raminqaf opened a new pull request, #28293: URL: https://github.com/apache/flink/pull/28293
## What is the purpose of the change
`REGEXP_REPLACE` declared its output via `nullableIfArgs`, so the planner
inferred a `NOT NULL` result when all three arguments were `NOT NULL`. But the
runtime returns `null` for a
non-literal regex that fails to compile: a column reference or a
`CONCAT(...)` result is only validated at runtime, not at planning time. That
let a `null` flow through a column the
planner believed was non-null, which is unsound.
This is a sub-task of the FLINK-39648 umbrella.
## Brief change log
- Change `BuiltInFunctionDefinitions.REGEXP_REPLACE` output type strategy
from `nullableIfArgs(explicit(STRING()))` to
`explicit(DataTypes.STRING().nullable())`, matching
`REGEXP_EXTRACT`, which is always-nullable for the same reason. (`REGEXP`
keeps `nullableIfArgs` because it returns `false`, not `null`, on an invalid
pattern.)
## Verifying this change
This change added a test and can be verified as follows:
- New `RegexpFunctionsITCase` case with `NOT NULL` arguments and a
non-literal invalid regex, asserting the output type stays nullable and the
value is `null`.
- Existing `RegexpFunctionsITCase`, `ScalarFunctionsTest`,
`SqlExpressionTest`, `MultiJoinTest` (plan digests unchanged), and
`ExpressionSerializationTest` regress the function.
## Does this pull request potentially affect one of the following parts:
- Dependencies (does it add or upgrade a dependency): no
- The public API, i.e., is any changed class annotated with
`@Public(Evolving)`: no
- The serializers: no
- The runtime per-record code paths (performance sensitive): no
- Anything that affects deployment or recovery: no
- The S3 file system connector: no
## Documentation
- Does this pull request introduce a new feature? no
- If yes, how is the feature documented? not applicable
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