Do not consider uninhabited constructors when performing exhaustive match checking

Author: Alex1005a

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -649,8 +649,13 @@ object SpaceEngine {
         //    <== refineUsingParent(NatT, class Succ, []) = Succ[NatT]
         //    <== isSub(Succ[NatT] <:< Succ[Succ[<?>]]) = false
         def getAppliedClass(tp: Type): Type = tp match
+          case tp @ AppliedType(tycon: TypeRef, _) if tycon.symbol.isClass =>
+            // apply arguments if they contain Nothing,
+            // because it may affect further selection of children of the class
+            val argsNotContainsNothing = tp.args.forall(_.classSymbol != defn.NothingClass)
+            if argsNotContainsNothing then tp
+            else getAppliedClass(tycon.superType.applyIfParameterized(tp.args))
           case tp @ AppliedType(_: HKTypeLambda, _)                        => tp
-          case tp @ AppliedType(tycon: TypeRef, _) if tycon.symbol.isClass => tp
           case tp @ AppliedType(tycon: TypeProxy, _)                       => getAppliedClass(tycon.superType.applyIfParameterized(tp.args))
           case tp                                                          => tp
         val tp = getAppliedClass(tpOriginal)
@@ -660,7 +665,11 @@ object SpaceEngine {
             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 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 noFieldsWithTypeNothing = child.typeRef.fields.forall(_.symbol.typeRef.classSymbol != defn.NothingClass)
+              if noFieldsWithTypeNothing then List(child) else Nil
           }
         val children = trace(i"getChildren($tp)")(getChildren(tp.classSymbol))
 

tests/pos/patmat-nothing-exhaustive.scala

@@ -0,0 +1,12 @@
+//> using options -Xfatal-warnings -deprecation -feature
+
+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