Feature GADT support for class type parameters

- Register class type parameters from outer contexts in typedDefdef.
- Remove a condition to support gadt approximation for class type parameters.
- Add test cases in tests/pos/class-gadt.
- Ignore test cases tests/pos/i4345.scala and i5735.scala (issue scala#11220 and scala#11221)

Author: Ang9876

compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -250,8 +250,7 @@ object Inferencing {
       *     approx, see gadt-approximation-interaction.scala).
       */
     def apply(tp: Type): Type = tp.dealias match {
-      case tp @ TypeRef(qual, nme) if (qual eq NoPrefix)
-                                   && variance != 0
+      case tp @ TypeRef(qual, nme) if variance != 0
                                    && ctx.gadt.contains(tp.symbol)
                                    =>
         val sym = tp.symbol

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2030,6 +2030,15 @@ class Typer extends Namer
     val tparamss = paramss1.collect {
       case untpd.TypeDefs(tparams) => tparams
     }
+
+    // Register GADT constraint for class type parameters from outer to inner class definition. (Useful when nested classes exist.) But do not cross a function definition.
+    if sym.flags.is(Method) then
+      rhsCtx.setFreshGADTBounds
+      ctx.outer.outersIterator.takeWhile(!_.owner.is(Method))
+        .filter(ctx => ctx.owner.isClass && ctx.owner.typeParams.nonEmpty)
+        .toList.reverse
+        .map(ctx => rhsCtx.gadt.addToConstraint(ctx.owner.typeParams))
+
     if tparamss.nonEmpty then
       rhsCtx.setFreshGADTBounds
       val tparamSyms = tparamss.flatten.map(_.symbol)

tests/pos/class-gadt/basic.scala

@@ -0,0 +1,14 @@
+object basic {
+  enum Expr[A] {
+    case IntExpr(value: Int) extends Expr[Int]
+    case Other[T](value: T) extends Expr[T]
+  }
+
+  class C[A] {
+    def eval(e: Expr[A]): A =
+      e match {
+        case Expr.IntExpr(i) => i + 2
+        case Expr.Other(v) => v
+      }
+  }
+}

tests/pos/class-gadt/inheritance.scala

@@ -0,0 +1,15 @@
+object inheritance{
+	enum SUB[-A, +B]:
+		case Refl[S]() extends SUB[S, S]
+
+	class A[T](val v: T) {
+		val foo1: T = v
+	}
+
+	class C[T](val v1: T) extends A[T](v1) {
+		def eval1(t: T, e: SUB[T, Int]): Int =
+			e match {
+				case SUB.Refl() => foo1
+			}
+	}
+}

tests/pos/class-gadt/member.scala

@@ -0,0 +1,11 @@
+object member{
+	enum SUB[-A, +B]:
+		case Refl[S]() extends SUB[S, S]
+
+	class C[T] {
+		def eval1(t: T, e: SUB[T, Int]): Int =
+			e match {
+				case SUB.Refl() => t + 2
+			}
+	}
+}

tests/pos/class-gadt/nestedClass.scala

@@ -0,0 +1,59 @@
+// Nested class for GADT constraining
+object nestedClass{
+  enum Expr[A] {
+    case IntExpr(value: Int) extends Expr[Int]
+    case Other[T](value: T) extends Expr[T]
+  }
+
+  class Outer1[C] {
+    class Inner1 {
+      def eval(e: Expr[C]): C =
+        e match {
+          case Expr.IntExpr(i) => i + 2
+          case Expr.Other(v) => v
+        }
+    }
+  }
+
+  def foo2[C](): Unit = 
+    class Outer2 {
+      class Inner2 {
+        def eval(e: Expr[C]): C =
+          e match {
+            case Expr.IntExpr(i) => i + 2
+            case Expr.Other(v) => v
+          }
+      }
+    }
+
+  class Outer3[C] {
+    def foo3(): Unit = 
+      class Inner3 {
+          def eval(e: Expr[C]): C =
+            e match {
+              case Expr.IntExpr(i) => i + 2
+              case Expr.Other(v) => v
+            }
+      }
+  }
+
+  trait Outer4[C] {
+    class Inner4 {
+      def eval(e: Expr[C]): C =
+        e match {
+          case Expr.IntExpr(i) => i + 2
+          case Expr.Other(v) => v
+        }
+    }
+  }
+
+  class Outer5[C] {
+    object Inner5 {
+      def eval(e: Expr[C]): C =
+        e match {
+          case Expr.IntExpr(i) => i + 2
+          case Expr.Other(v) => v
+        }
+    }
+  }
+}

tests/pos/class-gadt/variance.scala

@@ -0,0 +1,77 @@
+object variance {
+	enum SUB[-A, +B]:
+		case Refl[S]() extends SUB[S, S]
+	// Covariant
+	class C1[+T](v: T){
+		def foo(ev: T SUB Int): Int =
+			ev match {
+				case SUB.Refl() => v
+			}
+	}
+
+	// Contravariant
+	class B2[-T](v: T){}
+
+	class C2[-T](v: T){
+		def foo(ev: Int SUB T): B2[T] =
+			ev match {
+				case SUB.Refl() => new B2(v)
+			}
+	}
+
+	// Variance with inheritance
+
+	// superclass covariant and subclass covariant
+
+	class A3[+T](v: T) {
+		val value = v
+	}
+
+	class C3[+T](v: T) extends A3[T](v){
+		def foo(ev: T SUB Int): Int =
+			ev match {
+				case SUB.Refl() => value
+			}
+	}
+
+
+	// superclass covariant and subclass invariant
+	class A4[+T](v: T) {
+		val value = v
+	}
+
+	class C4[T](v: T) extends A4[T](v){
+		def foo(ev: T SUB Int): Int =
+			ev match {
+				case SUB.Refl() => value
+			}
+	}
+
+	// superclass contravariant and subclass contravariant
+	class B5[-T](v: T){}
+
+	class A5[-T](v: T) {
+		val value = new B5(v)
+	}
+
+	class C5[-T](v: T) extends A5[T](v){
+		def foo(ev: Int SUB T): B5[T] =
+			ev match {
+				case SUB.Refl() => value
+			}
+	}
+
+	// superclass contravariant and subclass invariant
+	class B6[-T](v: T){}
+
+	class A6[-T](v: T) {
+		val value = new B6(v)
+	}
+
+	class C6[-T](v: T) extends A6[T](v){
+		def foo(ev: Int SUB T): B6[T] =
+			ev match {
+				case SUB.Refl() => value
+			}
+	}
+}