Generic constraints and compiler errors

[Paco Soberón]

I just found that, while this code raises a compiler error, as expected:

    value tuple = Tuple<Integer, Boolean>(true, []); // Boolean is not assignable to upper bound Integer

This other code compiles without problems:

    void foo(Tuple<Integer, Boolean> tuple) {}
    Tuple<Integer, Boolean> bar() => nothing;

Narrowing my observation to a simpler generic class, this code doesn't compile:

    class Generic<Parent, Child>()
        given Child satisfies Parent {}
   
    value generic = Generic<Integer, Boolean>();

But this code does:

    void foo(Generic<Integer, Boolean> generic) {}
    Generic<Integer, Boolean> bar() => nothing;

Of course, I'm aware that even if it compiles, the code is completely useless, since you're not going to be able to create a valid object to pass to foo() or return from bar().

But I am still curious... why the compiler doesn't complain? Doesn't it check the generic constraints when declaring a type? Does it only do it when instantiating an object?

Regards.

[Lucas Werkmeister]

Looks like this is https://github.com/ceylon/ceylon/issues/3641 (originally https://github.com/ceylon/ceylon-spec/issues/535)

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[Paco Soberón]

Interesting stuff. Thanks for answering!

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