sql server - How does T-SQL implicit conversion / overload resolution work? -

i've been toying around t-sql in attempt understand implicit conversion , overload resolution rules, somehow seems work bit strange...

context:

• data type conversion: https://msdn.microsoft.com/en-us/library/ms191530.aspx
• data type precedence: https://msdn.microsoft.com/en-us/library/ms190309.aspx
• abs: https://msdn.microsoft.com/en-us/library/ms189800.aspx

basically latter tells abs work on int, float, decimal, etc. let's see how works:

declare @foo2 sql_variant; set @foo2 = abs(4); select sql_variant_property(@foo2, 'basetype')  -- result: int. ok, apparently have int overload. expected.  declare @foo2 sql_variant; set @foo2 = abs(cast(4.0 float)); select sql_variant_property(@foo2, 'basetype')  -- result: float. ok, apparently have float overload. expected. 

now, according implicit type conversion table, allowed implicitly convert stuff. we're going check converting varbinary int, should happen according type precedence rules:

declare @foo varbinary(4); set @foo = cast(4 varbinary(4)); select @foo + 2;  -- result: int. ok, expected. 

from result expect following work well:

declare @foo varbinary(4); set @foo = cast(4 varbinary(4)); select abs(@foo); -- result: error: operand type clash: varbinary incompatible float 

stuff don't understand here (the question):

• why implicit conversion pick 'float' overload of 'abs'? random overload? or perhaps overload that's highest in precedence list (which happens 'float')?
• why isn't implicit conversion applied varbinary -> int? after all, it's valid conversion.

from msdn abs:

arguments  numeric_expression      expression of exact numeric or approximate numeric data type category. 

i guess calling abs( varbinary ) try convert varbinary exact numeric or approximate numeric data type.

float @ top of data type precedence these types there problem guess.

update logic

declare @foo varbinary(4); 

declare varbinary variable.

set @foo = cast( 4 varbinary(4)); 

set variable value performing explicit cast int varbinary doable.

select sql_variant_property(@foo, 'basetype') 

this shows variable of type varbinary.

select abs(@foo); 

try run abs on varbinary. error message clear:

operand type clash: varbinary incompatible float 

so guess abs trying implicit convert varbinary first of data type precedence exact numeric or approximate numeric data types float.

this conversion fails according cast , convert chart.