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

[Sergey Bylokhov]

It is also interesting how the new 'var' should work in case of 
conditional expression:

String s = true ? new String() : new int[1]; // incompatible types: bad 
type in conditional expression
int[] s = true ? new String() : new int[1];  // incompatible types:
bad type in conditional expression

var s = true ? new String() : new int[1]; // works fine
Is it expected that this code compiles?


One related example:
         var s = true ? new String() : new int[1];
         s[0]=123;

testVar.java:11: error: array required, but Serializable found
         s[0]=123;
          ^
1 error

Looks like a surprise - 's' is of type "Serializable"?

On 9/18/17 15:44, 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
>>
>>
> 
> 


-- 
Best regards, Sergey.