• Eric Seidel's avatar
    Rework the Implicit CallStack solver to handle local lets. · 3ec8288a
    Eric Seidel authored and Ben Gamari's avatar Ben Gamari committed
    We can't just solve CallStack constraints indiscriminately when they
    occur in the RHS of a let-binder. The top-level given CallStack (if
    any) will not be in scope, so I've re-worked the CallStack solver as
    1. CallStacks are treated like regular IPs unless one of the following
       two rules apply.
    2. In a function call, we push the call-site onto a NEW wanted
       CallStack, which GHC will solve as a regular IP (either directly from a
       given, or by quantifying over it in a local let).
    3. If, after the constraint solver is done, any wanted CallStacks
       remain, we default them to the empty CallStack. This rule exists mainly
       to clean up after rule 2 in a top-level binder with no given CallStack.
    In rule (2) we have to be careful to emit the new wanted with an
    IPOccOrigin instead of an OccurrenceOf origin, so rule (2) doesn't fire
    again. This is a bit shady but I've updated the Note to explain the
    Test Plan: validate
    Reviewers: simonpj, austin, bgamari, hvr
    Reviewed By: simonpj, bgamari
    Subscribers: thomie
    Differential Revision: https://phabricator.haskell.org/D1422
    GHC Trac Issues: #10845
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