Add type parameter restriction to closed hierarchies #9560
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mattwar
approved these changes
Jul 29, 2025
jnm2
reviewed
Jul 30, 2025
proposals/closed-hierarchies.md
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| @@ -55,6 +55,19 @@ public class C1 : CC { ... } // Error, 'CC' is closed and in a different ass | |||
| public class C2 : CO { ... } // Ok, 'CO' is not closed | |||
| ``` | |||
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| ### Type parameter restriction | |||
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| If a generic class directly derives from a closed class, then all if its type parameters must be used in the base class specification: | |||
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| If a generic class directly derives from a closed class, then all if its type parameters must be used in the base class specification: | |
| If a generic class directly derives from a closed class, then all of its type parameters must be used in the base class specification: |
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Should we define "used in" precisely? What about:
closed class C<T1, T2>;
class D1<T1, T2> : C<int, Tuple<T1, T2>>;
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Should we define "used in" precisely? What about:
closed class C<T1, T2>; class D1<T1, T2> : C<int, Tuple<T1, T2>>;
This still works. The type parameters in the two types do not have to be the same, the type parameters from the sub-type just need to be used in the reference to the base type.
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Got it; when switching over a given C<...> in that case, T1 and T2 are held constant across the switch and exhaustivity doesn't have to worry about a varying type parameter in the derived class.
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Prevent classes that derive from closed classes from having "free" type parameters, since that would prevent an exhaustive switch.