RequirementMachine: Fix request evaluator cycle if a typealias requirement becomes redundant #42043
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…tingTypealias() Diagnostics are emitted from weird places sometimes, and as a result the emission is fragile for two reasons: - Kicking off requests can trigger cycles - Doing anything that can re-entrantly diagnose something else might crash In this case, the problem is getUnderlyingType(), which might kick off type resolution on the typealias. Since we already have a Type, just check that instead of getting a new one.
A protocol typealias 'typealias T = X' introduces a same-type requirement 'Self.T == Self.X'. However the right hand side of the requirement was wrapped in a sugared TypeAliasType. This meant if the requirement surfaced in a redundancy diagnostic, it would print as 'Self.T == Self.T'. It could also trigger a request cycle via isInterestingTypealias() in DiagnosticEngine.cpp.
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If a typealias introduces a same-type requirement that is redundant (see the test case), we would produce a diagnostic with a sugared TypeAliasType. The diagnostic engine would call getUnderlyingType() on this typealias, which triggers computing the requirement signature of the protocol we're currently diagnosing redundancies for.
This sort of thing is a fundamental problem with our diagnostics engine, but we can re-arrange things a little to avoid this scenario in this test case at least, by unwrapping the TypeAliasType when introducing implied same-type requirements from protocol typealiases.