On Wed, Jul 27, 2016 at 8:05 AM, Jonathan Wakely <jwak...@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
