Expected Behavior

Flux.thenMany should ignore all emissions of a Flux.concat

Actual Behavior

It only ignores emissions of the last concatted publisher due to an incorrect optimization.

Steps to Reproduce

@Test
void reproCase() {
  Flux.concat(Flux.just("a"), Flux.just("b"))
            // adding a .hide() here fixes the test case
            .thenMany(Flux.just("c"))
            .as(StepVerifier::create)
            .expectNext("c")
            .verifyComplete();
}

Result:

java.lang.AssertionError: expectation "expectNext(c)" failed (expected value: c; actual value: a)

Adding the .hide() or enabling Hooks.onOperatorDebug() fixes the test case.

Possible Solution

Flux.thenMany uses an incorrect optimization:

    if (this instanceof FluxConcatArray) {
	FluxConcatArray<T> fluxConcatArray = (FluxConcatArray<T>) this;
	return fluxConcatArray.concatAdditionalIgnoredLast(other); // <- incorrect!
    }

Your Environment

• Reactor version(s) used: 3.6.1
• JVM version (java -version): openjdk version "21.0.1" 2023-10-17 LTS