[SPARK-9068][SQL] refactor the implicit type cast code

[cloud-fan]

based on #7348

[SparkQA]

Test build #37365 has finished for PR 7420 at commit 5cca5ee.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • abstract class BinaryOperator extends BinaryExpression

[cloud-fan]

Why we need isSameType? I think the logic should be:

1. first check whether the input type is acceptable(i.e. expectedType.acceptsType(inType) returns true). If it is, do nothing here.
2. If input type is not acceptable, follow cast rules below to do implicit type cast.

It looks to me we only need acceptsType.

[rxin]

there was a reason, but i can't remember why right now. let me think about it.

cc @marmbrus do you remember why?

[cloud-fan]

cc @rxin

[SparkQA]

Test build #37368 has finished for PR 7420 at commit 03b70da.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • abstract class BinaryOperator extends BinaryExpression

[cloud-fan]

I think it's better to generate a different error message for BinaryOperator, as 2 children have same type and same expected type, we don't need to follow ExpectsInputTypes to report type error for both left and right.

[SparkQA]

Test build #37376 has finished for PR 7420 at commit fe169b0.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • abstract class BinaryOperator extends BinaryExpression

[SparkQA]

Test build #37384 has finished for PR 7420 at commit 5775081.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #37392 has finished for PR 7420 at commit 7633fa9.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[rxin]

Is this correct? Imagine we have TypeCollection(LongType, StringType), and the input is StringType

Wouldn't this just cast input to a longtype?

[cloud-fan]

the first rule is: case _ if expectedType.acceptsType(inType) => e.
So when we reach here, input type is not acceptable for any types in this type collection. see the inline comments above.

[rxin]

LGTM - @marmbrus and I can't really think of why anymore we needed it - maybe we don't need it anymore. Going to merge this.