[Evaluator] Enforce consistent results for cyclic requests #84745
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hamishknight
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slavapestov and
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slavapestov
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Oct 7, 2025
Factor out the common logic from `getPlaceholderGenericSignature`.
Avoid conformance failures and member lookup errors for generic signatures with invalid same-type requirements.
This helps avoid producing more downstream errors. This changes `GenericSignature::forInvalid` to produce the same signature as e.g `<T where T == Undefined>`. This subsumes the need to introduce conformance requirements for invertible protocols.
Record when we encounter a request cycle, and enforce that the outer step of the cycle also returns the default value. This fixes a couple of crashers where we were ending up with conflicting values depending on whether the request was queried from within the cycle or from outside it.
Remove the now-unnecessary hack where we could build recursive ErrorTypes.
Avoid type-checking the `where` clause when computing the interface type of a variable bound in the pattern of the loop as otherwise that will trigger cycles and give us an ErrorType result.
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Record when we encounter a request cycle, and enforce that the outer step of the cycle also returns the default value. This fixes a couple of crashers where we were ending up with conflicting values depending on whether the request was queried from within the cycle or from outside it. This also allows us to flatten recursive ErrorTypes, which were previously load-bearing.
To help avoid additional downstream diagnostics for recursive generic signature computation, this PR also changes the placeholder generic signature logic to use same-type ErrorType requirements, and adds a checks to avoid downstream diagnostics in type resolution and conformance checking.
Resolves #57045