Introduce a compiler option to store generic type information about a lambda expression using the Signature Attribute

[Joel Borggrén-Franck]

> On 7 jan 2015, at 21:18, Alex Buckley <alex.buckley at oracle.com> wrote:
> 
> On 1/7/2015 7:22 AM, Timo Walther wrote:
>> the Java Reflection API allows to access the generic signature of
>> methods through the java.lang.reflect.Method#getGenericReturnType()
>> method. However, this is not yet supported for Lambda expressions.
>> 
>> Given the following classes:
>> 
>> class Tuple2<F0,F1> {
>>   F0 field0;
>>   F1 field1;
>> 
>>   public Tuple2(F0 f0, F1 f1) {
>>     this.field0 = f0;
>>     this.field1 = f1;
>>   }
>> }
>> 
>> interface Map<IN, OUT> {
>>   OUT map(IN in);
>> }
>> 
>> Currently, there is no solution that allows to get further type
>> information about expressions like:
>> 
>> Map<String, Tuple2<String, Integer>> map = (str) -> new Tuple2<>(str, 1);
>> System.out.println(getReturnType(map)) // can only print Tuple2 =>
>> information about the Tuple2's fields are always lost
> 
> map is a variable, so it doesn't have a return type.
> 
> If map is a class variable or an instance variable, then Field#getGenericType should help.
> 
> Expressions (including lambda expressions) aren't generally exposed through Core Reflection. If an expression (such as a lambda expression) happens to be implemented as a method, then relying on the [generic] signature of that method is dangerous since it may change from release to release.
> 

One could argue that we should generate more attributes for synthetic methods. We did for example add annotations to bridges [1]. While we do want to it discourage depending on a specific translation strategy, I’m not sure adding Signature will make more code break once/if we change translation strategy.

cheers
/Joel