yet hash.Hash32 implements hash.Hash (https://golang.org/pkg/hash/#Hash32)

Just my two cents here:

According to Go's assignment semantic (https://golang.org/ref/spec#Assignability), functions are expected to be of identical type in assignment context, even though hash.Hash32 is indeed assignment compatible to hash.Hash.

(func() hash.Hash32) and (func() hash.Hash) are distinct, non-interface types. That Hash32 happens to implement Hash doesn't make it possible to assign a value of the first type to a variable of the other type. You can assign a value of Hash32 to a variable of type Hash, but you can not assign a value of type (func() hash.Hash32) to a variable of type (func() hash.Hash).

According to Go's assignment semantic (golang.org/ref/spec#Assignability), functions are expected to be of identical type in assignment context, even though hash.Hash32 is indeed assignment compatible to hash.Hash.

This is rather unfortunate as I feel it should be an allowed assignment, though since the behavior is documented I'm willing to close the issue if this ends up being the final say.

You can assign a value of Hash32 to a variable of type Hash, but you can not assign a value of type (func() hash.Hash32) to a variable of type (func() hash.Hash).

This is the root if why I believe this is an error.

The compiler is behaving correctly according to the language definition, so closing the issue. See https://golang.org/doc/faq#covariant_types. For further discussion of why the language works this way, please use a forum; see https://golang.org/wiki/Questions. Thanks.