RFR: 8177466: Add compiler support for local variable type-inference

Vicente Romero vicente.romero at oracle.com
Mon Sep 25 14:20:31 UTC 2017

Hi Maurizio,

Thanks for the updated version. I don't have any major comments. Looks 
good to me,


On 09/22/2017 06:52 AM, Maurizio Cimadamore wrote:
> Here's another revision:
> http://cr.openjdk.java.net/~mcimadamore/8177466_v3/
> diagnostics:
> http://cr.openjdk.java.net/~mcimadamore/8177466_v3/diags.html
> Changes:
> * Rewrote logic of type projection in Types.java to use an enum
> * Added several javadoc on Types.java
> * added call to baseType() in Attr::attribLazyConstantValue
> * fixed an issue in TypeHarness where type var was created with null 
> lower bound (as of this patch, javac is stricter and will assert on 
> null bounds, as they are problematic to deal with when doing projections)
> If there are no major comments, my plan is to push this on 
> jdk10/master early next week.
> Thanks!
> Maurizio
> On 19/09/17 16:04, Maurizio Cimadamore wrote:
>> Hi,
>> I have put together a slightly updated webrev:
>> * as pointed out, one diagnostic used to say '9' instead of '10' 
>> (this will likely need to change again because of new release 
>> cadence, but good enough for now)
>> * the previous webrev contained a spurious added file (TypeHarness) 
>> which was caused by a glitch in the lvti branch in the amber repo.
>> New webrev:
>> http://cr.openjdk.java.net/~mcimadamore/8177466_v2/
>> New diags:
>> http://cr.openjdk.java.net/~mcimadamore/8177466_v2/diags.html
>> Maurizio
>> On 18/09/17 17:14, Maurizio Cimadamore wrote:
>>> Hi,
>>> this change adds support for local variable type inference (JEP 286 
>>> [1]). A webrev of the change is available here:
>>> http://cr.openjdk.java.net/~mcimadamore/8177466
>>> The patch is relatively straightforward: implicitly typed locals are 
>>> modeled in a similar fashion to implicit lambda parameters: their 
>>> AST node is a JCVariableDecl whose 'vartype' field is not set (e.g. 
>>> null).
>>> There are few tricky parts to this changeset:
>>> 1) tweak the parser to give 'var' special meaning depending on the 
>>> version number and context
>>> 2) Add logic in name resolution to catch bad reference to types 
>>> named 'var'
>>> 3) add logic to map initializer type back to a suitable variable 
>>> declared type
>>> As for (1), the parser has been extended so as to special case local 
>>> variables with special name 'var', so that the type will be left out 
>>> of the corresponding AST representing the variable declaration. This 
>>> behavior will only affect latest source version.
>>> The parser has a number of extra checks to prevent 'var to be used 
>>> in places where it does not belong (according to the spec draft 
>>> [2]); for instance, declaring a class whose name is 'var' is 
>>> rejected in the parser. As a general rule, I tried to implement all 
>>> such checks in the parser, as that gives very early and precise 
>>> feedback about what's wrong with the code. The changes are 
>>> implemented in Parser.java.
>>> There are however errors which cannot be caught in the parser, and 
>>> that's why (2) is needed. Basically, whenever 'var' is used in a 
>>> position where it could be either a type or a package name, the 
>>> parser can't simply rule that out, so we have to accept the code, 
>>> and give an error if, later on, we discover that 'var' was really 
>>> used in a type position (see changes in Resolve.java).
>>> As far as (3) is concerned, we need to 'uncapture' captured types 
>>> from initializers. That means that if we have a 'var' like this:
>>> class Foo {
>>>     void test() {
>>>         var x = getClass().getSuperClass();
>>>     }
>>> }
>>> The initializer type will be something like Class<? super #CAP>, 
>>> where #CAP <: Foo
>>> In this case, the compiler will project this type back to the less 
>>> specific type Class<?>, and use that as the declared type for 'x'. 
>>> This logic is defined in Types.java. As this logic is the same logic 
>>> needed by jshell to render type of standalone expressions, jshell 
>>> class VarTypePrinter has been removed and jshell has been rewired to 
>>> point at the (now) official routine in Types. Jshell also needed 
>>> several other tweaks to (i) accept 'var' and (ii) to deal with 
>>> non-denotable types (intersection types and anonymous class types) 
>>> that can be produced by the LVTI machinery (many thanks to Jan for 
>>> doing those changes!)
>>> As far as testing is concerned, I wrote several tests to check that 
>>> the parser was behaving as expected; to check the behavior of the 
>>> LVTI inference machinery, I wrote a test harness which leverages 
>>> annotation on 'var' so that we can write down assertions such as:
>>> @InferredType("java.util.List<? extends java.lang.String>")
>>> var s = extString();
>>> Regarding compiler diagnostics, for those interested, a 
>>> comprehensive list of examples of new diagnostics triggered by the 
>>> LVTI compiler can be found here:
>>> http://cr.openjdk.java.net/~mcimadamore/8177466/lvti-diags.html
>>> Finally, a finder has been added to detect local variable decls 
>>> whose declared type can be replaced by 'var' - to enable it, the 
>>> hidden option -XDfind=local should be used.
>>> Thanks
>>> Maurizio
>>> [1] - http://openjdk.java.net/jeps/286
>>> [2] - http://cr.openjdk.java.net/~dlsmith/local-var-inference.html

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