Do not consider uninhabited constructors when performing exhaustive match checking (scala#21750)

Author: dwijnand

Diff for: compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -648,21 +648,38 @@ object SpaceEngine {
         // we get
         //    <== refineUsingParent(NatT, class Succ, []) = Succ[NatT]
         //    <== isSub(Succ[NatT] <:< Succ[Succ[<?>]]) = false
-        def getAppliedClass(tp: Type): Type = tp match
-          case tp @ AppliedType(_: HKTypeLambda, _)                        => tp
-          case tp @ AppliedType(tycon: TypeRef, _) if tycon.symbol.isClass => tp
+        def getAppliedClass(tp: Type): (Type, List[Type]) = tp match
+          case tp @ AppliedType(_: HKTypeLambda, _)                        => (tp, Nil)
+          case tp @ AppliedType(tycon: TypeRef, _) if tycon.symbol.isClass => (tp, tp.args)
           case tp @ AppliedType(tycon: TypeProxy, _)                       => getAppliedClass(tycon.superType.applyIfParameterized(tp.args))
-          case tp                                                          => tp
-        val tp = getAppliedClass(tpOriginal)
-        def getChildren(sym: Symbol): List[Symbol] =
+          case tp                                                          => (tp, Nil)
+        val (tp, typeArgs) = getAppliedClass(tpOriginal)
+        // This function is needed to get the arguments of the types that will be applied to the class.
+        // This is necessary because if the arguments of the types contain Nothing, 
+        // then this can affect whether the class will be taken into account during the exhaustiveness check
+        def getTypeArgs(parent: Symbol, child: Symbol, typeArgs: List[Type]): List[Type] =
+          val superType = child.typeRef.superType
+          if typeArgs.exists(_.isBottomType) && superType.isInstanceOf[ClassInfo] then
+            val parentClass = superType.asInstanceOf[ClassInfo].declaredParents.find(_.classSymbol == parent).get
+            val paramTypeMap = Map.from(parentClass.argTypes.map(_.typeSymbol).zip(typeArgs))
+            val substArgs = child.typeRef.typeParamSymbols.map(param => paramTypeMap.getOrElse(param, WildcardType))
+            substArgs
+          else Nil
+        def getChildren(sym: Symbol, typeArgs: List[Type]): List[Symbol] =
           sym.children.flatMap { child =>
             if child eq sym then List(sym) // i3145: sealed trait Baz, val x = new Baz {}, Baz.children returns Baz...
             else if tp.classSymbol == defn.TupleClass || tp.classSymbol == defn.NonEmptyTupleClass then
               List(child) // TupleN and TupleXXL classes are used for Tuple, but they aren't Tuple's children
-            else if (child.is(Private) || child.is(Sealed)) && child.isOneOf(AbstractOrTrait) then getChildren(child)
-            else List(child)
+            else if (child.is(Private) || child.is(Sealed)) && child.isOneOf(AbstractOrTrait) then
+              getChildren(child, getTypeArgs(sym, child, typeArgs))
+            else
+              val childSubstTypes = child.typeRef.applyIfParameterized(getTypeArgs(sym, child, typeArgs))
+              // if a class contains a field of type Nothing,
+              // then it can be ignored in pattern matching, because it is impossible to obtain an instance of it
+              val existFieldWithBottomType = childSubstTypes.fields.exists(_.info.isBottomType)
+              if existFieldWithBottomType then Nil else List(child)
           }
-        val children = trace(i"getChildren($tp)")(getChildren(tp.classSymbol))
+        val children = trace(i"getChildren($tp)")(getChildren(tp.classSymbol, typeArgs))
 
         val parts = children.map { sym =>
           val sym1 = if (sym.is(ModuleClass)) sym.sourceModule else sym

Diff for: tests/init-global/pos/i18629.scala

@@ -1,6 +1,6 @@
 object Foo {
   val bar = List() match {
     case List() => ???
-    case _ => ???
+    case null => ???
   }
 }

Diff for: tests/patmat/i13931.scala

@@ -3,5 +3,5 @@ class Test:
     case Seq() => println("empty")
     case _ => println("non-empty")
 
-  def test2 = IndexedSeq() match { case IndexedSeq() => case _ => }
+  def test2 = IndexedSeq() match { case IndexedSeq() => case null => }
   def test3 = IndexedSeq() match { case IndexedSeq(1) => case _ => }

Diff for: tests/warn/patmat-nothing-exhaustive.scala

@@ -0,0 +1,10 @@
+enum TestAdt:
+  case Inhabited
+  case Uninhabited(no: Nothing)
+
+def test1(t: TestAdt): Int = t match
+  case TestAdt.Inhabited => 1
+
+def test2(o: Option[Option[Nothing]]): Int = o match
+  case Some(None) => 1
+  case None => 2