Thodoris Sotiropoulos created GROOVY-10913:
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Summary: Type inference yields a wrong type even if the type of
the associated variable is given
Key: GROOVY-10913
URL: https://issues.apache.org/jira/browse/GROOVY-10913
Project: Groovy
Issue Type: Bug
Components: Static Type Checker
Reporter: Thodoris Sotiropoulos
The following code demonstrates a limitation of type inference in a real-world
example.
I have the following program
{code}
import java.util.*;
import java.util.stream.*;
class Test {
void test() {
Stream<Number> x = Arrays.stream(new Integer[]{1}).flatMap((y) ->
Arrays.stream(new Long[]{(long) 1}));
}
}
{code}
h3. Actual behavior
{code}
org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
test.groovy: 6: [Static type checking] - Incompatible generic argument types.
Cannot assign java.util.stream.Stream<java.lang.Long> to:
java.util.stream.Stream<java.lang.Number>
@ line 6, column 26.
Stream<Number> x = Arrays.stream(new Integer[]{1}).flatMap((y) ->
Arrays.stream(new Long[]{(long) 1}));
^
1 error
{code}
h3. Expected behavior
Compile successfully
The code should pass because the signature of the "flatMap" function is "<R>
Stream<R> flatMap(Function<? super T,? extends Stream<? extends R>>
mapper)". And therefore, Function<Integer, Stream<Long>> is a subtype of
Function<? super Integer, ? extends Stream<? extends Number>.
Tested against master (commit: 5a5726342adeb37a6fbaa8cdcbe2d47dee8dc56c)
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