Compilation Issue when passing map[string]struct{}

[Pranay Agarwal]

Hello,

I am using Go 1.20.4 on Ubuntu 22.04. I am facing a compilation error when I attempt to pass a map from a string to a struct to a function that expects a map from string to an interface.

Here is some example code demonstrating the issue: https://goplay.tools/snippet/GrPfTWfycdx

Here, the struct is Helper, and it conforms to both interface definitions - Helper1 and Helper2. I am able to pass in an instance of Helper directly, however when I attempt to pass a map[string]Helper the compiler throws a type error IncompatibleAssign.

I was wondering if anyone else has faced something similar? And if so, how did you solve this issue. Would appreciate any input, thank you.

Regards,

Pranay Agarwal

[Ian Lance Taylor]

Go doesn't work that way. You can't freely interchange a type with an
interface type, even if the type implements the interface. For a
related scenario, see https://go.dev/doc/faq#covariant_types. The
only fix is to be consistent: if you need a map to an interface, build
a map to an interface type. If you need to support two different
interfaces, build a map to any, and use a type assertion.

Ian