Is it legal to write:

   extern thread_local int var;
   template<int& r> void Tpl() { ... }
   void Foo() {
     Tpl<var>();
   }

? N3000 has, in [temp.arg.nontype] p1:

   "A template-argument for a non-type, non-template template-parameter
shall be one of:
    ...
          the address of an ob ject or function with external linkage,
including function templates and function
template-id s but excluding non-static class members, expressed as &
id-expression where the & is
optional if the name refers to a function or array, or if the
corresponding template-parameter is a
reference; or"

thread_local objects have thread storage duration but can also have
external linkage, so it appears this is legal. It's surprising because
it means that uses of &r in Tpl() may or may not be
constant-expressions depending on the template argument and because
non-template uses of a thread_local name immediately bind to the
current thread's instance, rather than delaying the binding to the use
of the reference.

I believe the current standard is implementable, but undesirable. I'd
propose to change "with external linkage" to "with external linkage
and static storage duration".

Jeffrey Yasskin


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