[llvm-bugs] [Bug 43087] New: Over-eager template instantiation breaks libstdc++ variant from gcc 10 trunk

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43087

            Bug ID: 43087
           Summary: Over-eager template instantiation breaks libstdc++
                    variant from gcc 10 trunk
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++17
          Assignee: unassignedclangbugs at nondot.org
          Reporter: David.Truby at arm.com
                CC: blitzrakete at gmail.com, erik.pilkington at gmail.com,
                    llvm-bugs at lists.llvm.org, richard-llvm at metafoo.co.uk

Created attachment 22414
  --> https://bugs.llvm.org/attachment.cgi?id=22414&action=edit
Simple reproducing case

libstdc++ 10 has changes to its variant implementation that cause compilation
to fail with clang. I've attached a file that works with clang and libc++ and
also with gcc and libstdc++. It seems that clang is over-eager when
instantiating the templates and goes looking for a constructor further down the
nested stack when there's a valid one higher up.

The same problem did manifest with libc++, but was fixed by this commit:
https://github.com/llvm-mirror/libcxx/commit/301501fb78f90d65cec68f22144f3446a571e553#diff-9308c8fc46861ae969c076f9b7637013
However, since clang has had the same problem with both libc++ and libstdc++,
and gcc doesn't have the same problem with libstdc++, I suspect this represents
a bug at the clang end.

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