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

Maurizio Cimadamore maurizio.cimadamore at oracle.com
Tue Sep 19 15:04:34 UTC 2017

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:


New diags:



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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