This test:

import java.util.function.*;

class Test {
    <Z> void m(Consumer<Z> c, Class<Z> cl) {

    }

    void test(boolean b, int i) {
        m(s -> s.length(), String.class);
        m(b ? s -> s.length() : s -> s.length(), String.class);
        m(switch (i) {
            case 0 -> s -> s.length();
            default -> s -> s.length();
        }, String.class);
    }
}


Fails to compile:

Test.java:12: error: cannot find symbol
            case 0 -> s -> s.length();
                            ^
  symbol: method length()
  location: variable s of type Z
  where Z is a type-variable:
    Z extends Object declared in method <Z>m(Consumer<Z>,Class<Z>)
Test.java:13: error: cannot find symbol
            default -> s -> s.length();
                             ^
  symbol: method length()
  location: variable s of type Z
  where Z is a type-variable:
    Z extends Object declared in method <Z>m(Consumer<Z>,Class<Z>)
Test.java:11: error: incompatible types: inference variable Z has incompatible equality constraints String,Z
        m(switch (i) {
         ^
  where Z is a type-variable:
    Z extends Object declared in method <Z>m(Consumer<Z>,Class<Z>)
3 errors


Note that the first call and the second compile without issues.

The problem is caused by the fact that DeferredAttr.PolyScanner does not look inside switch expression nodes. As a result, DeferredAttr.OverloadStuckPolicy and DefrredAttr.CheckStuckPolicy do not collect "stuck" type variables from functional interface targets required inside the arms of a switch expression (unlike what happens e.g. for conditionals). Because of that, Consumer<Z> is not detected as a non-concrete type; as such the compiler just tries to use it as a target, which generates the spurious errors seen above.