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* [Bug c++/110441] New: c++17 deferred materialization of temporaries fails when calling class static with member syntax
@ 2023-06-27 18:48 eric.niebler at gmail dot com
2023-06-28 12:47 ` [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision gasper.azman at gmail dot com
` (6 more replies)
0 siblings, 7 replies; 8+ messages in thread
From: eric.niebler at gmail dot com @ 2023-06-27 18:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110441
Bug ID: 110441
Summary: c++17 deferred materialization of temporaries fails
when calling class static with member syntax
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: eric.niebler at gmail dot com
Target Milestone: ---
struct immovable {
immovable() = default;
immovable(immovable &&) = delete;
};
struct S {
static immovable f() {
return {};
}
};
immovable g() {
return S().f();
}
compile with -std=c++17. Result:
<source>: In function 'immovable g()':
<source>:14:15: error: use of deleted function
'immovable::immovable(immovable&&)'
14 | return S().f();
| ~~~~~^~
<source>:4:3: note: declared here
4 | immovable(immovable &&) = delete;
| ^~~~~~~~~
https://godbolt.org/z/Y1h9bPrf3
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision
2023-06-27 18:48 [Bug c++/110441] New: c++17 deferred materialization of temporaries fails when calling class static with member syntax eric.niebler at gmail dot com
@ 2023-06-28 12:47 ` gasper.azman at gmail dot com
2023-06-28 12:48 ` gasper.azman at gmail dot com
` (5 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: gasper.azman at gmail dot com @ 2023-06-28 12:47 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110441
Gašper Ažman <gasper.azman at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |gasper.azman at gmail dot com
--- Comment #1 from Gašper Ažman <gasper.azman at gmail dot com> ---
I hit this in gcc 10 as well when implementing sender/receiver. Was not able to
reduce it this nicely, so I didn't report. Nice work, Eric.
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision
2023-06-27 18:48 [Bug c++/110441] New: c++17 deferred materialization of temporaries fails when calling class static with member syntax eric.niebler at gmail dot com
2023-06-28 12:47 ` [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision gasper.azman at gmail dot com
@ 2023-06-28 12:48 ` gasper.azman at gmail dot com
2023-06-28 14:27 ` ppalka at gcc dot gnu.org
` (4 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: gasper.azman at gmail dot com @ 2023-06-28 12:48 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110441
--- Comment #2 from Gašper Ažman <gasper.azman at gmail dot com> ---
Some more color from twitter, courtesy of @matthewecross:
Interestingly both "return S::f();" and "auto s = S(); return s.f();" both
pass. It's only when you create a temporary instance of S in the return
statement that it fails.
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision
2023-06-27 18:48 [Bug c++/110441] New: c++17 deferred materialization of temporaries fails when calling class static with member syntax eric.niebler at gmail dot com
2023-06-28 12:47 ` [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision gasper.azman at gmail dot com
2023-06-28 12:48 ` gasper.azman at gmail dot com
@ 2023-06-28 14:27 ` ppalka at gcc dot gnu.org
2023-06-28 14:30 ` ppalka at gcc dot gnu.org
` (3 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: ppalka at gcc dot gnu.org @ 2023-06-28 14:27 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110441
Patrick Palka <ppalka at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Last reconfirmed| |2023-06-28
CC| |jason at gcc dot gnu.org,
| |ppalka at gcc dot gnu.org
--- Comment #3 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Confirmed, this never worked. The problem seems to be that because f is
static, 'S().f()' is represented as a COMPOUND_EXPR that evaluates the
otherwise unused object argument S() followed by a TARGET_EXPR for S::f(). And
this COMPOUND_EXPR foils the copy elision check in build_special_member_call
which looks only for an outermost TARGET_EXPR and doesn't look through
COMPOUND_EXPR.
In contrast, '(S(), S::f())' (which should be equivalent) is represented as a
TARGET_EXPR of a COMPOUND_EXPR rather than a COMPOUND_EXPR of a TARGET_EXPR,
and so copy elision is correctly avoided. So perhaps we could make
keep_unused_object_arg for a TARGET_EXPR result place the COMPOUND_EXPR inside
the TARGET_EXPR_INITIAL instead of around the TARGET_EXPR?
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision
2023-06-27 18:48 [Bug c++/110441] New: c++17 deferred materialization of temporaries fails when calling class static with member syntax eric.niebler at gmail dot com
` (2 preceding siblings ...)
2023-06-28 14:27 ` ppalka at gcc dot gnu.org
@ 2023-06-28 14:30 ` ppalka at gcc dot gnu.org
2023-06-28 15:23 ` matt.cross+gcc-bugzilla at gmail dot com
` (2 subsequent siblings)
6 siblings, 0 replies; 8+ messages in thread
From: ppalka at gcc dot gnu.org @ 2023-06-28 14:30 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110441
--- Comment #4 from Patrick Palka <ppalka at gcc dot gnu.org> ---
(In reply to Patrick Palka from comment #3)
> In contrast, '(S(), S::f())' (which should be equivalent) is represented as
> a TARGET_EXPR of a COMPOUND_EXPR rather than a COMPOUND_EXPR of a
> TARGET_EXPR, and so copy elision is correctly avoided.
oops, this should say "is correctly _performed_"
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision
2023-06-27 18:48 [Bug c++/110441] New: c++17 deferred materialization of temporaries fails when calling class static with member syntax eric.niebler at gmail dot com
` (3 preceding siblings ...)
