Dune Flag Propagation and OCaml -w Syntax: Overlapping Range Precedence

Answer first

Precedence for -w is last flag wins in the command line string that reaches the OCaml compiler. If a warning number is covered by multiple overlapping ranges or per-warning prefixes, the last occurrence of a flag that mentions that number determines its final enabled/disabled state. In Dune, flags from dune-project and per-stanza (flags ...) are forwarded to the compiler and appended to Dune's internal command. User-specified flags in a local dune file therefore typically appear later than Dune's own defaults and override them.

Confirmed facts

• The OCaml compiler processes -w arguments linearly left to right. Overlapping ranges are resolved by the last matching specification.
• Dune propagates -w settings from dune-project and dune files to ocamlc/ocamlopt by appending them to the build command.
• Warning numbers are not stable across major OCaml releases. A range such as 1-5 can refer to different diagnostics in different compiler versions.

Likely explanation vs uncertainty

Per-warning prefix syntax like -w:+a -w:-5 is supported in modern OCaml releases, while older releases required a single prefix for the entire list after -w. The exact version boundary for per-warning prefixes is not confirmed in the supplied data and should be verified for your toolchain.

Dune's default warning set is applied by Dune itself. Whether it merges with or is overridden by user flags depends on the order Dune constructs the command. In practice user flags placed in a local stanza appear later and win, but this is implementation-dependent and should be verified.

Steps to avoid conflicts in this case

1. Inspect the actual flags passed to the compiler.

dune build --verbose-json | jq '.[] | select(.target?) | .command[]'

Look for the sequence of -w arguments.

2. Pin the syntax to your compiler version.

ocamlc -version

Test per-warning prefix support with a minimal file:

ocamlc -w:+1-10 -w:-5 -c test.ml

If warning 5 is suppressed, last-wins behavior is active.

3. Structure Dune warnings explicitly.
• Define a single canonical -w string in dune-project or at the library/executable stanza, e.g. (flags (:standard -w +A-4-41-42-44-45-48-70)).
• Avoid mixing broad ranges with later narrow disables across different files. If you need exceptions, place them in the same -w string in the order you want them applied.
• Do not rely on Dune defaults. Override them explicitly so the build is reproducible across machines.

One missing diagnostic detail that changes the recommendation: the exact OCaml version and Dune version in use. Per-warning prefix support and Dune's flag ordering changed over time, so confirming ocaml -version and dune --version is required before locking a portable -w specification.