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Re: C++ PATCH: PR c++/17435, c++/18140


Giovanni Bajo wrote:

Mark Mitchell wrote:



* 18140

This was a rejects-valid PR caused by Giovanni's patch to make ">>"
be treated as the end of a template argument, as an attempt at
better error-recovery. That did not work in the case that the ">>"
was actually legitimately interpreted as a shift operator. A
consequence is that one of the tests in g++.dg/template/error10.C
now issues marginally inferior error messages, but correctness
first.



I have troubles understanding why we were handling this before correctly:



template <int N> struct IntHolder { static const int value = N; };

const int N = 4;
const int S = 4;

template <int> struct ShareHolder {
 static const int value = IntHolder< N>>S >::value;
};


Did you check this? It is not that we were stopping parsing expressions at the
first ">>" we saw.


No, not always.

I think the problem is related to the fact that we special-case the fact that a
non-type template parameters is, by itself, a value argument. This is of course
true, but those of integral or enumaration type can also be part of constant
expressions. We were taking care of this by checking if "the next token ends a
template argument", but this predicate does not have a empty intersection set
with "next token is an operator which is part of a constant expression".

In other words, applying this one-liner (after reverting your patchlet) seems
to fix the problem (but it is untested). What do you think of this?

I don't like the fact that ends_template_argument_p accepted ">>". That's going to lead to bugs because the predicate now doesn't do what it's name says.

I don't object to the idea of your proposed fix. But, the check for ">>" should be explicitly inserted *in those places where it is safe*.

--
Mark Mitchell
CodeSourcery, LLC
(916) 791-8304
mark@codesourcery.com


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