2023-06-28 14:30 ` ppalka at gcc dot gnu.org
@ 2023-06-28 15:23 ` matt.cross+gcc-bugzilla at gmail dot com
2023-07-15 13:53 ` cvs-commit at gcc dot gnu.org
2023-07-15 14:00 ` ppalka at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: matt.cross+gcc-bugzilla at gmail dot com @ 2023-06-28 15:23 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110441
Matt Cross <matt.cross+gcc-bugzilla at gmail dot com> changed:
What |Removed |Added
----------------------------------------------------------------------------
CC| |matt.cross+gcc-bugzilla@gma
| |il.com
--- Comment #5 from Matt Cross <matt.cross+gcc-bugzilla at gmail dot com> ---
I have also found that
* Making the function f() non-static works. https://godbolt.org/z/jn6Ms1n5h
* Making a unique_ptr to an S fails: "auto sp = std::make_unique<S>(); return
sp->f();" https://godbolt.org/z/85e9MW91b
I suspect it is the same root cause, but just in case there's wrinkles here I
thought these additional test cases might be helpful.
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision
2023-06-27 18:48 [Bug c++/110441] New: c++17 deferred materialization of temporaries fails when calling class static with member syntax eric.niebler at gmail dot com
` (4 preceding siblings ...)
2023-06-28 15:23 ` matt.cross+gcc-bugzilla at gmail dot com
@ 2023-07-15 13:53 ` cvs-commit at gcc dot gnu.org
2023-07-15 14:00 ` ppalka at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: cvs-commit at gcc dot gnu.org @ 2023-07-15 13:53 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110441
--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> ---
The master branch has been updated by Patrick Palka <ppalka@gcc.gnu.org>:
https://gcc.gnu.org/g:0de651db45c758f54e9ed917069795a3835499de
commit r14-2539-g0de651db45c758f54e9ed917069795a3835499de
Author: Patrick Palka <ppalka@redhat.com>
Date: Sat Jul 15 09:50:51 2023 -0400
c++: copy elision w/ obj arg and static memfn call [PR110441]
Here the call A().f() is represented as a COMPOUND_EXPR whose first
operand is the otherwise unused object argument A() and second operand
is the call result (both are TARGET_EXPRs). Within the return statement,
this outermost COMPOUND_EXPR ends up foiling the copy elision check in
build_special_member_call, resulting in us introducing a bogus call to the
deleted move constructor. (Within the variable initialization, which goes
through ocp_convert instead of convert_for_initialization, we've already
been eliding the copy -- despite the outermost COMPOUND_EXPR -- ever since
r10-7410-g72809d6fe8e085 made ocp_convert look through COMPOUND_EXPR).
In contrast I noticed '(A(), A::f())' (which should be equivalent to
the above call) is represented with the COMPOUND_EXPR inside the RHS's
TARGET_EXPR initializer thanks to a special case in cp_build_compound_expr.
So this patch fixes this by making keep_unused_object_arg use
cp_build_compound_expr as well.
PR c++/110441
gcc/cp/ChangeLog:
* call.cc (keep_unused_object_arg): Use cp_build_compound_expr
instead of building a COMPOUND_EXPR directly.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1z/elide8.C: New test.
^ permalink raw reply [flat|nested] 8+ messages in thread
* [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision
2023-06-27 18:48 [Bug c++/110441] New: c++17 deferred materialization of temporaries fails when calling class static with member syntax eric.niebler at gmail dot com
` (5 preceding siblings ...)
2023-07-15 13:53 ` cvs-commit at gcc dot gnu.org
@ 2023-07-15 14:00 ` ppalka at gcc dot gnu.org
6 siblings, 0 replies; 8+ messages in thread
From: ppalka at gcc dot gnu.org @ 2023-07-15 14:00 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110441
Patrick Palka <ppalka at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Assignee|unassigned at gcc dot gnu.org |ppalka at gcc dot gnu.org
Target Milestone|--- |14.0
Status|NEW |RESOLVED
Resolution|--- |FIXED
--- Comment #7 from Patrick Palka <ppalka at gcc dot gnu.org> ---
Should be fixed for GCC 14, thanks for the report.
^ permalink raw reply [flat|nested] 8+ messages in thread
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2023-06-27 18:48 [Bug c++/110441] New: c++17 deferred materialization of temporaries fails when calling class static with member syntax eric.niebler at gmail dot com
2023-06-28 12:47 ` [Bug c++/110441] c++17: temporary causes static member function call to confuse required copy elision gasper.azman at gmail dot com
2023-06-28 12:48 ` gasper.azman at gmail dot com
2023-06-28 14:27 ` ppalka at gcc dot gnu.org
2023-06-28 14:30 ` ppalka at gcc dot gnu.org
2023-06-28 15:23 ` matt.cross+gcc-bugzilla at gmail dot com
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