Report error instead of ICE for deeply nested constant chains

[tavian-dev]

Summary

Fixes #15739.

When inline assembly references a constant variable that sits at the end of a chain deeper than 256 levels, isConstantVariableRecursive would trigger a solAssert (ICE: "Recursion depth limit reached"). This replaces the assertion with graceful handling: when the depth limit is reached, the variable is signaled as cyclic so the caller reports a proper TypeError 3558: Constant variable is circular. instead of crashing.

Change

In libsolidity/ast/ASTUtils.cpp, the isConstantVariableRecursive visitor's solAssert(_depth < 256) is replaced with:

if (_depth >= 256)
{
    // Signal as cyclic so callers report a proper error instead of an ICE.
    _cycleDetector.run(_variable);
    return;
}

When depth reaches 256, calling _cycleDetector.run(_variable) on the currently-being-processed variable triggers cycle detection (the variable is already in the processing set), causing isConstantVariableRecursive to return true. The TypeChecker then reports the standard "Constant variable is circular" error.

Test plan

• Added syntax test inlineAssembly/deep_constant_chain_err.sol with a 257-variable constant chain that previously triggered the ICE
• All 193 inline assembly syntax tests pass
• All 3552 syntax tests pass
• All 1637 semantic tests pass
• Full build + style check pass locally

[github-actions]

Thank you for your contribution to the Solidity compiler! A team member will follow up shortly.

If you haven't read our contributing guidelines and our review checklist before, please do it now, this makes the reviewing process and accepting your contribution smoother.

If you have any questions or need our help, feel free to post them in the PR or talk to us directly on the #solidity-dev channel on Matrix.

[nikola-matic]

This needs a changelog entry (you can add it as a separate commit), but looks good otherwise.