Given an input, a.ml:

(** f docs *)
let f = ()

(** g docs *)
let g = ()

let h = g

When the point is on the reference to g, all is well:

$ ocamlmerlin single document -verbosity 0 -position 7:9 < a.ml | jq .value
"g docs"

However, if we change h to be let h = f, the documentation for f is not returned:

$ ocamlmerlin single document -verbosity 0 -position 7:9 < a.ml | jq .value
"No documentation available"

If we remove the documentation comment for g though (note the changed line number to correspond), the documentation for f is found:

$ ocamlmerlin single document -verbosity 1 -position 6:9 < a.ml | jq .value
"f docs"

It seems that, for lookups of things defined in the same file, merlin will not return documentation for any but the final documented definition.

(Originally reported in ocamllabs/vscode-ocaml-platform#519, where @ulugbekna suggested the actual failure pattern.)