fix: type check for constant arrays

[tserg]

What I did

Fix type checker issue in #2534 and #2130.

The type checking for Subscript VyperNode type currently assumes that an exact type can be derived for its value. Ordinarily, this does not raise an issue for array variables as the value of the Subscript node in this instance would be a Name node type, from which an exact type can be determined.

However, where the array is a constant, the value of the Subscript node is now a List node. When deriving the type for a List node, a list of possible types are returned instead of a specific type (e.g. an array of [0, 1, 2] will return a list of uint8, int128, int256 and uint256). As a result, a StructureException of 'Ambiguous type' will be thrown when there is more than one possible type for an array.

How I did it

Relax the assumption that an exact type can be derived for a Subscript node where its value is a List node:

• In vyper/semantics/validation/annotation.py, the visit_Subscript function is amended to get all possible types where value is a List node. If only one type is returned, it will be assigned as the base type. If more than one value is returned, we iterate through the list of possible types and return the type that matches the type_ argument supplied to this function.
• In vyper/semantics/validation/utils.py, the types_from_Subscript function is amended to return all possible types where value is a List node.

How to verify it

See test cases.

Description for the changelog

Fix type checker issue for constant/literal arrays

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[codecov-commenter]

Codecov Report

Merging #2580 (c73c185) into master (255d74c) will decrease coverage by 0.03%.
The diff coverage is 84.61%.

@@            Coverage Diff             @@
##           master    #2580      +/-   ##
==========================================
- Coverage   85.70%   85.67%   -0.04%     
==========================================
  Files          90       90              
  Lines        9040     9052      +12     
  Branches     2329     2336       +7     
==========================================
+ Hits         7748     7755       +7     
- Misses        812      814       +2     
- Partials      480      483       +3     

Impacted Files	Coverage Δ
vyper/semantics/validation/annotation.py	94.40% <80.00%> (-1.12%)	⬇️
vyper/semantics/validation/utils.py	91.21% <100.00%> (+0.13%)	⬆️
vyper/builtin_functions/functions.py	88.20% <0.00%> (-0.35%)	⬇️

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