Synchronize ClsInst.doTyConApp with TcTypeable validity checks (#15862)

Issue #15862 (closed) demonstrated examples of type constructors on which TcTypeable.tyConIsTypeable would return False, but the Typeable constraint solver in ClsInst (in particular, doTyConApp) would try to generate Typeable evidence for anyway, resulting in disaster. This incongruity was caused by the fact that doTyConApp was using a weaker validity check than tyConIsTypeable to determine if a type constructor warrants Typeable evidence or not. The solution, perhaps unsurprisingly, is to use tyConIsTypeable in doTyConApp instead.

To avoid import cycles between ClsInst and TcTypeable, I factored out tyConIsTypeable into its own module, TcTypeableValidity.

Fixes #15862 (closed).

added Typeable label

mentioned in issue #15862 (closed)

added 3 commits

• 3f298aa7...5279dda8 - 2 commits from branch ghc:master
• 57c7e898 - Synchronize ClsInst.doTyConApp with TcTypeable validity checks (#15862 (closed))

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Looks good to me.

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Merged in 25ee60cd

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