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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Incomplete implementation of the P1330R0 proposal / [expr.const]p5 and [class.union]p6 (N4892)"
href="https://bugs.llvm.org/show_bug.cgi?id=51260">51260</a>
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<th>Summary</th>
<td>Incomplete implementation of the P1330R0 proposal / [expr.const]p5 and [class.union]p6 (N4892)
</td>
</tr>
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<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
</td>
</tr>
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<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
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<th>Component</th>
<td>C++
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>matthewjbarichello@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>blitzrakete@gmail.com, dgregor@apple.com, erik.pilkington@gmail.com, llvm-bugs@lists.llvm.org, richard-llvm@metafoo.co.uk
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<pre>Created <span class=""><a href="attachment.cgi?id=25087" name="attach_25087" title="Patch to fix the issue in 'releases/12.x'">attachment 25087</a> <a href="attachment.cgi?id=25087&action=edit" title="Patch to fix the issue in 'releases/12.x'">[details]</a></span>
Patch to fix the issue in 'releases/12.x'
All of the references and quotes in this report are concerned with the changes
in P1330R0 and the current working standard document, N4892:
- P1330R0: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1330r0.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2018/p1330r0.pdf</a>
- N4892: <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/n4892.pdf">http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/n4892.pdf</a>
Affected versions:
- trunk
- 12.0.1
- 12.0.0
- 11.0.1
- 11.0.0
- 10.0.1
- 10.0.0
- 9.0.1
- 9.0.0
Driver cmdline:
Note: For versions not supporting -std=c++20, -std=c++2a was used.
clang++ -std=c++20
Consider the following examples:
[Example A:
struct A {};
constexpr auto test() {
union U {
int i;
A a;
} u{.i = 0};
u.a = {};
return 0;
}
int main() {
constexpr auto r = test();
(void)r;
}
[Example B:
struct A {
int i;
constexpr A() noexcept : i(0) {}
constexpr A(int i) noexcept : i(i) {}
constexpr A& operator=(A const& other) noexcept {
i = other.i;
return *this;
}
};
constexpr auto test() {
union U {
int i;
A a;
} u{.i = 0};
u.a = {123};
return u.a.i;
}
int main() {
constexpr auto r = test();
(void)r;
}
According to [expr.constant]p5
An expression E is a core constant expression unless the evaluation of E,
following the rules of the abstract machine (6.9.1), would evaluate one of the
following:
... an invocation of an implicitly-defined copy/move constructor or
copy/move assignment operator for a union whose active member (if any) is
mutable, unless the lifetime of the union object began within the evaluation of
E ...
, and [class.union]p6
When the left operand of an assignment operator involves a member access
expression (7.6.1.5) that nominates a union member, it may begin the lifetime
of that union member ...
, the lifetime of `u.a` ("Example A":8) should start at the invocation of
`A`'s implicitly defined copy-assignment operator, however, under all listed
clang versions this is not the case as compilation yields the following error:
"Example A":13:17: error: constexpr variable 'r' must be initialized by a
constant expression
constexpr auto r = test();
^ ~~~~~~
"Example A":8:6: note: member call on member 'a' of union with active member
'i' is not allowed in a constant expression
u.a = {};
^
"Example A":13:21: note: in call to 'test()'
constexpr auto r = test();
^
1 error generated.
Likewise, the lifetime of `u.a` ("Example B":18) should begin at the
invocation of `A`'s user-defined constexpr copy-assignment operator, however
compilation yields the same error.
Addressing the issue:
I plan to formally submit a properly tested patch to trunk and backport it to
all affected major clang releases. Until then, I've attached a patch to address
this bug, based on the `release/12.x` tag:
commit fed41342a82f5a3a9201819a82bf7a48313e296b (grafted, HEAD ->
release/12.x, tag: llvmorg-12.0.1-rc4, tag: llvmorg-12.0.1,
origin/release/12.x)
Author: Tom Stellard <<a href="mailto:tstellar@redhat.com">tstellar@redhat.com</a>>
Date: Mon Jun 28 09:23:38 2021 -0700
Revert "Revert "[Coverage] Fix branch coverage merging in
FunctionCoverageSummary::get() for instantiation""
This reverts commit 33d312b2d731507327252fd597bac1b738870330.
The original patch was correct, so we need to restore it in the
release branch.
Notes:
- In testing both examples against other major compilers, such as MSVC and
GCC, it would seem that they compile and behave as expected.</pre>
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