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What Happened

When parsing T-SQL code like:

MERGE INTO dbo.target USING ( SELECT 1 AS i ) AS source ON source.i = target.i WHEN MATCHED THEN UPDATE SET target.i = source.i;

I'm getting parsing error:

L: 1 | P: 2 | PRS | Line 1, Position 2: Found unparsable section: 'MERGE | INTO dbo.target USING \n\t(\tSELECT 1...'

If I add alias to target table, everything is good:

MERGE INTO dbo.target t USING ( SELECT 1 AS i ) AS source ON source.i = target.i WHEN MATCHED THEN UPDATE SET target.i = source.i;

T-SQL dialect supports both options, and I see that in the SQL Fluff code it's configured correctly as well ("Ref("AliasExpressionSegment", optional=True)") in dialect_tsql.py, MergeStatementSegment class.
However, looks like this "optional" value is being ignored in this particular case. If I just delete this "Ref" line - it parses the code well.

Expected Behaviour

No parsing error for this MERGE statement

Observed Behaviour

`L: 1 | P: 2 | PRS | Line 1, Position 2: Found unparsable section: 'MERGE
| INTO dbo.target USING \n\t(\tSELECT 1...'

How to reproduce

run sqlfluff parse --dialect tsql 'script_for_sqlfluff.sql'

script_for_sqlfluff.sql:

MERGE INTO dbo.target USING ( SELECT 1 AS i ) AS source ON source.i = target.i WHEN MATCHED THEN UPDATE SET target.i = source.i;

Dialect

T-SQL

Version

Python 3.11.9, SQL Fluff 3.0.7

Configuration

default config, no extra files

Are you willing to work on and submit a PR to address the issue?

• Yes I am willing to submit a PR!

Code of Conduct

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