What does the ocamlc -strict-formats option do?

Ecosystem
1

It doesn’t seem documented anywhere except for this one blurb in the -help output.

  -strict-formats  Reject invalid formats accepted by legacy implementations
     (Warning: Invalid formats may behave differently from
      previous OCaml versions, and will become always-rejected
      in future OCaml versions. You should always use this flag
      to detect invalid formats so you can fix them.)

I notice because dune turns it on for compilation by default. What thing’s formats is it referring to? What do these errors look like if you run into one?

2

Here “formats” refers to format strings, as in, those strings passed to functions in the Format and Printf modules of the Stdlib.

3

Oh! Those formats. Thanks!

4

A possibly somewhat silly question but still: which formats exactly are deprecated?

5

Essentially, formats with conflicting or redundant options. For examples,

Format.printf "%04.3d" 0 (* pads with 4 and 3 zeros*);;
Format.printf "%++d" 0 (* redundant + *);;
Format.printf "%2.+3f" 0 (* the precision must be an integer *);;