goto-symex: assumed pointer equalities must update value set

[tautschnig]

Value sets and constant propagation are our only sources of points-to information when resolving dereferences in goto-symex. Therefore, assuming or branching on equalities of pointer-typed variables must have us consider both of their value sets to be equal. Else we may end up with spurious counterexamples as seen with the enclosed regression test (where we previously did not have any known value for a).

Fixes: #8492

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[codecov]

Codecov Report

Attention: Patch coverage is 64.70588% with 6 lines in your changes missing coverage. Please review.

Project coverage is 78.89%. Comparing base (018c61c) to head (2a27074).

Files with missing lines	Patch %	Lines
src/goto-symex/goto_state.cpp	64.70%	6 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #8494      +/-   ##
===========================================
- Coverage    78.90%   78.89%   -0.01%     
===========================================
  Files         1727     1727              
  Lines       198425   198452      +27     
  Branches     18523    18499      -24     
===========================================
+ Hits        156561   156568       +7     
- Misses       41864    41884      +20     

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[remi-delmas-3000]

What would happen in a situation like this ?

p = malloc(4);// o1
q = malloc(4);// o2
__CPROVER_assume(p == q);
// now p and q have both &o1 and &o2 in their value sets
*p = 1; // what happens here ?
*q = 2; // what happens here ?

[kroening]

This is problematic, and needs a deeper discussion.

I appreciate this is meant as a fix for #8492, but I'd advise to do a contract-specific fix.