sql: correctly type CASE expressions that result in tuples

[mgartner]

Consider the query:

SELECT CASE WHEN true THEN ('a', 2) ELSE NULL::RECORD END

As of 22.1, the query results in an error:

ERROR: CASE ELSE type tuple cannot be matched to WHEN type tuple{string, int}
SQLSTATE: 42804

The CASE is typed as a tuple{string, int} and the NULL::RECORD is typed as types.AnyTuple (what the RECORD type resolves to). The error occurs because there is no valid case from types.AnyTuple to any specific tuple type. The fix is to type the CASE expression as types.AnyTuple, following the logic described in Postgres's type conversion documentation:

5. Select the first non-unknown input type as the candidate type, then consider each other non-unknown input type, left to right. [12] If the candidate type can be implicitly converted to the other type, but not vice-versa, select the other type as the new candidate type. Then continue considering the remaining inputs. If, at any stage of this process, a preferred type is selected, stop considering additional inputs.

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Here's another example that errors in CRDB but succeeds in Postgres:

defaultdb> SELECT CASE WHEN false THEN ('a', 2) ELSE (1, 3) END;
ERROR: incompatible value type: tuples [('a', 2) (1, 3)] are not the same type: could not parse "a" as type int: strconv.ParseInt: parsing "a": invalid syntax
SQLSTATE: 22P02

The error suggests that some changes to the parser might be required in order to get this working correctly.

Epic CRDB-2474
Jira issue: CRDB-14072