[patch] Fix PR c++/27547: ICE on invalid operator=

[Gabriel Dos Reis]

Mark Mitchell <mark@codesourcery.com> writes:

[...]

| It's very hard to make the front-end robust if we must constantly
| account for the possible presence of things that make no sense; it
| requires that, as people knowledgeable about what constitutes valid C++,
| we constantly think about the possibly that we've got invalid C++
| around.  IMO, we would be better off just issuing an error about this
| invalid non-member operator, and dropping it on the floor.

Totally agreed.

-- Gaby