Re: RFC: Make diagnostics for C++ reference binding more consistent

[Jason Merrill <jason at redhat dot com>]

On Wed, Jul 27, 2016 at 8:05 AM, Jonathan Wakely <jwakely@redhat.com> wrote:
> Consider:
>
> template<typename T> T declval();
>
> int& r1 = declval<int&&>();
> int&& r2 = declval<int&>();
> int& r3 = declval<const int&>();
>
>
> This produces three quite different errors:
>
>
> refs.cc:3:25: error: invalid initialization of non-const reference of type
> 'int&' from an rvalue of type 'int'
> int& r1 = declval<int&&>();
>           ~~~~~~~~~~~~~~^~
> refs.cc:4:25: error: cannot bind 'int' lvalue to 'int&&'
> int&& r2 = declval<int&>();
>            ~~~~~~~~~~~~~^~
> refs.cc:5:30: error: binding 'const int' to reference of type 'int&'
> discards qualifiers
> int& r3 = declval<const int&>();
>           ~~~~~~~~~~~~~~~~~~~^~
>
>
> The first one uses the order to/from, but the other two are from/to.
>
> The first and second are saying the same thing (wrong value category)
> but very differently.
>
> The first and third mention references but the second doesn't.
>
> The standard talks about binding a reference to something, but the
> second and third diagnostics talk about binding something to a
> reference (and the first doesn't mention binding at all).
>
> The first diagnostic is specific to lvalue references (it wouldn't be
> invalid if it was binding a non-const _rvalue_ reference to an
> rvalue), but doesn't use the word "lvalue".
>
> (FWIW Clang and EDG both have their own inconsistencies for the same
> example, but that shouldn't stop us trying to improve.)
>
>
> IMHO the first should use the words "bind" and "lvalue reference":
>
> refs.cc:3:25: error: cannot bind non-const lvalue reference of type 'int&'
> to an rvalue of type 'int'
> int& r1 = declval<int&&>();
>           ~~~~~~~~~~~~~~^~
>
> The second should use the phrasing "bind ref to value" instead of
> "bind value to ref", and mention "rvalue reference":
>
> refs.cc:4:25: error: cannot bind rvalue reference of type 'int&&' to an
> lvalue of type 'int'
> int&& r2 = declval<int&>();
>            ~~~~~~~~~~~~~^~
>
> The third should use the same phrasing (but doesn't need to mention
> lvalue/rvalue because the problem is related to cv-qualifiers not
> value categories):
>
> refs.cc:5:30: error: binding reference of type 'int&' to 'const int'
> discards qualifiers
> int& r3 = declval<const int&>();
>           ~~~~~~~~~~~~~~~~~~~^~
>
>
> I've only considered trivial examples here, is there some reason to
> prefer to current diagnostics for more complex cases?
>
> The attached patch makes that change, but there are probably lots of
> tests that would need updating which I haven't done until I know if
> the change is worthwhile.

Sounds good to me.

Jason