[PATCH] c++: Fix hashing and testing for equality of ATOMIC_CONST_EXPRs

[Jason Merrill]

On 2/12/20 5:15 PM, Patrick Palka wrote:
> Two equal atomic constraint expressions do not necessarily share the same tree,
> so we can't assume that two ATOMIC_CONST_EXPRs are equal if and only if they
> point to the same tree.

This is incorrect; comparison of atomic constraints is based on them 
coming from the same actual tokens, which is why we use pointer comparison.

In your concepts-partial-spec7.C, test2 is properly rejected.  If you 
want to be able to write the same constraint in multiple places, you 
need to use a concept.

13.5.1.2:

Two atomic constraints are identical if they are formed from the same 
expression and the targets of the parameter mappings are equivalent 
according to the rules for expressions described in 13.7.6.1.

Jason