5.2.0~alpha1: Issues with (x : Subtype.t list :> Type.t list) in presence of multiple destructive substitutions

[kit-ty-kate]

When testing 5.2.0~alpha1 against opam-repository i saw these two Janestreet packages that compiles just fine on 5.1 but fails with that 5.2 release:

#=== ERROR while compiling ecaml.v0.16.0 ======================================#
# context              2.2.0~beta2~dev | linux/x86_64 | ocaml-variants.5.2.0+trunk | file:///home/opam/opam-repository
# path                 ~/.opam/5.2/.opam-switch/build/ecaml.v0.16.0
# command              ~/.opam/5.2/bin/dune build -p ecaml -j 1
# exit-code            1
# env-file             ~/.opam/log/ecaml-19-467667.env
# output-file          ~/.opam/log/ecaml-19-467667.out
### output ###
# (cd _build/default && /home/opam/.opam/5.2/bin/ocamlc.opt -w -40 -g -bin-annot -I ecaml_value/src/.ecaml_value.objs/byte -I /home/opam/.opam/5.2/lib/async_kernel -I /home/opam/.opam/5.2/lib/async_kernel/config -I /home/opam/.opam/5.2/lib/async_kernel/eager_deferred -I /home/opam/.opam/5.2/lib/async_kernel/read_write_pair -I /home/opam/.opam/5.2/lib/async_unix -I /home/opam/.opam/5.2/lib/async_unix/thread_pool -I /home/opam/.opam/5.2/lib/async_unix/thread_safe_ivar -I /home/opam/.opam/5.2/lib/base -I /home/opam/.opam/5.2/lib/base/base_internalhash_types -I /home/opam/.opam/5.2/lib/base/caml -I /home/opam/.opam/5.2/lib/base/md5 -I /home/opam/.opam/5.2/lib/base/shadow_stdlib -I /home/opam/.opam/5.2/lib/base_bigstring -I /home/opam/.opam/5.2/lib/base_quickcheck -I /home/opam/.opam/5.2/lib/base_quickcheck/ppx_quickcheck/runtime -I /home/opam/.opam/5.2/lib/bin_prot -I /home/opam/.opam/5.2/lib/bin_prot/shape -I /home/opam/.opam/5.2/lib/core -I /home/opam/.opam/5.2/lib/core/base_for_tests -I /home/opam/.opam/5.2/lib/core/command -I /home/opam/.opam/5.2/lib/core/filename_base -I /home/opam/.opam/5.2/lib/core/heap_block -I /home/opam/.opam/5.2/lib/core/univ_map -I /home/opam/.opam/5.2/lib/core/validate -I /home/opam/.opam/5.2/lib/core_kernel/bounded_int_table -I /home/opam/.opam/5.2/lib/core_kernel/caml_threads -I /home/opam/.opam/5.2/lib/core_kernel/caml_unix -I /home/opam/.opam/5.2/lib/core_kernel/enum -I /home/opam/.opam/5.2/lib/core_kernel/flags -I /home/opam/.opam/5.2/lib/core_kernel/iobuf -I /home/opam/.opam/5.2/lib/core_kernel/moption -I /home/opam/.opam/5.2/lib/core_kernel/pairing_heap -I /home/opam/.opam/5.2/lib/core_kernel/sexp_hidden_in_test -I /home/opam/.opam/5.2/lib/core_kernel/thread_pool_cpu_affinity -I /home/opam/.opam/5.2/lib/core_kernel/thread_safe_queue -I /home/opam/.opam/5.2/lib/core_kernel/timing_wheel -I /home/opam/.opam/5.2/lib/core_kernel/tuple_pool -I /home/opam/.opam/5.2/lib/core_kernel/univ -I /home/opam/.opam/5.2/lib/core_kernel/uopt -I /home/opam/.opam/5.2/lib/core_kernel/uuid -I /home/opam/.opam/5.2/lib/core_unix -I /home/opam/.opam/5.2/lib/core_unix/bigstring_unix -I /home/opam/.opam/5.2/lib/core_unix/core_thread -I /home/opam/.opam/5.2/lib/core_unix/error_checking_mutex -I /home/opam/.opam/5.2/lib/core_unix/filename_unix -I /home/opam/.opam/5.2/lib/core_unix/iobuf_unix -I /home/opam/.opam/5.2/lib/core_unix/linux_ext -I /home/opam/.opam/5.2/lib/core_unix/nano_mutex -I /home/opam/.opam/5.2/lib/core_unix/ocaml_c_utils -I /home/opam/.opam/5.2/lib/core_unix/signal_unix -I /home/opam/.opam/5.2/lib/core_unix/squeue -I /home/opam/.opam/5.2/lib/core_unix/sys_unix -I /home/opam/.opam/5.2/lib/core_unix/time_float_unix -I /home/opam/.opam/5.2/lib/core_unix/time_ns_unix -I /home/opam/.opam/5.2/lib/core_unix/time_stamp_counter -I /home/opam/.opam/5.2/lib/core_unix/uuid -I /home/opam/.opam/5.2/lib/fieldslib -I /home/opam/.opam/5.2/lib/int_repr -I /home/opam/.opam/5.2/lib/jane-street-headers -I /home/opam/.opam/5.2/lib/ocaml/threads -I /home/opam/.opam/5.2/lib/ocaml/unix -I /home/opam/.opam/5.2/lib/ocaml_intrinsics -I /home/opam/.opam/5.2/lib/parsexp -I /home/opam/.opam/5.2/lib/ppx_assert/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_bench/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_compare/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_enumerate/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_expect/collector -I /home/opam/.opam/5.2/lib/ppx_expect/common -I /home/opam/.opam/5.2/lib/ppx_expect/config -I /home/opam/.opam/5.2/lib/ppx_expect/config_types -I /home/opam/.opam/5.2/lib/ppx_hash/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_here/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_inline_test/config -I /home/opam/.opam/5.2/lib/ppx_inline_test/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_log/types -I /home/opam/.opam/5.2/lib/ppx_module_timer/runtime -I /home/opam/.opam/5.2/lib/ppx_sexp_conv/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_stable_witness/runtime -I /home/opam/.opam/5.2/lib/ppx_stable_witness/stable_witness -I /home/opam/.opam/5.2/lib/sexplib -I /home/opam/.opam/5.2/lib/sexplib/unix -I /home/opam/.opam/5.2/lib/sexplib0 -I /home/opam/.opam/5.2/lib/spawn -I /home/opam/.opam/5.2/lib/splittable_random -I /home/opam/.opam/5.2/lib/stdio -I /home/opam/.opam/5.2/lib/time_now -I /home/opam/.opam/5.2/lib/timezone -I /home/opam/.opam/5.2/lib/typerep -I /home/opam/.opam/5.2/lib/variantslib -I nested_profile/src/.nested_profile.objs/byte -intf-suffix .ml -no-alias-deps -open Ecaml_value__ -o ecaml_value/src/.ecaml_value.objs/byte/ecaml_value__Form.cmo -c -impl ecaml_value/src/form.pp.ml)
# File "ecaml_value/src/form.ml", line 84, characters 19-54:
# 84 |       |> (fun x -> (x : Symbol.t list :> Value.t list))
#                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# Error: This expression has type "Ecaml_value__.Value.t list"
#        but an expression was expected of type "Ecaml_value__.Symbol.t list"
#        Type "Ecaml_value__.Value.t" = "Ecaml_value__Value0.t"
#        is not compatible with type
#          "Ecaml_value__.Symbol.t" = "Ecaml_value__Symbol.t"
# (cd _build/default && /home/opam/.opam/5.2/bin/ocamlopt.opt -w -40 -g -I ecaml_value/src/.ecaml_value.objs/byte -I ecaml_value/src/.ecaml_value.objs/native -I /home/opam/.opam/5.2/lib/async_kernel -I /home/opam/.opam/5.2/lib/async_kernel/config -I /home/opam/.opam/5.2/lib/async_kernel/eager_deferred -I /home/opam/.opam/5.2/lib/async_kernel/read_write_pair -I /home/opam/.opam/5.2/lib/async_unix -I /home/opam/.opam/5.2/lib/async_unix/thread_pool -I /home/opam/.opam/5.2/lib/async_unix/thread_safe_ivar -I /home/opam/.opam/5.2/lib/base -I /home/opam/.opam/5.2/lib/base/base_internalhash_types -I /home/opam/.opam/5.2/lib/base/caml -I /home/opam/.opam/5.2/lib/base/md5 -I /home/opam/.opam/5.2/lib/base/shadow_stdlib -I /home/opam/.opam/5.2/lib/base_bigstring -I /home/opam/.opam/5.2/lib/base_quickcheck -I /home/opam/.opam/5.2/lib/base_quickcheck/ppx_quickcheck/runtime -I /home/opam/.opam/5.2/lib/bin_prot -I /home/opam/.opam/5.2/lib/bin_prot/shape -I /home/opam/.opam/5.2/lib/core -I /home/opam/.opam/5.2/lib/core/base_for_tests -I /home/opam/.opam/5.2/lib/core/command -I /home/opam/.opam/5.2/lib/core/filename_base -I /home/opam/.opam/5.2/lib/core/heap_block -I /home/opam/.opam/5.2/lib/core/univ_map -I /home/opam/.opam/5.2/lib/core/validate -I /home/opam/.opam/5.2/lib/core_kernel/bounded_int_table -I /home/opam/.opam/5.2/lib/core_kernel/caml_threads -I /home/opam/.opam/5.2/lib/core_kernel/caml_unix -I /home/opam/.opam/5.2/lib/core_kernel/enum -I /home/opam/.opam/5.2/lib/core_kernel/flags -I /home/opam/.opam/5.2/lib/core_kernel/iobuf -I /home/opam/.opam/5.2/lib/core_kernel/moption -I /home/opam/.opam/5.2/lib/core_kernel/pairing_heap -I /home/opam/.opam/5.2/lib/core_kernel/sexp_hidden_in_test -I /home/opam/.opam/5.2/lib/core_kernel/thread_pool_cpu_affinity -I /home/opam/.opam/5.2/lib/core_kernel/thread_safe_queue -I /home/opam/.opam/5.2/lib/core_kernel/timing_wheel -I /home/opam/.opam/5.2/lib/core_kernel/tuple_pool -I /home/opam/.opam/5.2/lib/core_kernel/univ -I /home/opam/.opam/5.2/lib/core_kernel/uopt -I /home/opam/.opam/5.2/lib/core_kernel/uuid -I /home/opam/.opam/5.2/lib/core_unix -I /home/opam/.opam/5.2/lib/core_unix/bigstring_unix -I /home/opam/.opam/5.2/lib/core_unix/core_thread -I /home/opam/.opam/5.2/lib/core_unix/error_checking_mutex -I /home/opam/.opam/5.2/lib/core_unix/filename_unix -I /home/opam/.opam/5.2/lib/core_unix/iobuf_unix -I /home/opam/.opam/5.2/lib/core_unix/linux_ext -I /home/opam/.opam/5.2/lib/core_unix/nano_mutex -I /home/opam/.opam/5.2/lib/core_unix/ocaml_c_utils -I /home/opam/.opam/5.2/lib/core_unix/signal_unix -I /home/opam/.opam/5.2/lib/core_unix/squeue -I /home/opam/.opam/5.2/lib/core_unix/sys_unix -I /home/opam/.opam/5.2/lib/core_unix/time_float_unix -I /home/opam/.opam/5.2/lib/core_unix/time_ns_unix -I /home/opam/.opam/5.2/lib/core_unix/time_stamp_counter -I /home/opam/.opam/5.2/lib/core_unix/uuid -I /home/opam/.opam/5.2/lib/fieldslib -I /home/opam/.opam/5.2/lib/int_repr -I /home/opam/.opam/5.2/lib/jane-street-headers -I /home/opam/.opam/5.2/lib/ocaml/threads -I /home/opam/.opam/5.2/lib/ocaml/unix -I /home/opam/.opam/5.2/lib/ocaml_intrinsics -I /home/opam/.opam/5.2/lib/parsexp -I /home/opam/.opam/5.2/lib/ppx_assert/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_bench/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_compare/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_enumerate/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_expect/collector -I /home/opam/.opam/5.2/lib/ppx_expect/common -I /home/opam/.opam/5.2/lib/ppx_expect/config -I /home/opam/.opam/5.2/lib/ppx_expect/config_types -I /home/opam/.opam/5.2/lib/ppx_hash/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_here/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_inline_test/config -I /home/opam/.opam/5.2/lib/ppx_inline_test/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_log/types -I /home/opam/.opam/5.2/lib/ppx_module_timer/runtime -I /home/opam/.opam/5.2/lib/ppx_sexp_conv/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_stable_witness/runtime -I /home/opam/.opam/5.2/lib/ppx_stable_witness/stable_witness -I /home/opam/.opam/5.2/lib/sexplib -I /home/opam/.opam/5.2/lib/sexplib/unix -I /home/opam/.opam/5.2/lib/sexplib0 -I /home/opam/.opam/5.2/lib/spawn -I /home/opam/.opam/5.2/lib/splittable_random -I /home/opam/.opam/5.2/lib/stdio -I /home/opam/.opam/5.2/lib/time_now -I /home/opam/.opam/5.2/lib/timezone -I /home/opam/.opam/5.2/lib/typerep -I /home/opam/.opam/5.2/lib/variantslib -I nested_profile/src/.nested_profile.objs/byte -I nested_profile/src/.nested_profile.objs/native -intf-suffix .ml -no-alias-deps -open Ecaml_value__ -o ecaml_value/src/.ecaml_value.objs/native/ecaml_value__Form.cmx -c -impl ecaml_value/src/form.pp.ml)
# File "ecaml_value/src/form.ml", line 84, characters 19-54:
# 84 |       |> (fun x -> (x : Symbol.t list :> Value.t list))
#                         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# Error: This expression has type "Ecaml_value__.Value.t list"
#        but an expression was expected of type "Ecaml_value__.Symbol.t list"
#        Type "Ecaml_value__.Value.t" = "Ecaml_value__Value0.t"
#        is not compatible with type
#          "Ecaml_value__.Symbol.t" = "Ecaml_value__Symbol.t"

