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

[Maurizio Cimadamore]

On 19/09/17 01:03, Sergey Bylokhov wrote:
> On 9/18/17 16:35, Maurizio Cimadamore wrote:
>> So, I'd suggest that we deal with this as a separate 
>> diagnostic-related issue in JDK 10 (I could also reproduce it in 8 
>> and 9).
>
> Sure, it was just a random example.
>
> Your comment about jdk8/9 just highlighted one more place, in:
> http://cr.openjdk.java.net/~mcimadamore/8177466/src/jdk.compiler/share/classes/com/sun/tools/javac/resources/compiler.properties.sdiff.html 
>
>
> "as of release 9, 'var' is a restricted local variable type and cannot 
> be used for type declarations"
>
> I guess it is restricted from jdk 10?
Whoops - correct, well spotted

Maurizio
>
>>
>> Maurizio
>>
>> On 18/09/17 23:44, Sergey Bylokhov wrote:
>>> Hi, Maurizio.
>>> I am not sure is it expected or not, but in some cases the new 'var' 
>>> produce some non-easy to read error messages:
>>>
>>> var s = true ? new ArrayList<String>() : new ArrayList<Integer>();
>>> s.add(new String());
>>>
>>> testVar.java:9: error: no suitable method found for add(String)
>>>         s.add(new String());
>>>          ^
>>>     method Collection.add(CAP#1) is not applicable
>>>       (argument mismatch; String cannot be converted to CAP#1)
>>>     method List.add(CAP#1) is not applicable
>>>       (argument mismatch; String cannot be converted to CAP#1)
>>>     method AbstractCollection.add(CAP#1) is not applicable
>>>       (argument mismatch; String cannot be converted to CAP#1)
>>>     method AbstractList.add(CAP#1) is not applicable
>>>       (argument mismatch; String cannot be converted to CAP#1)
>>>     method ArrayList.add(CAP#1) is not applicable
>>>       (argument mismatch; String cannot be converted to CAP#1)
>>>   where CAP#1 is a fresh type-variable:
>>>     CAP#1 extends INT#1 from capture of ? extends INT#1
>>>   where INT#1,INT#2 are intersection types:
>>>     INT#1 extends Object,Serializable,Comparable<? extends INT#2>
>>>     INT#2 extends Object,Serializable,Comparable<?>
>>> Note: Some messages have been simplified; recompile with 
>>> -Xdiags:verbose to get full output
>>>
>>>
>>> On 9/18/17 09:14, 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
>>>>
>>>>
>>>
>>>
>>
>
>