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

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Mar 2 22:56:49 PST 2017


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

> 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;

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