How to combine date and time to datetime2 in SQL Server?

Question

Given the following components

DECLARE @D DATE = '2013-10-13'
DECLARE @T TIME(7) = '23:59:59.9999999'

What is the best way of combining them to produce a DATETIME2(7) result with value '2013-10-13 23:59:59.9999999'?

Some things which don't work are listed below.

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SELECT @D + @T 

Operand data type date is invalid for add operator.

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SELECT CAST(@D AS DATETIME2(7)) + @T 

Operand data type datetime2 is invalid for add operator.

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SELECT DATEADD(NANOSECOND,DATEDIFF(NANOSECOND,CAST('00:00:00.0000000' AS TIME),@T),@D)

The datediff function resulted in an overflow. The number of dateparts separating two date/time instances is too large. Try to use datediff with a less precise datepart.

* The overflow can be avoided in Azure SQL Database and SQL Server 2016, using DATEDIFF_BIG.

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SELECT CAST(@D AS DATETIME) + @T 

The data types datetime and time are incompatible in the add operator.

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SELECT CAST(@D AS DATETIME) + CAST(@T AS DATETIME)

Returns a result but loses precision 2013-10-13 23:59:59.997