RFR: 8263642: javac emits duplicate checkcast for first bound of intersection type in cast

[Guoxiong Li]

Hi all,

Before the phase `desugar`, the demo `(C1 & I1) o` has **one** type-cast sub-tree, `JCTypeIntersection C1&I1`, at the syntax tree. After the phase `desugar`, the `(C1 & I1) o` becomes **three** type-cast sub-trees: `JCTypeIntersection C1&I1`, `I1`, `C1`. So at phase `Gen`, `javac` generates **three** `checkcast` according to the **three** type-cast sub-trees which causes this bug.

This patch doesn't generate `checkcast` when the type is `JCTypeIntersection` so that the problem can be solved. And a corresponding test case is added.

Another way to solve this issue is that the `TransTypes` of  the `desugar` should be modified and **two** type-cast sub-trees should be generated instead of **three**. But this way may change more code than my original patch and may cause other regression.

Thank you for taking the time to review.

Best Regards.
--Guoxiong

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Commit messages:
 - 8263642: javac emits duplicate checkcast for first bound of intersection type in cast

Changes: https://git.openjdk.java.net/jdk/pull/3399/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3399&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263642
  Stats: 95 lines in 2 files changed: 94 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3399.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3399/head:pull/3399

PR: https://git.openjdk.java.net/jdk/pull/3399