Fix TypeError in IRVisitor when expressions are undefined due to parse errors

[Copilot]

• I have read and I understand the Note on AI-assisted contributions
• Changes manually tested locally and confirmed to work as described
  (including screenshots is helpful)
• Tests added for any new code
• Documentation added for any new functionality
• Entries added to the respective CHANGELOG.md for any new functionality

Description

The IRVisitor crashed with TypeError: Cannot read properties of undefined (reading 'kind') when encountering malformed IR from parse errors. The visitor unconditionally called walkExpression() on fields that can be undefined.

Changes

quint/src/ir/IRVisitor.ts

• Added null checks before walkExpression() calls in three locations:
  • OpDef.expr (line 381)
  • Assume.assumption (line 402)
  • Instance.overrides expressions (line 303)

quint/test/ir/IRVisitor.test.ts

• Added test suite "handles malformed IR gracefully" with coverage for all three cases

Example

Before this change, malformed IR like:

const malformedDef = {
  kind: 'def',
  name: 'badDef',
  expr: undefined  // Can happen with parse errors
}
walkDefinition(visitor, malformedDef) // TypeError

Now handles gracefully by skipping the undefined expression walk.

Original prompt

Eu gostaria que investigasse o seguinte bug e criasse um fiz em inglês mesmo

{
Typecheck Output:

TypeError: Cannot read properties of undefined (reading 'kind')
at walkExpression (/opt/homebrew/Cellar/quint/0.29.1/libexec/lib/node_modules/@informalsystems/quint/dist/src/ir/IRVisitor.js:322:18)
at walkDefinition (/opt/homebrew/Cellar/quint/0.29.1/libexec/lib/node_modules/@informalsystems/quint/dist/src/ir/IRVisitor.js:302:13)
at walkDeclaration (/opt/homebrew/Cellar/quint/0.29.1/libexec/lib/node_modules/@informalsystems/quint/dist/src/ir/IRVisitor.js:201:13)
at /opt/homebrew/Cellar/quint/0.29.1/libexec/lib/node_modules/@informalsystems/quint/dist/src/ir/IRVisitor.js:55:46
at Array.forEach ()
at walkModule (/opt/homebrew/Cellar/quint/0.29.1/libexec/lib/node_modules/@informalsystems/quint/dist/src/ir/IRVisitor.js:55:30)
at NameResolver.enterModule (/opt/homebrew/Cellar/quint/0.29.1/libexec/lib/node_modules/@informalsystems/quint/dist/src/names/resolver.js:68:36)
at walkModule (/opt/homebrew/Cellar/quint/0.29.1/libexec/lib/node_modules/@informalsystems/quint/dist/src/ir/IRVisitor.js:53:17)
at /opt/homebrew/Cellar/quint/0.29.1/libexec/lib/node_modules/@informalsystems/quint/dist/src/names/resolver.js:30:36
at Array.forEach ()

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