No IMPLICIT_CONVERSION warning in this query plan Take the following minimally complete and verifiable example1 code:

USE tempdb;

DROP TABLE IF EXISTS dbo.t;
GO
CREATE TABLE dbo.t
(
    t_id int NOT NULL
        CONSTRAINT t_pk
        PRIMARY KEY 
        CLUSTERED
        IDENTITY(1,1)
    , k sysname NOT NULL
        INDEX t_001
    , s sysname NOT NULL
        INDEX t_002
    , somedata varchar(1000) NOT NULL
        CONSTRAINT t_somedata_df
        DEFAULT REPLICATE('A', 1000)
    , INDEX t_003 (k, s)
);

INSERT INTO dbo.t (s, k)
SELECT sc1.name, sc2.name
FROM sys.syscolumns sc1
    CROSS JOIN sys.syscolumns sc2;

CREATE STATISTICS t_st001 ON dbo.t (k) WITH FULLSCAN;
CREATE STATISTICS t_st002 ON dbo.t (s) WITH FULLSCAN;


The following query purposely uses the wrong variable types.  I was hoping the query plan would include an implicit conversion warning, but it doesn't.

DECLARE @k char(128) = 'a';
DECLARE @s char(128) = '';
SELECT s 
FROM dbo.t 
    LEFT JOIN sys.syscolumns sc ON t.s = sc.name
WHERE dbo.t.s = @s
    OR dbo.t.k = @k;


Why is that?

The query plan is here.



1 - that's my website, BTW
Your parameters have a lower Data Type Precedence than the columns they are compared to.  In this case the parameter values will be implicitly converted.  This is a good thing.  Converting the column values to the parameter’s type is problematic and will trigger the warning.