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

Maurizio Cimadamore maurizio.cimadamore at oracle.com
Mon Sep 18 23:35:46 UTC 2017

Uhm - this is a bug in the diagnostic generation for overload resolution 
- although not a bug in the 'var' support.

I could reproduce same issue with this code:

import java.util.*;

class Test {

void test() {
    make(new ArrayList<String>(), new ArrayList<Integer>()).add("");

<Z> Z make(Z z1, Z z2) {
    return null;

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).


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

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