RFR: 8244681: Add a warning for possibly lossy conversion in compound assignments

[Adam Sotona]

On Mon, 9 May 2022 15:56:35 GMT, Adam Sotona <asotona at openjdk.org> wrote:

> Please review this patch adding new lint option, **lossy-conversions**, to javac to warn about type casts in compound assignments with possible lossy conversions.
> 
> The new lint warning is shown if the type of the right-hand operand of a compound assignment is not assignment compatible with the type of the variable.
> 
> The implementation of the warning is based on similar check performed to emit "possible lossy conversion" compilation error for simple assignments. 
> 
> Proposed patch also include complex matrix-style test with positive and negative test cases of lossy conversions in compound assignments.
> 
> Proposed patch also disables this new lint option in all affected JDK modules and libraries to allow smooth JDK build. Individual cases to address possibly lossy conversions warnings in JDK are already addressed in a separate umbrella issue and its sub-tasks.
> 
> Thanks for your review,
> Adam

I agree with the priority to keep java.base and java.desktop clean from possibly lossy conversions, so the related issues should probably raise from P4 priority level.

However this lint warning as a part of the javac is critical to confirm that the situations have been correctly addressed.
If we want to avoid "blind" patching, we only two possible scenarios:
1. big homogenous patch including hundreds of fixed lines of code across many "moving-target" classes, together with lint warning implemented and enabled 
2. javac lint patch (disabled for affected JDK modules build) goes first, so each case can be resolved, reviewed and validated in individual patch 

>From complexity and cost perspective I prefer the second scenario.

-------------

PR: https://git.openjdk.java.net/jdk/pull/8599