[GROOVY-10699] Unable to infer type argument from the parameter type of a lambda

• Type: Bug
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 4.0.5
• Component/s: Static Type Checker
• Assignee: Eric Milles
• Reporter: Thodoris Sotiropoulos
• Created: 21/Jul/22 13:07
• Updated: 10/Feb/23 21:39
• Resolved: 24/Jul/22 15:07

Description

I have the following program:

import java.util.function.Function;
import java.util.function.Consumer;
import java.util.function.Supplier;


class A<T> {
  A(C<T> x) {}
}

class C<T> {
  C(T x ) {}
}

class B<X> {
  B(Consumer<X> x) {}
}

class D<X> {
  D(Supplier<X> x) {}
}


class Test {
  void test() {
    A<String> x = new A<>(new C<>("str")); // type inference works (type inferred based on the constant "str")
    D<String> z = new D<>(() -> "fda"); // type inference works (type inferred based on the return type of the lambda)
    B<String> y = new B<>((String e) -> {return;} ); // type inference doesn't work
  }
}

Actual behavior

org.codehaus.groovy.control.MultipleCompilationErrorsException: startup failed:
test.groovy: 27: [Static type checking] - Expected type java.lang.Object for lambda parameter: e
 @ line 27, column 28.
       B<String> y = new B<>((String e) -> {return;} ); // type inference doesn't work
                              ^

1 error

Expected behavior

Compile successfully

Tested against master (commit: 63bcab1bf13fb3811626fb1727c22e86528feb7f)

*Notes*: As indicated in the accompanying test case, the compiler is unable to infer the type argument of a parameterized constructor call from the parameter of a lambda, although it works for any other case (including the return type of a lambda).