Pexp_constraint expression locations are ghost

[aantron]

Essentially, why are they ghost? Is this a bug?

This appears to be affecting Bisect_ppx (the coverage tool) in aantron/bisect_ppx#377 (comment). Here is a smaller example than in the issue:

let () =
  if true then
    (() : unit)
  else
    ()

with OCaml 4.12.0 and Bisect_ppx master (or any recent version), this gets instrumented as

Note the lack of point on the expression in the then branch, which is suppressed by having () wrapped in a constraint expression. This is because Bisect currently deliberately avoids instrumenting expressions with ghost locations. If the contraint expression is removed, the remaining () will be instrumented, as expected by users, and as shown by the else branch.

The uninstrumented AST for this program:

[
  structure_item (foo.ml[1,0+0]..[5,47+6])
    Pstr_value Nonrec
    [
      <def>
        pattern (foo.ml[1,0+4]..[1,0+6])
          Ppat_construct "()" (foo.ml[1,0+4]..[1,0+6])
          None
        expression (foo.ml[2,9+2]..[5,47+6])
          Pexp_ifthenelse
          expression (foo.ml[2,9+5]..[2,9+9])
            Pexp_construct "true" (foo.ml[2,9+5]..[2,9+9])
            None
          expression (foo.ml[3,24+4]..[3,24+15]) ghost
            Pexp_constraint
            expression (foo.ml[3,24+5]..[3,24+7])
              Pexp_construct "()" (foo.ml[3,24+5]..[3,24+7])
              None
            core_type (foo.ml[3,24+10]..[3,24+14])
              Ptyp_constr "unit" (foo.ml[3,24+10]..[3,24+14])
              []
          Some
            expression (foo.ml[5,47+4]..[5,47+6])
              Pexp_construct "()" (foo.ml[5,47+4]..[5,47+6])
              None
    ]
]

The ghost on the location of the Pexp_constraint is in question in this issue.

I see a potentially related discussion in #1850.

cc @gkaracha @dorranh

[nojb]

Thanks for the report, this looks like a bug at first sight.