fix ungrounded error when destructuring with don't-cares

[quentin]

Name unnamed variables that appear in record and branch inits before the resolution of aliases.

fix #2482

After the ResolveAliasesTransformer, all occurences of t are replaced with [ta,_], and the groundedness constraint system cannot give a bounded value to [ta,_] in the head (var([ta,_])->0 in the Solution).

Clause: pred([ta,_],ta) :-
   fact([ta,_]).
Problem:
{
        var([ta,_]) ⇒ var(ta),
        var([ta,_]) ⇒ var(_),
        var(ta) ∧ var(_) ⇒ var([ta,_]),
        var([ta,_]) is true,
        var([ta,_]) ⇒ var(ta),
        var([ta,_]) ⇒ var(_),
        var(ta) ∧ var(_) ⇒ var([ta,_])
}
Solution:
{var(_)->1,var(_)->0,var([ta,_])->1,var([ta,_])->0,var(ta)->1}

When you provide an explicit variable for the second part of the record, the constraint system can be solved because the second member of the record has is bound by the variable (var(_fix)->1 and var([ta,_fix])->1):

Clause: pred([ta,_fix],ta) :-
   fact([ta,_fix]).
Problem:
{
        var([ta,_fix]) ⇒ var(ta),
        var([ta,_fix]) ⇒ var(_fix),
        var(ta) ∧ var(_fix) ⇒ var([ta,_fix]),
        var([ta,_fix]) is true,
        var([ta,_fix]) ⇒ var(ta),
        var([ta,_fix]) ⇒ var(_fix),
        var(ta) ∧ var(_fix) ⇒ var([ta,_fix])
}
Solution:
{var([ta,_fix])->1,var([ta,_fix])->1,var(ta)->1,var(_fix)->1}

To fix this, we need to transform _ into unique variables before the ResolveAliasesTransformer.

[quentin]

This change allows some form of match/case:

case_adt(t, case) :-
  fact_adt(t),
  ( t = $Some(_,_), case = "some"
  ; t = $None(), case = "none"
  ).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.83%. Comparing base (57f104d) to head (bd052b5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2483      +/-   ##
==========================================
+ Coverage   77.80%   77.83%   +0.03%     
==========================================
  Files         492      492              
  Lines       33189    33205      +16     
==========================================
+ Hits        25822    25846      +24     
+ Misses       7367     7359       -8     

Files	Coverage Δ
src/ast/transform/ResolveAliases.cpp	98.65% <100.00%> (+0.10%)	⬆️

... and 4 files with indirect coverage changes

[julienhenry]

Thank you, this is useful.