https://gcc.gnu.org/bugzilla/show_bug.cgi?id=89576
Bug ID: 89576
Summary: constexpr not working if implicitly captured in a
lambda in a function template (gcc 8.3+)
Product: gcc
Version: 8.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: wielkiegie at gmail dot com
Target Milestone: ---
There are many similar bugs that I have found in the bugzilla (see below), but
this one differs from the other ones:
1. It happens only in gcc 8.3 and trunk (gcc 8.2 and earlier are fine)
2. The lambda is not generic (but it is contained in a function template which
might be similar internally)
3. The capture is implicit
Example to reproduce:
------------------------------------------------
template <class T>
void foo()
{
constexpr int x = 0;
[&] {
if constexpr (x) {}
};
}
------------------------------------------------
Produces the following in gcc 8.3 and trunk:
------------------------------------------------
<source>: In lambda function:
<source>:6:24: error: lambda capture of 'x' is not a constant expression
if constexpr (x) {}
^
------------------------------------------------
The following changes to the example make it compile:
1. Adding static to the constexpr int
2. Making it a regular function and not a template
3. Removing the [&] capture
Changing the capture from [&] to [=] doesn't help.
If I change it to an explicit capture [x], it fails in all gcc versions except
gcc 7.4 and passes on clang.
The other similar bug reports: bug 86429, bug 82643
