[llvm-bugs] [Bug 49657] New: Needless instantiation in requires-expression

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49657

            Bug ID: 49657
           Summary: Needless instantiation in requires-expression
           Product: clang
           Version: trunk
          Hardware: PC
               URL: https://godbolt.org/z/3q4frc
                OS: Linux
            Status: NEW
          Keywords: compile-fail
          Severity: enhancement
          Priority: P
         Component: C++2a
          Assignee: unassignedclangbugs at nondot.org
          Reporter: johelegp at gmail.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    johelegp at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

See https://godbolt.org/z/3q4frc.
```C++
#include<span>
struct X;
struct Y { std::span<Y> s; };
Y y{};
Y y2 = y;
```
When generating the copy constructor for `Y`, candidate
`span::span(contiguous_range auto&& r)` checks for `ranges::end(r)`. `r.end()`
ends up being instantiated, even though there's nothing in the immediate
context that requires instantiating it.
```
In file included from <source>:1:
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/span:292:38:
error: arithmetic on a pointer to an incomplete type 'Y'
      { return iterator(this->_M_ptr + this->size()); }
                        ~~~~~~~~~~~~ ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:179:17:
note: in instantiation of member function 'std::span<Y,
18446744073709551615>::end' requested here
            return __t.end();
                       ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:582:2:
note: in instantiation of function template specialization
'std::ranges::__cust_access::_End::operator()<const std::span<Y,
18446744073709551615> &>' requested here
        ranges::end(__t);
        ^
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:582:2:
note: in instantiation of requirement here
        ranges::end(__t);
        ^~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:579:21:
note: while substituting template arguments into constraint expression here
    concept range = requires(_Tp& __t)
                    ^~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:639:27:
note: while checking the satisfaction of concept 'range<const std::span<Y,
18446744073709551615> &>' requested here
    concept input_range = range<_Tp> && input_iterator<iterator_t<_Tp>>;
                          ^~~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/bits/ranges_base.h:639:27:
note: (skipping 9 contexts in backtrace; use -ftemplate-backtrace-limit=0 to
see all)
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/span:202:11:
note: while checking the satisfaction of concept 'contiguous_range<const
std::span<Y, 18446744073709551615> &>' requested here
        requires ranges::contiguous_range<_Range> &&
ranges::sized_range<_Range>
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/span:202:19:
note: while substituting template arguments into constraint expression here
        requires ranges::contiguous_range<_Range> &&
ranges::sized_range<_Range>
                         ^~~~~~~~~~~~~~~~~~~~~~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/span:104:11:
note: while checking constraint satisfaction for template 'span<const
std::span<Y, 18446744073709551615> &>' required here
    class span
          ^~~~
/opt/compiler-explorer/gcc-snapshot/lib/gcc/x86_64-linux-gnu/11.0.1/../../../../include/c++/11.0.1/span:104:11:
note: in instantiation of function template specialization 'std::span<Y,
18446744073709551615>::span<const std::span<Y, 18446744073709551615> &>'
requested here
<source>:3:8: note: while declaring the implicit copy constructor for 'Y'
struct Y { std::span<Y> s; };
       ^
<source>:3:8: note: definition of 'Y' is not complete until the closing '}'
1 error generated.
Compiler returned: 1
```

-- 
You are receiving this mail because:
You are on the CC list for the bug.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-bugs/attachments/20210320/b443a4c1/attachment-0001.html>