assertAll

Runs every assertion, collecting every failure instead of stopping at the first, then reports all of them together.

What it throws depends on what it collected:

• Only AssertionErrors — one aggregate AssertionError naming every failure: 2 assertion(s) failed:\n - …\n - ….

• Anything else in the mix — a NoSuchElementException from a single(), an IllegalStateException from production code — then that throwable is what propagates, as itself: the original object, its own type, its own message, its own stack. Every other failure collected rides along on its suppressedExceptions, so no diagnosis is lost. If more than one non-assertion throwable is collected, the first is the primary and the rest ride along too.

• A CancellationException — propagates immediately and undecorated, ahead of any collection, per the repo's standing exception discipline. It should be unreachable: assertAll takes non-suspend lambdas, so no Flow.first() or runBlocking can appear inside one. Treat a hit as a bug to surface, not a case to aggregate.

Carrying the diagnoses on the primary rather than folding the primary into an aggregate is deliberate, and is where this differs from JUnit 5's Assertions.assertAll and Kotest's assertSoftly: both throw an aggregate and demote a genuine production error to a suppressed entry, costing you its type and its stack (#2283). Suppression is measured to survive and to render on every kuilt target — JVM, Android, wasmJs, iOS and macOS — by ThrowableSuppressionTest.

What this replaced, because the old advice is still in a lot of comments (#1823, #2283). assertAll used to catch only AssertionError, so a single() / first() / last() / getValue() / !! / [i] on the collection under test aborted the whole block exactly when that collection was empty or short — the broken state you were trying to diagnose. Every later assertion was skipped and every failure already collected was discarded, so you were shown NoSuchElementException: List is empty. instead of the named failures you wrote. That is fixed here, for all 210 blocks the #2283 sweep found, rather than site by site.

So writing an assertion list-shaped is now good style, not a safety requirement: it still reads better and still gives you a named failure printing both sides, but it is no longer what stands between you and losing the diagnosis.

assertAll(
    { assertEquals(1, received.size, "one frame delivered") },
    { assertEquals(listOf(origin), received.map { it.sender }) },
)