[jdk17] RFR: 8254571: Erroneous generic type inference in a lambda expression with a checked exception

[Jan Lahoda]

On Thu, 17 Jun 2021 16:35:45 GMT, Vicente Romero <vromero at openjdk.org> wrote:

>> Hi,
>> 
>> The compiler is issuing an error for code like:
>> 
>> 
>> class JDK8254571 {
>>     public static void test() {
>>         outer(nested(x -> mightThrow()));
>>     }
>>     
>>     static <A> void outer(Object o) {}
>> 
>>     static <B, C, E extends Throwable> B nested(Fun<C,E> fun) {
>>         return null;
>>     }
>> 
>>     interface Fun<X, Y extends Throwable> {
>>         void m(X t) throws Y;
>>     }
>> 
>>     static void mightThrow() throws Exception {}
>> } 
>> 
>> 
>> there are several things at play here:
>> 
>> 1) for performance reasons we decided to minimize the number of inference variables popped up when inferring nested generic method invocations, see Infer::roots
>> 2) when an expression is stuck (like in this case this implicit lambda) we decide which inference variables should be present in the related inference context and thrown variables were not part of the equation
>> 
>> So there are three hypothesis here:
>> 1) there is a bug in the code that determines which variables are passed up to the nested inference context, meaning that it is passing less variables than the bare minimum needed
>> 2) there is a bug in determining which inference variables should be extracted from stuck expressions
>> 3) both are buggy
>> 
>> The root cause of this bug is that some types escape from Attr with non instantiated type variables in them. I modified the test case to double check if the issue was not related to lambdas, but then the test passed so in the example above I added:
>> 
>> 
>>      static <X, Y extends Throwable> Fun<X, Y> mightThrow2() throws Y { return null; }
>> 
>> 
>> which simulates the lambda and the compiler has no issues compiling this invocation:
>> 
>> 
>>     outer(nested(mightThrow2()));
>> 
>> 
>> so summing up, after double checking the logic of the code that minimized the inference context I realized that the issue is not there. The code is sound and its output only depend on its input, so the issue should be on determining the input. So IMO the issue is in what inference variables we extract from stuck expressions, which is one of the inputs to Infer::roots. Looking for what variables we extract from stuck expressions, I realized that the code currently ignores type variables in the throws clause and this is why my proposed fix is addressing this issue.
>> 
>> Comments?
>
> src/jdk.compiler/share/classes/com/sun/tools/javac/comp/DeferredAttr.java line 1242:
> 
>> 1240:                 stuckVars.addAll(freeArgVars);
>> 1241:                 depVars.addAll(inferenceContext.freeVarsIn(descType.getReturnType()));
>> 1242:                 depVars.addAll(inferenceContext.freeVarsIn(descType.getThrownTypes()));
> 
> did the same for method references given the similarities between both but I couldn't find a test case that reproduced the issue with a method reference

How about:

outer(nested(ConsiderExceptionTVarsInStuckExprs::mightThrow2));
...
static <C> void mightThrow2(C c) throws Exception {}

-------------

PR: https://git.openjdk.java.net/jdk17/pull/91