The documentation for (*bytes.Buffer).Grow says "If the buffer can't grow it will panic with ErrTooLarge."

However, the calculation of the new size in tryGrowByReslice has an unchecked overflow:

func (b *Buffer) tryGrowByReslice(n int) (int, bool) {
	if l := len(b.buf); l+n <= cap(b.buf) {
		b.buf = b.buf[:l+n]
		return l, true
	}
	return 0, false
}

When n is very large, l+n is negative and passes the cap check.

That results in an attempt to reslice with a negative capacity, panicking with runtime error: slice bounds out of range instead of the promised bytes.ErrTooLarge (https://play.golang.org/p/iwtlQtSehu).

(#19624 would report the erroneous overflow more directly.)