Github user srowen commented on the issue:

    https://github.com/apache/spark/pull/22063
  
    For those following along, the problem right now is this: functions like 
`udf()` have always declared that their type params have `TypeTag`s, meaning 
the caller has to have runtime info about types. This supports schema inference.
    
    Although the internals of the `udf()` methods changed in this PR, the 
signature did not. `SchemaReflection.schemaFor` was called before. However, 
with this change, suddenly it seems like the `TypeTag`s are needed by the 
compiler, and many instances of `udf()` fail now because the call site does not 
have one.
    
    I worked around a few, but am having more trouble with others.
    I am still not clear why it is that only with this change do the `TypeTag`s 
seem to matter; they were always required?


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