[cfe-dev] integer constant expression oh my!

[Doug Gregor]

On Mon, Nov 17, 2008 at 3:10 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Mon, Nov 17, 2008 at 10:47 AM, Doug Gregor <doug.gregor at gmail.com> wrote:
>> We have to instantiate the correct template *declaration* as part of
>> type-checking. We don't have to instantiation the template
>> *definition* until we try to evaluate the value of the expression.
>
> Right... but I'm pretty sure we're still going to instantiate it
> before we try to evaluate the value of the expression.  Take the
> following program:
>
> template <int i> int f(int (*a)[-i]) {
>  int b[i]; return 2;
> }
> int b() {return 0 ? f<-1>(0) : 0;}
>
> I'm pretty sure we're still supposed to emit a diagnostic (at the very
> least, g++ does).

Yes, it emits a diagnostic. However, the diagnostic actually comes
later on, since GCC (like many compilers) delays the instantiation of
function templates until the end of the translation unit. (Or later,
if one is using a prelinker-based approach like the one EDG supports).

>  In -fsyntax-only mode, there isn't anything after
> Sema; therefore, Sema has to force the instantiation of the definition
> at some point.  The only reasonable place to force the instantiation
> of the definition is when Sema processes the call to f<-1>.  This is
> before the call exists in the AST, so the template will be
> instantiated before we might want to evaluate it.

If you add a bit of bogus code that doesn't involve templates to the
end of your example and run it through GCC, you'll see that the error
message for the problem inside f<-1> comes *after* the error messages
for that bogus code. That's the delay in performing the instantiation.
It can be helpful: for one, it means that error messages due to failed
instantiations of function templates don't come from inside the bodies
of other functions that just happen to use the broken template.

> I don't really want to attempt to drag this out before we actually
> have a plan for how we're going to implement templates, but if there
> really is an architectural issue here, I would prefer to solve it
> sooner rather than later.

I don't think it's a major architectural issue, I just think that
we'll eventually have to move the evaluation code to Sema to deal with
constexpr function templates.

  - Doug