Filtered index is not used when the variable in WHERE condition

Why MS SQL Server refuse using supporting filtered index in this scenario?

-- demo data CREATE TABLE #Test ( ID INT IDENTITY(1,1) NOT NULL CONSTRAINT PK_Test_ID PRIMARY KEY ,Col1 NVARCHAR(36) NOT NULL DEFAULT NEWID() ,Col2 NVARCHAR(20) NOT NULL DEFAULT N'' -- !! ); WITH L0 AS(SELECT 1 AS C UNION ALL SELECT 1 AS O), -- 2 rows L1 AS(SELECT 1 AS C FROM L0 AS A CROSS JOIN L0 AS B), -- 4 rows L2 AS(SELECT 1 AS C FROM L1 AS A CROSS JOIN L1 AS B), -- 16 rows L3 AS(SELECT 1 AS C FROM L2 AS A CROSS JOIN L2 AS B), -- 256 rows L4 AS(SELECT 1 AS C FROM L3 AS A CROSS JOIN L3 AS B), -- 65,536 rows L5 AS(SELECT 1 AS C FROM L4 AS A CROSS JOIN L4 AS B), -- 4,294,967,296 rows Nums AS(SELECT ROW_NUMBER() OVER(ORDER BY (SELECT NULL)) AS N FROM L5) INSERT INTO #Test(Col2) SELECT TOP 100000 N'' FROM Nums; INSERT INTO #Test(Col2) VALUES(N'ABC'); -- FILTERED index to support filter predicate of a query CREATE NONCLUSTERED INDEX IX_Test_Col2_filtered ON #Test (Col2 ASC) WHERE Col2 <> N''; -- just checking statistics DBCC SHOW_STATISTICS('#Test', 'IX_Test_Col2_filtered') -- condition on variable = index scan :-( DECLARE @Filter NVARCHAR(20) = N'ABC' SELECT Col1 FROM #Test WHERE Col2 = @Filter AND Col2 <> N'';

Everything goes as expected when using literals.

-- condition on literal value - index seek + key lookup :-) SELECT Col1 FROM #Test WHERE Col2 = N'ABC';