Generic Last Year Measure

that depends on your data structure, and how you specify PY

if you have a slicer that selects CY/PY then you could do something like this:

Smart Sales v1 =
SWITCH (
    SELECTEDVALUE ( Slicer[CY_PY] ),
    "CY", SUM ( Sales[Value] ),
    "PY", CALCULATE ( SUM ( Sales[Value] ), SAMEPERIODLASTYEAR ( Calendar[Date] ) ),
    BLANK ()
)

if you have a data structure like this:
MeasureValueDate

you could do something like this

SmartValue =
SWITCH (
    SELECTEDVALUE ( Slicer[CY_PY] ),
    "CY", SUM ( Table[Value] ),
    "PY", CALCULATE ( SUM ( Table[Value] ), SAMEPERIODLASTYEAR ( Calendar[Date] ) ),
    BLANK ()
)

and then sales would be just:

Sales = CALCULATE([SmartValue], Table[Measure] = Sales)

this solution has the benefit that you can also add YTD, QTD, MTH calculation within same logic