[premium-concern]

  (int64, error)

gives you exactly four possibilities.

Either gives you exactly the two you want.

[weberc2]

Please feel free to enumerate them.

[premium-concern]

  (value, no error)
  (value, error)
  (no value, error)
  (no value, no error)

[leaveyou]

The convention is that if the err == nil then the value is not nil. The exceptions to this rule are very few and usually specified in the documentation. Normally you only have to check for error.

[bluejekyll]

The fact that it's a convention, and not a compiler error is the entire issue at debate here.

[weberc2]

There is no "no value" representation for int64. The only two cases are "<int64>, nil" or "<int64>, <error>".