR c++/83167
> +// { dg-do compile { target c++11 } }
> +
> +int main() {
> + int x;
> + const int y = 42;
> +
> + [] {
> + using ty1 = decltype((x));
> + using ty1 = int&;
> +
> + using ty2 = decltype((y));
> + using ty2 = const int&;
> + };
> +}
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." */
+ 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));
+ type = cp_build_qualified_type (type, quals);
+ type = build_reference_type (type);
+ }
}
else if (error_operand_p (expr))
type = error_mark_node;
@@ -11878,7 +11904,8 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
/* If the expression is just "this", we want the
cv-unqualified pointer for the "this" type. */
type = TYPE_MAIN_VARIANT (TREE_TYPE (expr));
- else
+
+ if (!type)
{
/* Otherwise, where T is the type of e, if e is an lvalue,
decltype(e) is defined as T&; if an xvalue, T&&; otherwise, T. */
@@ -12767,60 +12794,6 @@ apply_deduced_return_type (tree fco, tree return_type)
}
}
-/* DECL is a local variable or parameter from the surrounding scope of a
- lambda-expression. Returns the decltype for a use of the capture field
- for DECL even if it hasn't been captured yet. Or NULL_TREE if we can't give
- a correct answer at this point and we should build a DECLTYPE_TYPE. */
-
-static tree
-capture_decltype (tree decl)
-{
- tree lam = CLASSTYPE_LAMBDA_EXPR (DECL_CONTEXT (current_function_decl));
- tree cap = lookup_name (DECL_NAME (decl), LOOK_where::BLOCK,
- LOOK_want::HIDDEN_LAMBDA);
- tree type;
-
- if (cap && is_capture_proxy (cap))
- type = TREE_TYPE (cap);
- else
- switch (LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lam))
- {
- case CPLD_NONE:
- error ("%qD is not captured", decl);
- return error_mark_node;
-
- case CPLD_COPY:
- type = TREE_TYPE (decl);
- if (TYPE_REF_P (type)
- && TREE_CODE (TREE_TYPE (type)) != FUNCTION_TYPE)
- type = TREE_TYPE (type);
- break;
-
- case CPLD_REFERENCE:
- type = TREE_TYPE (decl);
- if (!TYPE_REF_P (type))
- type = build_reference_type (TREE_TYPE (decl));
- break;
-
- default:
- gcc_unreachable ();
- }
-
- if (!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. */
- return NULL_TREE;
- int quals = cp_type_quals (type);
- if (INDIRECT_TYPE_P (obtype))
- quals |= cp_type_quals (TREE_TYPE (obtype));
- type = cp_build_qualified_type (type, quals);
- type = build_reference_type (type);
- }
- return type;
-}
-
/* Build a unary fold expression of EXPR over OP. If IS_RIGHT is true,
this is a right unary fold. Otherwise it is a left unary fold. */
diff --git a/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C
new file mode 100644
index 00000000000..0062d7b8672
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/lambda/lambda-decltype4.C
@@ -0,0 +1,15 @@
+// PR c++/83167
+// { dg-do compile { target c++11 } }
+
+int main() {
+ int x;
+ const int y = 42;
+
+ [] {
+ using ty1 = decltype((x));
+ using ty1 = int&;
+
+ using ty2 = decltype((y));
+ using ty2 = const int&;
+ };
+}
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
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
>
>
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
@@ -11732,7 +11732,8 @@ finish_decltype_type (tree expr, bool id_expression_or_member_access_p,
/* If the expression is just "this", we want the
cv-unqualified pointer for the "this" type. */
type = TYPE_MAIN_VARIANT (TREE_TYPE (expr));
- else
+
+ if (!type)
{
/* Otherwise, where T is the type of e, if e is an lvalue,
decltype(e) is defined as T&; if an xvalue, T&&; otherwise, T. */
@@ -12639,8 +12640,7 @@ capture_decltype (tree decl)
switch (LAMBDA_EXPR_DEFAULT_CAPTURE_MODE (lam))
{
case CPLD_NONE:
- error ("%qD is not captured", decl);
- return error_mark_node;
+ return NULL_TREE;
case CPLD_COPY:
type = TREE_TYPE (decl);
new file mode 100644
@@ -0,0 +1,15 @@
+// PR c++/83167
+// { dg-do compile { target c++11 } }
+
+int main() {
+ int x;
+ const int y = 42;
+
+ [] {
+ using ty1 = decltype((x));
+ using ty1 = int&;
+
+ using ty2 = decltype((y));
+ using ty2 = const int&;
+ };
+}