xuyangzhong opened a new pull request #17444:
URL: https://github.com/apache/flink/pull/17444


   ## What is the purpose of the change
   
   Before, the result of the sql "select 1 = '1'" is false, which is wrong.  
And when "1 = '1'" is in join conditions, it will throw an exception, instead 
of returning a true or false result. 
   
   "=" should have the same behavior with ">" and "<", which have the correct 
results. So before calcite solves this bug or Flink supports this kind of 
implicit type conversion, we'd better temporarily forbidding this implicit type 
conversion in "=" and "<>" and all throwing a clear exception, instead of 
giving a wrong result.
   
   ## Brief change log
   
     - In the codegen function, check the validity of implicit type conversion 
to avoid conversion between numeric and (var)char.
     - According to flip-154, add many test cases to test implicit type 
conversion now supported.
     - Change the existing test cases
   
   
   ## Verifying this change
   
   This change added tests to verify this change.
   
   
   ## 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: JobManager (and its 
components), Checkpointing, Kubernetes/Yarn, ZooKeeper: no
     - The S3 file system connector: no
   
   ## Documentation
   
     - Does this pull request introduce a new feature? no
     - If yes, how is the feature documented? 
   


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