dune build --profile=release
         ppx gig.pp.ml (exit 2)
(cd _build/default && .ppx/0821ca0fdc728d392e71f384bacfe25b/ppx.exe -o gig.pp.ml --impl gig.ml -corrected-suffix .ppx-corrected -diff-cmd - -dump-ast)
Fatal error: exception File "gig.ml", line 24, characters 4-8: let operators are not supported before OCaml 4.08
Raised at file "src/migrate_parsetree_408_407_migrate.ml", line 7, characters 2-49
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 57, characters 8-40
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 75, characters 10-30
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 57, characters 8-40
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 75, characters 10-30
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 57, characters 8-40
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 84, characters 10-30
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 57, characters 8-40
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 253, characters 8-34
Called from file "list.ml", line 92, characters 20-23
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 74, characters 10-42
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 57, characters 8-40
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 84, characters 10-30
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 57, characters 8-40
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 253, characters 8-34
Called from file "list.ml", line 92, characters 20-23
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 494, characters 10-42
Called from file "src/migrate_parsetree_408_407_migrate.ml", line 478, characters 8-44
Called from file "list.ml", line 92, characters 20-23
Called from file "list.ml", line 92, characters 32-39
Called from file "list.ml", line 92, characters 32-39
Called from file "list.ml", line 92, characters 32-39
Called from file "list.ml", line 92, characters 32-39
Called from file "list.ml", line 92, characters 32-39
Called from file "src/migrate_parsetree_versions.ml" (inlined), line 356, characters 22-27
Called from file "src/migrate_parsetree_versions.ml", line 360, characters 31-86
Called from file "src/migrate_parsetree_versions.ml" (inlined), line 356, characters 22-27
Called from file "src/migrate_parsetree_versions.ml", line 360, characters 31-86
Called from file "src/migrate_parsetree_driver.ml", line 248, characters 21-67
Called from file "src/migrate_parsetree_driver.ml", line 255, characters 4-82
Called from file "src/driver.ml", line 893, characters 53-82
Called from file "src/driver.ml", line 1292, characters 6-24
Re-raised at file "parsing/location.ml", line 898, characters 14-25
Called from file "src/driver.ml", line 1295, characters 4-59
Called from file ".ppx/0821ca0fdc728d392e71f384bacfe25b/_ppx.ml", line 1, characters 9-36
*** Error code 1