[kddejong]

Fixes #4693

Fn::Select with a hardcoded index that is greater than or equal to the length of a hardcoded list passed linting with 0 errors, then failed at deploy time:

Template error: Fn::Select cannot select nonexistent value at index 5

Root cause

select() in _resolvers_cfn.py simply continues when the index is out of bounds, yielding nothing. Even if it emitted a resolution error, BaseFn.resolve() short-circuits when the target property's schema has no constraining keywords (_has_schema_constraints), so an out-of-bounds !Select in an unconstrained context (e.g. an Outputs value) would still pass silently.

Fix

Detect the out-of-bounds case directly in the E1017 rule (Select.py). The length of a literal list is fixed at authoring time — functions used as list items still each count as one element — so a hardcoded, non-negative integer index >= len(list) is always a definite deploy-time failure. Because the check lives in the rule (not the resolver), it fires regardless of the target property's schema.

Only fires when both the index is a hardcoded non-negative integer and the list is a hardcoded array; a Ref/function index or a function-produced list is left alone (it can resolve to anything at deploy time).

$ cfn-lint template.yaml
E1017 5 is not a valid index for a list of length 2
template.yaml:7:7

Tests

• Updated an existing case that incorrectly asserted an out-of-bounds !Select [1, ["bar"]] produced no error.
• Added cases for a plain literal out-of-bounds index and one where the list contains functions as items.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.65%. Comparing base (3a3a240) to head (6458c43).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4695   +/-   ##
=======================================
  Coverage   94.64%   94.65%           
=======================================
  Files         434      434           
  Lines       15543    15558   +15     
  Branches     3015     3018    +3     
=======================================
+ Hits        14711    14726   +15     
  Misses        454      454           
  Partials      378      378           

Flag	Coverage Δ
unittests	94.65% <100.00%> (+<0.01%)	⬆️

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