Mockgen can't generate mocks for interfaces whose method param or return types are unnamed non-empty structs. For example:

Suppose I have a foo.go file whose package import path is foo:

package foo

type Client interface {
	Get() struct{ x int }
}

Running source mode mockgen -source=foo/foo.go gives

 failed parsing returns: foo/foo.go:4:8: can't handle non-empty unnamed struct types

Running reflect mode mockgen foo Client gives

Reflection: can't yet turn struct { x int } (struct) into a model.Type