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Pin current object-field mutability behavior ahead of representation cleanup
Add behavior-pinning tests for the structural-object mutability semantics (currently encoded via phantom "x#=" setter members), ahead of the staged representation cleanup proposed in #8584. tests/tests/src/object_mutability_pin.res pins the compiling cases: closed mutable-to-immutable covariance, open-source and open-target coercions, assignment- and coercion-driven strengthening of open rows, and a generalized getter used at both mutabilities. Two cases are marked EXPECTED TO FLIP with the rationale in place: the unequal-type coercion and the unrelated-type assignment (getter int acquiring setter string), which today produces a value of declared type int that is the string "hello" at runtime - the type-preservation failure the cleanup closes. Seven super_errors fixtures pin the rejecting directions: closed-row writes, both-open invariance, readonly-to-mutable coercions, mutable-to-mutable invariance with unequal types, read-only callers against strengthened rows, and writes after a closed-source-to-open-target coercion. Part of #8584. Signed-Off-By: Cristiano Calcagno <ccrisccris@gmail.com> Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01PCtQiaDijUqA2fujQXvKUw
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‎tests/build_tests/super_errors/expected/object_coercion_mutable_unequal.res.expected‎

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We've found a bug for you!
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/.../fixtures/object_coercion_mutable_unequal.res:8:35-57
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6 │ type wide = {"a": int, "b": int}
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7 │ type narrow = {"a": int}
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8 │ let p = (v: {@set "x": wide}) => (v :> {@set "x": narrow})
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9 │
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Type {"x": wide, "x#=": wide => unit} is not a subtype of
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{"x": narrow, "x#=": narrow => unit}
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Type narrow = {"a": int} is not a subtype of wide = {"a": int, "b": int}

‎tests/build_tests/super_errors/expected/object_coercion_open_open.res.expected‎

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We've found a bug for you!
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/.../fixtures/object_coercion_open_open.res:9:32-51
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7 │ type wide = {"a": int, "b": int}
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8 │ type narrow = {"a": int}
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9 │ let p = (o: {.."x": wide}) => (o :> {.."x": narrow})
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10 │
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Type {.."x": wide} is not a subtype of {.."x": narrow}
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Type wide = {"a": int, "b": int} is not compatible with type
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narrow = {"a": int}
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The second object is expected to have a field "b" of type int, but it does not.

‎tests/build_tests/super_errors/expected/object_coercion_promote_readonly_caller.res.expected‎

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We've found a bug for you!
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/.../fixtures/object_coercion_promote_readonly_caller.res:11:11-18
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9 │ let f = (o: {.."x": wide}) => (o :> {@set "x": wide})
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10 │ @val external readonly: {"x": wide} = "readonly"
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11 │ let _ = f(readonly)
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12 │
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This has type: {"x": wide}
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But this function argument is expecting: {.."x": wide, "x#=": wide => unit}
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The first object is expected to have a field "x#=" of type wide => unit, but it does not.

‎tests/build_tests/super_errors/expected/object_coercion_readonly_to_mutable.res.expected‎

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We've found a bug for you!
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/.../fixtures/object_coercion_readonly_to_mutable.res:6:20-39
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4 │ See docs/object_representation_cleanup.md. */
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5 │ type t = {"x": int}
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6 │ let p = (v: t) => (v :> {@set "x": int})
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7 │
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Type t = {"x": int} is not a subtype of {"x": int, "x#=": int => unit}

‎tests/build_tests/super_errors/expected/object_open_write_readonly_caller.res.expected‎

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We've found a bug for you!
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/.../fixtures/object_open_write_readonly_caller.res:8:11-18
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6 │ let f = (o: {.."x": int}) => o["x"] = 1
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7 │ @val external readonly: {"x": int} = "readonly"
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8 │ let _ = f(readonly)
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9 │
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This has type: {"x": int}
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But this function argument is expecting: {.."x": int, "x#=": int => unit}
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The first object is expected to have a field "x#=" of type int => unit, but it does not.

‎tests/build_tests/super_errors/expected/object_write_after_open_target_coercion.res.expected‎

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We've found a bug for you!
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/.../fixtures/object_write_after_open_target_coercion.res:12:3
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10 │ let p = (v: {"x": wide}) => {
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11 │ let r = (v :> {.."x": narrow})
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12 │ r["x"] = {"a": 1}
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13 │ }
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14 │
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This expression has type {"x": narrow}
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It has no field x#=

‎tests/build_tests/super_errors/expected/object_write_closed_row.res.expected‎

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We've found a bug for you!
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/.../fixtures/object_write_closed_row.res:6:28
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4 │ See docs/object_representation_cleanup.md; compiling counterparts in
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5 │ tests/tests/src/object_mutability_pin.res. */
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6 │ let g = (o: {"x": int}) => o["x"] = 1
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7 │
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This expression has type {"x": int}
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It has no field x#=

‎tests/build_tests/super_errors/fixtures/object_coercion_mutable_unequal.res‎

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/* Pin (object mutability cleanup): mutable-to-mutable coercion is invariant
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in the field type — with unequal types it is rejected (today the setter
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member demands contravariance while the getter demands covariance). Must
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stay an error under the new model (Mutable A <: Mutable B iff A = B).
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See docs/object_representation_cleanup.md. */
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type wide = {"a": int, "b": int}
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type narrow = {"a": int}
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let p = (v: {@set "x": wide}) => (v :> {@set "x": narrow})

‎tests/build_tests/super_errors/fixtures/object_coercion_open_open.res‎

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/* Pin (object mutability cleanup): when BOTH rows are open, object fields
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are invariant — this covariant coercion is rejected. Principled, not an
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artifact: an open result is a promotable result, and a covariantly
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weakened field must never remain promotable (a later write at the narrow
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type would reach readers at the wide type). Must stay an error under the
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new model. See docs/object_representation_cleanup.md. */
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type wide = {"a": int, "b": int}
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type narrow = {"a": int}
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let p = (o: {.."x": wide}) => (o :> {.."x": narrow})

‎tests/build_tests/super_errors/fixtures/object_coercion_promote_readonly_caller.res‎

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/* Pin (object mutability cleanup): COERCION-driven strengthening (as
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opposed to the assignment-driven case in
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object_open_write_readonly_caller.res): coercing an open-row parameter to
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a same-type mutable target constrains the row, so a read-only caller is
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rejected. Both halves must survive the new model (the coercion promotes
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the open source's field; the demand becomes a Mutable field).
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See docs/object_representation_cleanup.md. */
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type wide = {"a": int, "b": int}
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let f = (o: {.."x": wide}) => (o :> {@set "x": wide})
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@val external readonly: {"x": wide} = "readonly"
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let _ = f(readonly)

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