Why can I not use func literal as custom func type despite matching parameters in golang?

Question

Package google.golang.org/grpc defines type UnaryClientInterceptor as:

type UnaryClientInterceptor func(ctx context.Context, method string, req, reply interface{}, cc *ClientConn, invoker UnaryInvoker, opts ...CallOption) error

In my code I'd like to do something like:

func clientInterceptor() grpc.UnaryClientInterceptor {
    return func(ctx context.Context, method string, req, reply interface{}, cc *grpc.ClientConn, invoker grpc.UnaryInvoker, opts ...grpc.CallOption) error {
        //intercept stuff
        return invoker(ctx, method, req, reply, cc, opts...)
    }
}

However I get this error:

"cannot use func literal (type func("context".Context, string, interface {}, interface {}, *grpc.ClientConn, grpc.UnaryInvoker, ...grpc.CallOption) error) as type grpc.UnaryClientInterceptor in return argument"

Looking at Why can I type alias functions and use them without casting? I get the impression that the anonymous function returned in my clientInterceptor() should match the grpc.ClientInterceptor type.

Also, from the spec on Type Identity (http://golang.org/ref/spec#Type_identity)

Two function types are identical if they have the same number of parameters and result values, corresponding parameter and result types are identical, and either both functions are variadic or neither is. Parameter and result names are not required to match.

I've tried casting and making variables of type grpc.UnaryClientInterceptor but nothing works.

I also did essentially the exact same thing with the grpc.UnaryServerInterceptor and had no problems.

What am I missing here?

Answer 1

You may be referencing the context package differently than your version of grpc. As of Go 1.7, the package moved from golang.org/x/net/context to just context, and the compiler likely doesn't see them as equivalent.