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

[David Blaikie]

So I'm looking at addressing PR21216 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...

& 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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