Clean up node kinds in SymbolTableNode

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Assessment

Difficulty
5/5
Estimated time
Over a week
Newbie friendliness
25/100
Issue type
Refactor
Clarity
Needs clarification
Activity status
Stale
Tech stack
python
Domain
compilers

Research direction

Start by reading the SymbolTableNode handling described in the issue and the linked discussion in PR 4080. Map the current LDEF, GDEF, and MDEF kinds, module references, and type-alias attributes before deciding how the proposed SymbolNode subclasses and MypyFile marker fit. Done means the special kind handling and kind attribute are replaced by the proposed scope-based structure without losing type-alias behavior.

Written by the indexing model from the issue text.

Description

priority-0-high refactoring

It seems like we could remove the special handling of module references and other special node kinds in SymbolTableNode so that we could remove the kind attribute and replace it with a scope attribute, with only 3 possible values, corresponding to the current kinds LDEF, GDEF and MDEF. We'd create new SymbolNode subclasses for some of the node kinds. For module references we can perhaps just use a MypyFile target node as a marker. We could move some attributes that are specific to type aliases to live in the new SymbolNode subclass.

From https://github.com/python/mypy/pull/4080#discussion_r143480572:

@ilevkivskyi :

Type aliases are quite complicated (due to several corner cases). I tried to make some simplifications recently. It seems to me that we can introduce a dedicated SybmolNode (like Var or TypeInfo) and deprecate type_override and friends and just point .node to this new symbol node. This is major refactoring, but I think it may help.

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