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@@            Coverage Diff             @@##             main    #8558      +/-   ##==========================================- Coverage   88.10%   88.09%   -0.02%==========================================  Files         402      402                Lines       14029    14015      -14       Branches     4117     4112       -5     ==========================================- Hits        12360    12346      -14  Misses       1370     1370                Partials      299      299

is this not the case that this assertion is required?
Or are you saying that the logic isn't required because it's never hit?

@bradzacher this assertion is indeed required; however, no special handling is required for it (or, for any tuple types, I believe) in this lint rule becauseservices.getTypeAtLocation(node.expression) === castType returnsfalse.

Alrighty — I still need to clean up the tests a little bit, but I've reviewed (and added tests for):

• the examples (some of which are currently broken) from thecurrent docs, discussed further in#8458
• the examples fromthe original TSLint PR
• and all examples both invalid and valid, from the open issue[no-unnecessary-type-assertion] only checks non-null-assertion #4485

and I am confident that my logic is correct for detecting useful vs useless literal type assertions:

• First, as discussed above, any special logic for tuple type assertions is unnecessary. The base condition ofcastType === uncastType always returns false for any non-primitive type (in other words, this rule seems to do nothing at all for non-primitive types. Something to investigate for the future?)
• The new logic is in thewouldSameTypeBeInferred variable. There are two cases to consider, depending on whethercastType.isLiteral() returnstrue orfalse.
• Note thatcastType.isLiteral() only returnstrue if the type assertion is asingle literal. If it is a union of literals, for example, thencastType.isLiteral() will returnfalse.
• ifcastType.isLiteral() returnstrue, then we know that the same type will be inferred, and therefore we are OK to report, if and only if we are within aconst variable declaration. Thisincludes reporting on anas const of a primitive within aconst variable declaration.
• On the other hand, ifcastType.isLiteral() returns false, then we must be looking at a more complex type, and weshould report (assuming thattypeIsUnchanged is alsotrue)unless this is aconst assertion, in which case, do not report.

Things to investigate for the future:

• Make this rule work for contextual typing (already noted as a TODO)
• Make this rule work for non-primitives (did it ever in the past? unsure)