#=== ERROR while compiling file_path.v0.16.0 ==================================#
# context              2.2.0~beta2~dev | linux/x86_64 | ocaml-variants.5.2.0+trunk | file:///home/opam/opam-repository
# path                 ~/.opam/5.2/.opam-switch/build/file_path.v0.16.0
# command              ~/.opam/5.2/bin/dune build -p file_path -j 1
# exit-code            1
# env-file             ~/.opam/log/file_path-19-53c44e.env
# output-file          ~/.opam/log/file_path-19-53c44e.out
### output ###
# (cd _build/default && /home/opam/.opam/5.2/bin/ocamlc.opt -w -40 -g -bin-annot -I src/.file_path.objs/byte -I /home/opam/.opam/5.2/lib/base -I /home/opam/.opam/5.2/lib/base/base_internalhash_types -I /home/opam/.opam/5.2/lib/base/caml -I /home/opam/.opam/5.2/lib/base/md5 -I /home/opam/.opam/5.2/lib/base/shadow_stdlib -I /home/opam/.opam/5.2/lib/base_bigstring -I /home/opam/.opam/5.2/lib/base_quickcheck -I /home/opam/.opam/5.2/lib/base_quickcheck/ppx_quickcheck/runtime -I /home/opam/.opam/5.2/lib/bin_prot -I /home/opam/.opam/5.2/lib/bin_prot/shape -I /home/opam/.opam/5.2/lib/core -I /home/opam/.opam/5.2/lib/core/base_for_tests -I /home/opam/.opam/5.2/lib/core/command -I /home/opam/.opam/5.2/lib/core/filename_base -I /home/opam/.opam/5.2/lib/core/heap_block -I /home/opam/.opam/5.2/lib/core/univ_map -I /home/opam/.opam/5.2/lib/core/validate -I /home/opam/.opam/5.2/lib/core_kernel/nonempty_list -I /home/opam/.opam/5.2/lib/core_kernel/reversed_list -I /home/opam/.opam/5.2/lib/fieldslib -I /home/opam/.opam/5.2/lib/int_repr -I /home/opam/.opam/5.2/lib/jane-street-headers -I /home/opam/.opam/5.2/lib/parsexp -I /home/opam/.opam/5.2/lib/ppx_assert/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_bench/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_compare/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_enumerate/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_expect/collector -I /home/opam/.opam/5.2/lib/ppx_expect/common -I /home/opam/.opam/5.2/lib/ppx_expect/config -I /home/opam/.opam/5.2/lib/ppx_expect/config_types -I /home/opam/.opam/5.2/lib/ppx_hash/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_here/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_inline_test/config -I /home/opam/.opam/5.2/lib/ppx_inline_test/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_log/types -I /home/opam/.opam/5.2/lib/ppx_module_timer/runtime -I /home/opam/.opam/5.2/lib/ppx_sexp_conv/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_stable_witness/runtime -I /home/opam/.opam/5.2/lib/ppx_stable_witness/stable_witness -I /home/opam/.opam/5.2/lib/sexplib -I /home/opam/.opam/5.2/lib/sexplib0 -I /home/opam/.opam/5.2/lib/splittable_random -I /home/opam/.opam/5.2/lib/stdio -I /home/opam/.opam/5.2/lib/time_now -I /home/opam/.opam/5.2/lib/typerep -I /home/opam/.opam/5.2/lib/variantslib -intf-suffix .ml -no-alias-deps -open File_path__ -o src/.file_path.objs/byte/file_path__Relative.cmo -c -impl src/relative.pp.ml)
# File "src/relative.ml", line 20, characters 19-39:
# 20 | let of_part part = (part : Part.t :> t)
#                         ^^^^^^^^^^^^^^^^^^^^
# Error: This expression has type "t" = "File_path__Types.Relative.t"
#        but an expression was expected of type
#          "File_path__.Part.t" = "File_path__Types.Part.t"
# (cd _build/default && /home/opam/.opam/5.2/bin/ocamlopt.opt -w -40 -g -I src/.file_path.objs/byte -I src/.file_path.objs/native -I /home/opam/.opam/5.2/lib/base -I /home/opam/.opam/5.2/lib/base/base_internalhash_types -I /home/opam/.opam/5.2/lib/base/caml -I /home/opam/.opam/5.2/lib/base/md5 -I /home/opam/.opam/5.2/lib/base/shadow_stdlib -I /home/opam/.opam/5.2/lib/base_bigstring -I /home/opam/.opam/5.2/lib/base_quickcheck -I /home/opam/.opam/5.2/lib/base_quickcheck/ppx_quickcheck/runtime -I /home/opam/.opam/5.2/lib/bin_prot -I /home/opam/.opam/5.2/lib/bin_prot/shape -I /home/opam/.opam/5.2/lib/core -I /home/opam/.opam/5.2/lib/core/base_for_tests -I /home/opam/.opam/5.2/lib/core/command -I /home/opam/.opam/5.2/lib/core/filename_base -I /home/opam/.opam/5.2/lib/core/heap_block -I /home/opam/.opam/5.2/lib/core/univ_map -I /home/opam/.opam/5.2/lib/core/validate -I /home/opam/.opam/5.2/lib/core_kernel/nonempty_list -I /home/opam/.opam/5.2/lib/core_kernel/reversed_list -I /home/opam/.opam/5.2/lib/fieldslib -I /home/opam/.opam/5.2/lib/int_repr -I /home/opam/.opam/5.2/lib/jane-street-headers -I /home/opam/.opam/5.2/lib/parsexp -I /home/opam/.opam/5.2/lib/ppx_assert/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_bench/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_compare/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_enumerate/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_expect/collector -I /home/opam/.opam/5.2/lib/ppx_expect/common -I /home/opam/.opam/5.2/lib/ppx_expect/config -I /home/opam/.opam/5.2/lib/ppx_expect/config_types -I /home/opam/.opam/5.2/lib/ppx_hash/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_here/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_inline_test/config -I /home/opam/.opam/5.2/lib/ppx_inline_test/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_log/types -I /home/opam/.opam/5.2/lib/ppx_module_timer/runtime -I /home/opam/.opam/5.2/lib/ppx_sexp_conv/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_stable_witness/runtime -I /home/opam/.opam/5.2/lib/ppx_stable_witness/stable_witness -I /home/opam/.opam/5.2/lib/sexplib -I /home/opam/.opam/5.2/lib/sexplib0 -I /home/opam/.opam/5.2/lib/splittable_random -I /home/opam/.opam/5.2/lib/stdio -I /home/opam/.opam/5.2/lib/time_now -I /home/opam/.opam/5.2/lib/typerep -I /home/opam/.opam/5.2/lib/variantslib -intf-suffix .ml -no-alias-deps -open File_path__ -o src/.file_path.objs/native/file_path__Relative.cmx -c -impl src/relative.pp.ml)
# File "src/relative.ml", line 20, characters 19-39:
# 20 | let of_part part = (part : Part.t :> t)
#                         ^^^^^^^^^^^^^^^^^^^^
# Error: This expression