[llvm-bugs] [Bug 32115] constexpr variable template requires default initialization even though it's not instantiated

[via llvm-bugs]

http://bugs.llvm.org/show_bug.cgi?id=32115

Richard Smith <richard-llvm at metafoo.co.uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
         Resolution|---                         |INVALID
                 CC|                            |richard-llvm at metafoo.co.uk
             Status|NEW                         |RESOLVED

--- Comment #2 from Richard Smith <richard-llvm at metafoo.co.uk> ---
[temp.res]p8: "The program is ill-formed, no diagnostic required, if [...] no
valid specialization can be generated for a template and the template is not
instantiated."

> that would be consistent with a constexpr function for example that
> can remain undefined unless it's actually used.

Clang rejects this case too, for the same reason:

template<typename T> constexpr void f() {
  const int n;
}

> Eventually, I'm looking to specialize the tag for different type and
> letting the compile catch any untagged types.

This only triggers a warning (and a link error), but here's a valid way to get
something like what you want:

template<typename T> extern int kTag;
template<> constexpr int kTag<int> = 0;

-- 
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/20170303/273651ee/attachment.html>