[llvm-bugs] [Bug 38578] New: C++ friend Constructor with Templates Not Recognized

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=38578

            Bug ID: 38578
           Summary: C++ friend Constructor with Templates Not Recognized
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: C++
          Assignee: unassignedclangbugs at nondot.org
          Reporter: mattmiller1 at gmail.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org

Noticed the following behavior which isn't clear to me is standard compliant:

1. non-template friend function (not constructor): compiles in gcc and clang
2. non-template friend constructor: compiles in gcc and clang
3. template friend function (not constructor): compiles in gcc and clang
4. template friend constructor: compiles in gcc , but not clang
(compiled in clang 3.0.0 but not clang 3.1+)

Here is an example (use -DUSE_TEMPLATE to control whether the template
or non-template code is compiled):

https://godbolt.org/g/FT7Mzj

In the clang case for (4), it gives an error as if the friend
declaration was never made.

Looking at a C++ standard:
http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf

It looks like section 14.3.5 gives an explicit example of a
non-template friend constructor being allowed.  In 17.5.4, it didn't
look like there was anything to indicate template friend constructors
are treated different than template friend non-constructor functions,
but maybe I'm missing a subtle point in that section or elsewhere.

If there is a reason why (4) is not allowed by the standard but (3)
is, it might be good to have a more specific compile error message
indicating that.

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