Enforce function arity in type inclusion and coercion - #8559
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Add the arity guard (already present in unify) to the Tarrow cases of moregen, eqtype, and subtype_rec. Previously a curried implementation (int => int => int) could satisfy an uncurried interface ((int, int) => int) through signature inclusion or :> coercion; calls made through the interface type compile to direct JavaScript calls with the declared arity, so a first-class use of such a value miscompiled (e.g. returning a closure where an int was expected). The value-mismatch report in includemod now prints a dedicated hint when the two sides are functions of different arities, replacing the vestigial empty curry_kind slot. mcomp is deliberately left arity-lenient: it is an incompatibility oracle for pattern and GADT reasoning, where leniency errs toward "possibly compatible". Co-Authored-By: Claude Fable 5 <noreply@anthropic.com> Signed-off-by: Cristiano Calcagno <cristianoc@users.noreply.github.com>
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Codecov Report❌ Patch coverage is
Additional details and impacted files@@ Coverage Diff @@
## master #8559 +/- ##
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Coverage 75.54% 75.54%
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Files 476 476
Lines 62710 62732 +22
=======================================
+ Hits 47374 47393 +19
- Misses 15336 15339 +3
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Summary
Why
A curried implementation such as
int => int => intcould previously satisfy an uncurried interface such as(int, int) => int, or be coerced to that type. Calls through the declared interface are emitted as plain JavaScript calls with the declared arity, so first-class uses could return a closure where a value was expected.The root cause was that unification checked arrow arity, while the parallel
moregen,eqtype, andsubtype_recpaths did not.Validation
make testnode tests/build_tests/super_errors/input.jsnode tests/build_tests/super_errors_multi/input.jsmake checkformat