[llvm-bugs] [Bug 50717] New: std::ranges::size incorrectly uses perfect forwarding

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=50717

            Bug ID: 50717
           Summary: std::ranges::size incorrectly uses perfect forwarding
           Product: libc++
           Version: 12.0
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: All Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: zilla at kayari.org
                CC: llvm-bugs at lists.llvm.org, mclow.lists at gmail.com

test/std/ranges/range.access/range.prim/size.pass.cpp has this check:

  assert(std::ranges::size(MoveOnlySizeFunction()) == 42);

which relies on this function being called with an rvalue:

   friend constexpr size_t size(MoveOnlySizeFunction) { return 42; }

However, the relevant part of the standard says an lvalue should be used:

"then ranges::size(E) is expression-equivalent to decay-copy(size(t))"

where 't' is an lvalue, i.e. it should not be forwarded.

As a result, the test fails for libstdc++ trunk (since I fixed the same bug
earlier this month).

The spec was changed by https://wg21.link/p2091r0 to use lvalues everywhere
except ranges::cdata.

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