An idiomatic Go function will ensure that T is always useful, regardless of the error state. At very least it will return the zero value for T, to which the Go maxim states: Make the zero value useful. From the perspective of writing the function returning (T, error), there will always be four states, with T and error being independent of each other. Anything else is faulty design.

Unfortunately, some early Go code written before things were well understood left T to be undefined given certain error states, so one cannot assume that T will be valid in all cases for all functions. This does mean that the caller's perspective can only reliably consider three states absent of diving deeper (e.g. reading the documentation).

By 'implicit' I assume you mean 'by convention'? I say this because unless I've misread the go spec (and that's a distinct possibility), function returns are either a single value or a tuple and there is no specification on tuple returns and mutually exclusive values.

FWIW, I mostly like go and work with it practically every day. I absolutely loathe it's ideas on error handling. I have quite strong feelings on the subject that aren't fit for polite discussion.