Make --MLish --lax safer
#3123
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Make --MLish --lax safer
#3123
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This gives us more type safety in the compiler, since otherwise _any_ type equalities that could potentially be proven by SMT are accepted (since we are --lax too). This means for instance that every polymorphic variable can unify with a concrete type, and the typechecker would have accepted it. By running with smt_ok=false, the unifier will simply reject it earlier.
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We've usually been bitten in compiler code by running in lax mode, since the unifier could "accept" an equality such as
'a = int(where'ais some fresh polymorphic variable), since it would generate an SMT guard for it that would immediately be dropped. This leads to hard to find crashes when it does happen.This PR prevents the unifier from generating these equalities in
--MLishmode, and failing early instead. We had all the mechanism already implemented: thesmt_okflag, which we just force to false if we're using--MLish.This uncovers two typing errors in compiler code!
cc @nikswamy who had the idea