Why Does Type Assertion Fail When Casting to a Type Alias in Go?

Casting to Type Aliases in Go

Problem:

In the example snippet, the first type assertion succeeds by explicitly specifying the function signature. However, the second assertion, which attempts to cast to the type alias somethingFuncy, panics. Why is this the case, and is there a way to cast to a longer function signature?

Explanation:

Contrary to the misconception that type casting is involved here, Go only offers type assertions and type conversions. The focus is on type assertions, which are employed in the provided snippet.

A key distinction between type assertions and conversions lies in the criteria used for comparison. In conversions, such as the example with int and MyInt, the underlying types are considered. However, with type assertions, only the exact type is taken into account.

To elaborate, in the example where a is of type int, the assertion a.(MyInt) fails because int is not identical to MyInt, even though both share the same underlying type.

tl;dr:

Type assertions check for exact type identity, which means a type alias like somethingFuncy is distinct from func(int) bool. Thus, the second cast fails.

Bonus:

The code used to verify type identity in the snippet is provided for reference, showcasing the direct comparison involved in the assertion process.

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