Unapply with trivial union type crashes the compiler

[LPTK]

I noticed some strange logic in the handling of unapply return types, so I poked in the REPL a bit and it blew up:

Minimized code

scala> case class A(a: Int)
// defined case class A

scala> object C { def unapply(a: A): true | true = true }
// defined object C

scala> (A(1): A | A) match { case C() => "OK" }
Exception in thread "main" java.lang.AssertionError: assertion failed

Output (click arrow to expand)

Exception in thread "main" java.lang.AssertionError: assertion failed
	at dotty.DottyPredef$.assertFail(DottyPredef.scala:16)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.unapplyPlan$3$$anonfun$2(PatternMatcher.scala:340)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:106)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.unapplyPlan$1(PatternMatcher.scala:358)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.patternPlan(PatternMatcher.scala:384)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.caseDefPlan(PatternMatcher.scala:420)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan$$anonfun$2$$anonfun$1(PatternMatcher.scala:427)
	at scala.collection.immutable.List.foldRight(List.scala:324)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan$$anonfun$1(PatternMatcher.scala:427)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.letAbstract(PatternMatcher.scala:106)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.matchPlan(PatternMatcher.scala:428)
	at dotty.tools.dotc.transform.PatternMatcher$Translator.translateMatch(PatternMatcher.scala:980)
	at dotty.tools.dotc.transform.PatternMatcher.transformMatch(PatternMatcher.scala:41)
	at dotty.tools.dotc.transform.MegaPhase.goMatch(MegaPhase.scala:746)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:345)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:396)
	at dotty.tools.dotc.transform.MegaPhase.mapValDef$1(MegaPhase.scala:234)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:238)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:394)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:404)
	at dotty.tools.dotc.transform.MegaPhase.$anonfun$1(MegaPhase.scala:409)
	at scala.collection.immutable.List.mapConserve(List.scala:444)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:409)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:339)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:396)
	at dotty.tools.dotc.transform.MegaPhase.transformNamed$1(MegaPhase.scala:251)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:394)
	at dotty.tools.dotc.transform.MegaPhase.transformStat$2(MegaPhase.scala:404)
	at dotty.tools.dotc.transform.MegaPhase.$anonfun$1(MegaPhase.scala:409)
	at scala.collection.immutable.List.mapConserve(List.scala:444)
	at dotty.tools.dotc.transform.MegaPhase.transformStats(MegaPhase.scala:409)
	at dotty.tools.dotc.transform.MegaPhase.mapPackage$1(MegaPhase.scala:356)
	at dotty.tools.dotc.transform.MegaPhase.transformUnnamed$1(MegaPhase.scala:359)
	at dotty.tools.dotc.transform.MegaPhase.transformTree(MegaPhase.scala:396)
	at dotty.tools.dotc.transform.MegaPhase.transformUnit(MegaPhase.scala:415)
	at dotty.tools.dotc.transform.MegaPhase.run(MegaPhase.scala:427)
	at dotty.tools.dotc.core.Phases$Phase.runOn$$anonfun$1(Phases.scala:318)
	at scala.collection.immutable.List.map(List.scala:219)
	at dotty.tools.dotc.core.Phases$Phase.runOn(Phases.scala:319)
	at dotty.tools.dotc.Run.runPhases$4$$anonfun$4(Run.scala:165)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
	at scala.collection.ArrayOps$.foreach$extension(ArrayOps.scala:1323)
	at dotty.tools.dotc.Run.runPhases$5(Run.scala:175)
	at dotty.tools.dotc.Run.compileUnits$$anonfun$1(Run.scala:183)
	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
	at dotty.tools.dotc.util.Stats$.maybeMonitored(Stats.scala:64)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:190)
	at dotty.tools.dotc.Run.compileUnits(Run.scala:132)
	at dotty.tools.repl.ReplCompiler.runCompilationUnit(ReplCompiler.scala:156)
	at dotty.tools.repl.ReplCompiler.compile(ReplCompiler.scala:166)
	at dotty.tools.repl.ReplDriver.compile(ReplDriver.scala:229)
	at dotty.tools.repl.ReplDriver.interpret(ReplDriver.scala:193)
	at dotty.tools.repl.ReplDriver.loop$1(ReplDriver.scala:127)
	at dotty.tools.repl.ReplDriver.runUntilQuit$$anonfun$1(ReplDriver.scala:130)
	at dotty.tools.repl.ReplDriver.withRedirectedOutput(ReplDriver.scala:148)
	at dotty.tools.repl.ReplDriver.runUntilQuit(ReplDriver.scala:130)
	at dotty.tools.repl.Main$.main(Main.scala:6)
	at dotty.tools.repl.Main.main(Main.scala)