Why doesn’t @@TRANCOUNT increment after my IF statement?

output.txt

(1 row(s) affected)
unreachable
after1 0
after2 0
after3 1

SQLQuery28.sql

SET IMPLICIT_TRANSACTIONS ON
GO
IF OBJECT_ID('dbo.t1') IS NOT NULL
BEGIN
    DROP TABLE dbo.t1
    COMMIT
END
GO
CREATE TABLE dbo.t1 (x INT);
GO
INSERT INTO dbo.t1 VALUES (1);
COMMIT;
GO

IF CASE WHEN EXISTS (SELECT * FROM dbo.t1) THEN 1 ELSE 0 END = 1 PRINT 'unreachable';
PRINT 'after1 ' + CAST(@@TRANCOUNT AS VARCHAR(MAX)); -- after1 0

DECLARE @x INT = 2;
PRINT 'after2 ' + CAST(@@TRANCOUNT AS VARCHAR(MAX)); -- after2 0

DECLARE @y INT = CASE WHEN EXISTS (SELECT * FROM dbo.t1) THEN 1 ELSE 0 END;
PRINT 'after3 ' + CAST(@@TRANCOUNT AS VARCHAR(MAX)); -- after3 1
GO

WHILE @@TRANCOUNT > 0 ROLLBACK
GO

http://dba.stackexchange.com/q/168409