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WIP: Allow signatures without bindings

Soham Chowdhury requested to merge evertedsphere/ghc:es/defer-undeclared into master

Builds on !9999, from where I took the implementations of some of the helper functions.

The author of that MR mentioned that they got stuck actually emitting the binding for the type signature, and I wanted to take a crack at it myself following Simon's (very helpful) outline in the original MR, before getting this into a mergeable state. In particular, that means that this MR in its current state doesn't even add the flag that would control whether or not this feature was activated, which I intend to do once I get the feature actually working, which is the part I need some help with.

The current implementation doesn't work; GHC seems to ignore the binding that is generated:

==================== Renamer ====================
Main.x :: Int
Main.main :: IO ()
Main.main = print x
Main.x = Control.Exception.Base.undeclaredError ""#


binds []
sigs []
uses [(Nothing, {}), (Nothing, {})]

Main.hs:6:14: error: [GHC-88464] Variable not in scope: x
  |
6 | main = print x
  |

My best guess is that I need to make the generated RHS for the undeclared binding somehow participate in dependency analysis in rnValBindsRHS, so that GHC can figure out how to wire the references to the undeclared identifier to the newly generated binding, but I'm not sure how to do so. I felt that unioning them with mbinds before the call to depAnalBinds seemed like a reasonable way of doing so, but there I was stumped by the type indices not matching up (HsBindLR GhcPs GhcRn vs HsBindLR GhcRn GhcRn or something similar) and wanted to make sure I was on the right track to begin with. (Is genSimpleFunBind the right way to build the rhses, for instance?)

Edited by Soham Chowdhury

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