Simon Peyton Jones authored Jan 23, 2026 and Marge Bot committed Jan 28, 2026

There are two main changes here:

* Use `isSolvedWC` rather than `isEmptyWC` in `tryShortCutSolver`
  The residual constraint may have some fully-solved, but
  still-there implications, and we don't want them to abort short
  cut solving!  That bug caused #26805.

* In the short-cut solver, we abandon the fully-solved residual
  constraint; but we may thereby lose track of Givens that are
  needed, and either report them as redundant or prune evidence
  bindings that are in fact needed.

  This bug stopped the `constraints` package from compiling;
  see the trail in !15389.

The second bug led me to (another) significant refactoring
of the mechanism for tracking needed EvIds.   See the new
   Note [Tracking needed EvIds] in GHC.Tc.Solver.Solve
It's simpler and much less head-scratchy now.

Some particulars:

* An EvBindsVar now tracks NeededEvIds

* We deal with NeededEvIds for an implication only when it is
  fully solved.  Much simpler!

* `tryShortCutTcS` now takes a `TcM WantedConstraints` rather than
  `TcM Bool`, so that is can plumb the needed EvIds correctly.

* Remove `ic_need` and `ic_need_implic` from Implication (hooray),
  and add `ics_dm` and `ics_non_dm` to `IC_Solved`.

Pure refactor
* Shorten data constructor `CoercionHole` to `CH`, following
  general practice in GHC.
* Rename `EvBindMap` to `EvBindsMap` for consistency