SMT2: do not define a symbol by an expression that refers to it

[tautschnig]

smt2_convt::set_to turns the equality of a not yet declared symbol and an expression into a define-fun of that symbol. When the expression refers to the symbol itself, e.g., x == (x & y) | z, the result is a definition that refers to the symbol being defined, which solvers reject ("Symbol 'x' not declared as a variable" in cvc5). Such an equality is a constraint on x, and is now emitted as declare-fun followed by an assert, as it already was for back-ends that use declare-fun throughout.

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[Copilot]

🟢 Approval recommended

The focused fix correctly prevents recursive definitions while preserving existing behavior and includes appropriate regression coverage.

0 open findings

What changed in this PR

Prevents invalid self-referential SMT2 define-fun output by emitting such equalities as declarations plus assertions.

Changes:

• Detects references to the prospective definition symbol.
• Adds unit coverage for self-referential and ordinary equalities.

File	Description
src/​solvers/​smt2/​smt2_conv.cpp	Guards definition generation against self-reference.
unit/​solvers/​smt2/​smt2_conv.cpp	Verifies declaration/assertion and definition paths.

🧠 Review effort: Balanced

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 80.86%. Comparing base (4b26409) to head (ff9415c).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #9179   +/-   ##
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  Coverage    80.85%   80.86%           
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  Files         1717     1717           
  Lines       190161   190181   +20     
  Branches        73       73           
========================================
+ Hits        153759   153782   +23     
+ Misses       36402    36399    -3     

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