Better error reporting for inaccessible code

This patch fixes Trac #15558.  There turned out to be
two distinct problems

* In TcExpr.tc_poly_expr_nc we had

    tc_poly_expr_nc (L loc expr) res_ty
      = do { traceTc "tcPolyExprNC" (ppr res_ty)
           ; (wrap, expr')
               <- tcSkolemiseET GenSigCtxt res_ty $ \ res_ty ->
                  setSrcSpan loc $
                    -- NB: setSrcSpan *after* skolemising,
                    -- so we get better skolem locations
                  tcExpr expr res_ty

  Putting the setSrcSpan inside the tcSkolemise means that
  the location on the Implication constraint is the /call/
  to the function rather than the /argument/ to the call,
  and that is really quite wrong.

  I don't know what Richard's comment NB means -- I moved the
  setSrcSpan outside, and the "binding site" info in error
  messages actually improved.

  The reason I found this is that it affects the span reported
  for Trac #15558.

* In TcErrors.mkGivenErrorReporter we carefully munge the location
  for an insoluble Given constraint (Note [Inaccessible code]).
  But the 'implic' passed in wasn't necesarily the immediately-
  enclosing implication -- but for location-munging purposes
  it jolly well should be.

  Solution: use the innermost implication. This actually
  simplifies the code -- no need to pass an implication in to
  mkGivenErrorReporter.

0 files changed, 0 insertions, 0 deletions