[cfe-dev] PR21216 and non-type template parameters of reference type

[Richard Smith]

On Tue, Oct 14, 2014 at 1:31 PM, David Blaikie <dblaikie at gmail.com> wrote:

> So I'm looking at addressing PR21216
>

Are you sure that's the right bug number?


> and I've come across a few hiccups:
>
> * I can fix the bug as filed by using TemplateArgument's
> isDeclForReferenceParameter to either create a reference or a pointer type
> for the argument
>
> * But this doesn't include const-ness. How would I differentiate the types
> of the arguments to these template parameters: "template<const int&, int&>"
> ?
>
> * This also doesn't include rvalue ref-ness. How would I differentiate the
> types of the arguments to these template parameters: "template<int &,
> int&&>" ?
>
> * Clang prints the desugared name (which we use in the debug info, but can
> also appear in diagnostics) somewhat strange/incorrectly:
>
> int i;
> template<int& I, int *J, int&& K, const int &L>
> struct foo {
> };
>
> foo<i, &i, i, i> f;
>
> void f1(int);
>
> template<typename T>
> void f2(T t) {
>   f1(t);
> }
>
> int main() {
>   f1(f);
>   f2(f);
> }
>
> Clang produces (just cutting to the notes in question):
> templ.cpp:16:3: error: no matching function for call to 'f1'
>   f1(f);
>   ^~
> templ.cpp:8:6: note: candidate function not viable: no known conversion
> from 'foo<i, &i, i, i>' to 'int' for 1st argument
> void f1(int);
>      ^
> templ.cpp:12:3: error: no matching function for call to 'f1'
>   f1(t);
>   ^~
> templ.cpp:17:3: note: in instantiation of function template specialization
> 'f2<foo<&i, &i, &i, &i> >' requested here
>   f2(f);
>   ^
> templ.cpp:8:6: note: candidate function not viable: no known conversion
> from 'foo<&i, &i, &i, &i>' to 'int' for 1st argument
> void f1(int);
>      ^
>
> Note the desugaring of the template adds '&' to all the parameters, even
> the reference parameters.
>
> GCC gets something that looks ugly but probably correct:
> templ.cpp: In function ‘int main()’:
> templ.cpp:16:7: error: cannot convert ‘foo<(* & i), (& i), (* & i),
> ((const int&)(& i))>’ to ‘int’ for argument ‘1’ to ‘void f1(int)’
>    f1(f);
>        ^
>
> Though, weirdly, GCC doesn't reject the call to f2... which is weird.
>
> Anyway: where should I get the right type for my non-type template
> arguments? Do I have to go spelunking through the template parameters (can
> I get to them from the template arguments) & try to stitch the type
> together? That's probably not ideal...
>

Right now, I think you need the template parameters.


> & what can we do about the type printing for non-type template parameters?
> (I haven't spent too long searching for where those extra '&' are coming
> from yet - but pointers would be appreciated)
>
> Thanks,
> - David
>
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