Typechecker rejects callable argument in virtual method call

Author: BI71317Created Aug 26, 2026Updated Aug 28, 2026

The compiler fails to typecheck this program:

python

class A:
    def f(self, block: CallableTrait[[], bool]):
        return False

class B(A):
    def f(self, block: CallableTrait[[], bool]):
        return block()

def make_base() -> A:
    return B()

print(make_base().f(lambda: False or True))
print(make_base().f(lambda: True and False))

B inherits from A. The function make_base is annotated as returning A, but it actually returns an instance of B. So it should dispatch to B.f, execute the passed lambda, and return the lambda result. However, it looks like the program fails during typechecking before it can get as far as calling either A.f or B.f.

Result

bash

$ codon run -release 04_factory_base_return_callable.codon 
04_factory_base_return_callable.codon:15 (1-44): error: cannot typecheck 'print(make_base().f(lambda: False or True))'
04_factory_base_return_callable.codon:16 (1-45): error: cannot typecheck 'print(make_base().f(lambda: True and False))'

Expected to print:

True
False

Source: exaloop/codon

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