More complicated TYPEP tests are marginally transparent to type propagation
Expansions for TYPEP are wrapped in %typep-wrapper, a fancy identity. The
additional arguments track what value is being tested for what type, which
helps inform constraint propagation, as well as detecting redundant type
tests.
Such a wrapper is a hindrance to lower level control flow rewriting that are
essential for decent code generation. After a single pass of constraint
propagation, the wrapper evaporates and the TYPEP expansion becomes as opaque
as it is now.
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