has type "t" = "File_path__Types.Relative.t"
#        but an expression was expected of type
#          "File_path__.Part.t" = "File_path__Types.Part.t"
# (cd _build/default && /home/opam/.opam/5.2/bin/ocamlc.opt -w -40 -g -bin-annot -I src/.file_path.objs/byte -I /home/opam/.opam/5.2/lib/base -I /home/opam/.opam/5.2/lib/base/base_internalhash_types -I /home/opam/.opam/5.2/lib/base/caml -I /home/opam/.opam/5.2/lib/base/md5 -I /home/opam/.opam/5.2/lib/base/shadow_stdlib -I /home/opam/.opam/5.2/lib/base_bigstring -I /home/opam/.opam/5.2/lib/base_quickcheck -I /home/opam/.opam/5.2/lib/base_quickcheck/ppx_quickcheck/runtime -I /home/opam/.opam/5.2/lib/bin_prot -I /home/opam/.opam/5.2/lib/bin_prot/shape -I /home/opam/.opam/5.2/lib/core -I /home/opam/.opam/5.2/lib/core/base_for_tests -I /home/opam/.opam/5.2/lib/core/command -I /home/opam/.opam/5.2/lib/core/filename_base -I /home/opam/.opam/5.2/lib/core/heap_block -I /home/opam/.opam/5.2/lib/core/univ_map -I /home/opam/.opam/5.2/lib/core/validate -I /home/opam/.opam/5.2/lib/core_kernel/nonempty_list -I /home/opam/.opam/5.2/lib/core_kernel/reversed_list -I /home/opam/.opam/5.2/lib/fieldslib -I /home/opam/.opam/5.2/lib/int_repr -I /home/opam/.opam/5.2/lib/jane-street-headers -I /home/opam/.opam/5.2/lib/parsexp -I /home/opam/.opam/5.2/lib/ppx_assert/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_bench/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_compare/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_enumerate/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_expect/collector -I /home/opam/.opam/5.2/lib/ppx_expect/common -I /home/opam/.opam/5.2/lib/ppx_expect/config -I /home/opam/.opam/5.2/lib/ppx_expect/config_types -I /home/opam/.opam/5.2/lib/ppx_hash/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_here/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_inline_test/config -I /home/opam/.opam/5.2/lib/ppx_inline_test/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_log/types -I /home/opam/.opam/5.2/lib/ppx_module_timer/runtime -I /home/opam/.opam/5.2/lib/ppx_sexp_conv/runtime-lib -I /home/opam/.opam/5.2/lib/ppx_stable_witness/runtime -I /home/opam/.opam/5.2/lib/ppx_stable_witness/stable_witness -I /home/opam/.opam/5.2/lib/sexplib -I /home/opam/.opam/5.2/lib/sexplib0 -I /home/opam/.opam/5.2/lib/splittable_random -I /home/opam/.opam/5.2/lib/stdio -I /home/opam/.opam/5.2/lib/time_now -I /home/opam/.opam/5.2/lib/typerep -I /home/opam/.opam/5.2/lib/variantslib -intf-suffix .ml -no-alias-deps -open File_path__ -o src/.file_path.objs/byte/file_path__Path.cmo -c -impl src/path.pp.ml)
# File "src/path.ml", line 22, characters 27-55:
# 22 | let of_absolute absolute = (absolute : Absolute.t :> t)
#                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
# Error: This expression has type "t" = "File_path__Types.Path.t"
#        but an expression was expected of type
#          "File_path__.Absolute.t" = "File_path__Types.Absolute.t"

We've tried to reproduce in a smaller test-case with @Octachron but without success.
Does anyone know what type of change could be responsible for this type of failure?

The code for both of those can be seen in:

• https://github.com/janestreet/file_path/tree/dd4f505129a9fd9fa914f10b8f56fd041319b787/src
• https://github.com/janestreet/ecaml/tree/18b704626c6cec1b65a07615040a70ce2e8181c6/ecaml_value/src

Both codes seem to be pretty similar with a heavy use of includes, module types and destructive type substitutions

[gasche]

I can't make a better guess than you did, so I would just try bisecting typing/ changes between 5.1 and 5.2.

[kit-ty-kate]

@Octachron made some progress debugging this in the meantime and it seems to be related to the preview of ppxlib we are using. Either one of us will close this if it's confirmed to be ppxlib-only issue

[Octachron]

This seems indeed increasingly likely to be a ppxlib issue. Typically, my reproduction case at https://github.com/Octachron/bug-reproduction-archives/tree/main/ppxlib_and_include fails with the reported error whenever a ppxlib-based ppx is used (even for an identity ppx).

[kit-ty-kate]

mmh, that's weird though, your example doesn't use any ppxlib feature other than the driver.
Ast_mapper.default_mapper is a function from compiler-libs. Maybe something is missing from there in 5.2

[kit-ty-kate]

oh nevermind i read the ppx / manual_ppx directories the wrong way, sorry for the noise

[gasche]

Random thought: could this be some fallout from however ppxlib adapted to the changes in #12119, #12191? This was done in 5.1, but it may be that ppxlib changed it after the 5.1 release and we are only testing the change opam-wide now.

[kit-ty-kate]

I've just tried the main branch of ppxlib and it works just fine there if that was what you were thinking.

However I also managed to reduce @Octachron's test case even more: Octachron/bug-reproduction-archives#1
and it turns out the issue is in the way private types are translated in ppxlib.

I'll close this issue to avoid further noise. The ppxlib maintainers have been notified.