• Simon Peyton Jones's avatar
    Better error reporting for inaccessible code · ff29fc84
    Simon Peyton Jones authored
    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
TcErrors.hs 127 KB