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BETA_JAVA8: This bug is to record & track a clarification request to the 335 EG: // ---- 8< ------- The following lambda expression does not compile with 8b74, while the implementation of the method in a class does compile: // ------------ interface G1 { <E extends Exception> Object m(E p) throws E; } interface G2 { <F extends Exception> String m(F q) throws Exception; } interface G extends G1, G2 {} // G has descriptor <F extends Exception> ()->String throws F public class X { G g = (x) -> { // Elided type is inferred from descriptor to be F throw x; // ~== throw new F() }; } class Y implements G { public <T extends Exception> String m(T t) throws T { throw t; } } // ----------------------- Is this a bug in the implementation or the specification ? Since the lambda cannot refer to the type variables of the descriptor by name nor declare its own, if 8b74 behavior is correct, that would mean that, no checked exceptions could be thrown from a lambda if the descriptor of the target interface method mentions a type variable in throws clause. My analysis is that the lambda should be accepted as it does not manufacture any fresh exceptions and simply throws an exception that is known to have satisfied the constraints at the invocation site, i.e no additional/unexpected surprises are there due the lambda throwing the object it was handed.
org.eclipse.jdt.core.tests.compiler.regression.NegativeLambdaExpressionsTest.test041() encodes current Eclipse compiler behavior which differs from javac 8b74
Two observations from the EG: 1. A lambda cannot implement a generic method. While the syntactic limitations around not being able to provide type parameter are well know, even on an inferred basis, this is not allowed. If the descriptor is generic, then the compiler should refuse to compile. (For Reference expressions, it is ok, assuming other constraints hold) 2. Javac has a bug that results in accepting a lambda expression that implements a generic method. (in comment#0, the complaint is not about the attempt to use a lambda expression to implement a generic function descriptor, but about a throw) I have adjusted our code base to reflect the clarification. We now issue a message: "Illegal lambda expression: Method {0} of type {1} is generic"
(In reply to comment #2) > Two observations from the EG: > > 1. A lambda cannot implement a generic method. Whether to allow a lambda to implement a generic method is on the basis that it would infer unmentionable type variables is opened as a topic of discussion and I don't see much interest so far. The spec is being clarified to explicitly ban a lambda from implementing a generic method. javac's observed behavior has been acknowledged as a bug. Since ECJ's behavior has already been adjusted to ban this scenario, closing this bug. Will reopen if there is any change in spec.
