pmd / pmd / 725

79%

Build Type

push

github

Committed by web-flow

Commit Message

[java] Fix #5441: Resolve interdependent inference variables simultaneously (#6934)

* [java] Fix #5441: Resolve interdependent inference variables simultaneously

UseDiamondOperator reported a false positive on code where javac cannot
infer the type arguments, so following the suggestion did not compile:

    Cache<String, List<Integer>> cache =
        provider.<String, List<Integer>>newCacheBuilder("test")
                .buildAsync(new ConcreteCacheLoader<String, List<Integer>>() { ... })
                .synchronous();

The rule itself was fine; the underlying type inference was wrong. PMD
inferred the diamond as ConcreteCacheLoader<String, List<Integer>>, which
is identical to the explicit type arguments, so the rule concluded they
were redundant. javac infers ConcreteCacheLoader<Object, List<Integer>>
instead, which is why replacing the type arguments with a diamond breaks
compilation ("does not override abstract method fetch(Object)").

Cause: VarWalkStrategy merges strongly connected components of the ivar
dependency graph into a single batch, precisely so that mutually dependent
variables are solved together. But solveBatchProgressed instantiated only
the first acceptable variable of the batch and returned, so incorporation
ran again before the rest of the batch was solved.

That violates JLS 18.4, which requires the instantiations of such a batch
to all be *computed* from the same bound set and only then be incorporated.
The distinction matters because the resolution rules may only use proper
bounds, i.e. bounds that do not mention an inference variable. For the
snippet above the relevant context is

    'a { 'a <: String, 'a <: 'e }     # K1 of buildAsync
    'e { 'e <: Object, 'e >: 'a }     # K of the diamond

Solved sequentially, 'a := String by rule UPPER; incorporation then turns
'e >: 'a into the proper bound 'e >: String, rule LOWER applies and
'e := String. Solved simultaneously, 'e has no proper lower bound (its only
lower bound ... (continued)

Coverage Stats

19521 of 25584 branches covered (76.3%)

Branch coverage included in aggregate %.

12 of 12 new or added lines in 2 files covered. (100.0%)

2 existing lines in 2 files now uncovered.

42287 of 52350 relevant lines covered (80.78%)

0.82 hits per line