AssociatedTypeInference: Make a cycle breaking hack more principled #82159
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slavapestov
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We would return an ErrorType without diagnosing anything from NormalProtocolConformance::getTypeWitnessAndDecl() to signal a transient condition to associated type inference. But other callers that receive an ErrorType don't expect that this can happen, and they in turn won't diagnose anything. So we'd end up in SILGen with a broken AST. Instead, push the cycle break up into getWitnessTypeForMatching(), and remove the hack from getTypeWitnessAndDecl(). If someone else calls getTypeWitnessAndDecl() and that causes a request cycle, we will now diagnose the request cycle via the usual mechanism. Fixes rdar://120388028.
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We would return an ErrorType without diagnosing anything from NormalProtocolConformance::getTypeWitnessAndDecl() to signal a transient condition to associated type inference. But other callers that receive an ErrorType don't expect that this can happen, and they in turn won't diagnose anything. So we'd end up in SILGen with a broken AST.
Instead, push the cycle break up into getWitnessTypeForMatching(), and remove the hack from getTypeWitnessAndDecl(). If someone else calls getTypeWitnessAndDecl() and that causes a request cycle, we will now diagnose the request cycle via the usual mechanism.
Fixes rdar://120388028.