SELECT TOP COALESCE and bigint - sql

Question

Ignore the practicality of the following sql query
DECLARE #limit BIGINT
SELECT TOP (COALESCE(#limit, 9223372036854775807))
*
FROM
sometable
It warns that
The number of rows provided for a TOP or FETCH clauses row count parameter must be an integer.
Why doesn't it work but the following works?
SELECT TOP 9223372036854775807
*
FROM
sometable
And COALESCE(#limit, 9223372036854775807) is indeed 9223372036854775807 when #limit is null?
I know that changing COALESCE to ISNULL works but I want to know the reason.

Answer 1

https://technet.microsoft.com/en-us/library/aa223927%28v=sql.80%29.aspx
Specifying bigint Constants
Whole number constants that are outside the range supported by the int
data type continue to be interpreted as numeric, with a scale of 0 and
a precision sufficient to hold the value specified. For example, the
constant 3000000000 is interpreted as numeric. These numeric constants
are implicitly convertible to bigint and can be assigned to bigint
columns and variables:
DECLARE #limit bigint
SELECT SQL_VARIANT_PROPERTY(COALESCE(#limit, 9223372036854775807),'BaseType')
SELECT SQL_VARIANT_PROPERTY(9223372036854775807, 'BaseType') BaseType
shows that 9223372036854775807 is numeric, so the return value of coalesce is numeric. Whereas
DECLARE #limit bigint
SELECT SQL_VARIANT_PROPERTY(ISNULL(#limit, 9223372036854775807),'BaseType')
gives bigint. Difference being ISNULL return value has the data type of the first expression, but COALESCE return value has the highest data type.
SELECT TOP (cast(COALESCE(#limit, 9223372036854775807) as bigint))
*
FROM
tbl
should work.

Answer 2

DECLARE
#x AS VARCHAR(3) = NULL,
#y AS VARCHAR(10) = '1234567890';
SELECT
COALESCE(#x, #y) AS COALESCExy, COALESCE(#y, #x)
AS COALESCEyx,
ISNULL(#x, #y) AS ISNULLxy, ISNULL(#y, #x)
AS ISNULLyx;
Output:
COALESCExy COALESCEyx ISNULLxy ISNULLyx
---------- ---------- -------- ----------
1234567890 1234567890 123 1234567890
Notice that with COALESCE, regardless of which input is specified first, the type of the output is VARCHAR(10)—the one with the higher precedence. However, with ISNULL, the type of the output is determined by the first input. So when the first input is of a VARCHAR(3) data type (the expression aliased as ISNULLxy), the output is VARCHAR(3). As a result, the returned value that originated in the input #y is truncated after three characters.That means isnull would not change the type, but coalesce would.

Answer 3

Turns out that 9223372036854775807 is a numeric instead of a bigint
From https://technet.microsoft.com/en-us/library/aa223927(v=sql.80).aspx
Whole number constants that are outside the range supported by the int data type continue to be interpreted as numeric, with a scale of 0 and a precision sufficient to hold the value specified
So we need to explicitly cast it to bigint
DECLARE #limit BIGINT
SELECT TOP (COALESCE(#limit, CAST(9223372036854775807 AS BIGINT)))
*
FROM
sometable