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Eric Seidel authored
We originally wanted CallStacks to be opt-in, but dealing with let binders complicated things, forcing us to infer CallStacks. It turns out that the inference is actually unnecessary though, we can let the wanted CallStacks bubble up to the outer context by refusing to quantify over them. Eventually they'll be solved from a given CallStack or defaulted to the empty CallStack if they reach the top. So this patch prevents GHC from quantifying over CallStacks, getting us back to the original plan. There's a small ugliness to do with PartialTypeSignatures, if the partial theta contains a CallStack constraint, we *do* want to quantify over the CallStack; the user asked us to! Note that this means that foo :: _ => CallStack foo = getCallStack callStack will be an *empty* CallStack, since we won't infer a CallStack for the hole in the theta. I think this is the right move though, since we want CallStacks to be opt-in. One can always write foo :: (HasCallStack, _) => CallStack foo = getCallStack callStack to get the CallStack and still have GHC infer the rest of the theta. Test Plan: ./validate Reviewers: goldfire, simonpj, austin, hvr, bgamari Reviewed By: simonpj, bgamari Subscribers: bitemyapp, thomie Projects: #ghc Differential Revision: https://phabricator.haskell.org/D1912 GHC Trac Issues: #11573
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