Given v := constant.Make(big.NewInt(10)), constant.Int64Val(v) returns (10, false), but I'd expect it to return (10, true). See https://play.golang.org/p/SPzai9ii_7_f.

This is because go/constant expects that intVal will only be used to represent values that don't fit into an int64, because it always uses makeInt to convert a big.Int into a Value, which satisfies this expectation.

I see three possible solutions:

1. Change Make to use makeInt (and maybe makeRat and makeFloat) to ensure the internal representations match how go/constant would have represented them normally. This will require TestMake to be updated though, because Val(Make(big.NewInt(10))) would now return an int64 instead of *big.Int.

2. Change Int64Val to return Int64Val to return x.val.IsInt64() instead of false for intVal.

3. Document that callers of Make should use an int64 instead of big.Int when possible, and maybe panic if provided a big.Int that fits int64.

/cc @griesemer @stamblerre