#10845: Incorrect behavior when let binding implicit CallStack object -------------------------------------+------------------------------------- Reporter: nitromaster101 | Owner: gridaphobe Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 7.11 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: #10846 | Differential Rev(s): -------------------------------------+------------------------------------- Changes (by gridaphobe): * related: => #10846 Comment: Ok, the problem is that when we infer the type of a let-bound variable, we attempt to solve any constraints generated from the RHS without consulting the given constraints from the surrounding context. Specifically, `TcSimplify.simplifyInfer` tries to solve the wanteds with no givens. This is problematic for CallStacks because of the special rule that allows us to discharge a CallStack IP constraint without a given CallStack, by creating one out of thin air. So in {{{#!haskell f :: (?loc :: CallStack) => [(String, SrcLoc)] f = let y = getCallStack ?loc in y }}} the use of `getCallStack` tells GHC that `?loc :: CallStack`, and GHC immediately discharges the IP constraint without seeing the CallStack given by `f`'s context, while in {{{#!haskell f :: (?loc :: CallStack) => [(String, SrcLoc)] f = let y = ?loc in getCallStack y }}} we end up with a generic `?loc::t` constraint that GHC can't solve, until it emits an implication with `f`'s given constraints. I can work around this by splitting off any IP wanteds at the beginning of `simplifyInfer` and stitching them back onto the final implication constraint that `simplifyInfer` emits, but I'm not sure if this is the right solution. I don't think it should affect regular IPs since we always infer an IP constraint in local binders, but I still need to validate my patch. Also, I believe #10846 is closely related to this issue because `PartialTypeSignatures` triggers a similar code path through `TcBinds.tcPolyInfer` and `TcSimplify.simplifyInfer`, causing us to lose sight of the given IP constraint. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10845#comment:6 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler