Diagnostics: inline-record mutability coercion gives no reason, and `throw` in `finally` warns about unreachable code

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Avaliação

Dificuldade
4/5
Tempo estimado
3-5 dias
Facilidade para iniciantes
38/100
Tipo de issue
Bug
Clareza
Razoavelmente clara
Status de atividade
Ativa
Stack de tecnologia
ocaml
Domínio
compilers, developer-experience

Direção de pesquisa

The throw-in-finally case points to compiler/ml/typecore.ml around lines 2920-2922, where the finally block is typed as a statement; check how warning 21 is raised for non-returning statements. The inline-record coercion error is reached through a subtype path the payload does not name, so locate it by searching for the existing plain-record mutability explanation and compare the two. Run the two snippets from the issue to confirm the wrong output, and count done as the mutable-field explanation appearing for inline records and warning 21 no longer firing for throw in finally.

Escrita pelo modelo de indexação a partir do texto da issue.

Descrição

Found by differential semantic drift analysis from b123b7c3 to c10223e8.

What is broken

  • Coercion into an inline record with a mutable field gives no reason (#9086). The coercion is now rejected, correctly:

    type v1 = A({x: float})
type v2 = A({mutable x: float})
let a: v1 = A({x: 0.})
let b = (a :> v2)

    The error is only Type v1 is not a subtype of v2. For a field of a plain record type ({r: r1} into {r: r2}) the error explains: "The field x is mutable in record r2, but not in record r1. A coercion can't make a field writable."

  • throw in finally gives warning 21 (#8931):

    exception B
let k = () => try { 1 } finally { throw(B) }

    Warning 21: "This statement does not continue execution; following code is unreachable." No code follows the throw; the generated JavaScript is correct. The finally block is typed as a statement (typecore.ml L2920-L2922), and a non-returning statement triggers this warning.

Possible cause (speculation only)

The inline-record path reports the subtype failure before reaching the field comparison that produces the explanation for plain records.

🤖 Generated with Claude Code

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