add method, annotation and test cases

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1034,6 +1034,7 @@ class Definitions {
   @tu lazy val TransparentTraitAnnot: ClassSymbol = requiredClass("scala.annotation.transparentTrait")
   @tu lazy val NativeAnnot: ClassSymbol = requiredClass("scala.native")
   @tu lazy val RepeatedAnnot: ClassSymbol = requiredClass("scala.annotation.internal.Repeated")
+  @tu lazy val RuntimeCheckedAnnot: ClassSymbol = requiredClass("scala.annotation.internal.RuntimeChecked")
   @tu lazy val SourceFileAnnot: ClassSymbol = requiredClass("scala.annotation.internal.SourceFile")
   @tu lazy val ScalaSignatureAnnot: ClassSymbol = requiredClass("scala.reflect.ScalaSignature")
   @tu lazy val ScalaLongSignatureAnnot: ClassSymbol = requiredClass("scala.reflect.ScalaLongSignature")

compiler/src/dotty/tools/dotc/semanticdb/ExtractSemanticDB.scala

@@ -458,14 +458,15 @@ object ExtractSemanticDB:
       def unapply(tree: ValDef)(using Context): Option[(Tree, Tree)] = tree.rhs match
 
         case Match(Typed(selected: Tree, tpt: TypeTree), CaseDef(pat: Tree, _, _) :: Nil)
-        if tpt.span.exists && !tpt.span.hasLength && tpt.tpe.isAnnotatedByUnchecked =>
+        if tpt.span.exists && !tpt.span.hasLength && tpt.tpe.isAnnotatedByUncheckedOrRuntimeChecked =>
           Some((pat, selected))
 
         case _ => None
 
       extension (tpe: Types.Type)
-        private inline def isAnnotatedByUnchecked(using Context) = tpe match
-          case Types.AnnotatedType(_, annot) => annot.symbol == defn.UncheckedAnnot
+        private inline def isAnnotatedByUncheckedOrRuntimeChecked(using Context) = tpe match
+          case Types.AnnotatedType(_, annot) =>
+            annot.symbol == defn.UncheckedAnnot || annot.symbol == defn.RuntimeCheckedAnnot
           case _                             => false
 
       def collectPats(pat: Tree): List[Tree] =

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

@@ -794,6 +794,7 @@ object SpaceEngine {
       }
 
     !sel.tpe.hasAnnotation(defn.UncheckedAnnot)
+    && !sel.tpe.hasAnnotation(defn.RuntimeCheckedAnnot)
     && {
       ctx.settings.YcheckAllPatmat.value
       || isCheckable(sel.tpe)
@@ -903,7 +904,7 @@ object SpaceEngine {
   def checkMatch(m: Match)(using Context): Unit =
     checkMatchExhaustivityOnly(m)
     if reachabilityCheckable(m.selector) then checkReachability(m)
-  
+
   def checkMatchExhaustivityOnly(m: Match)(using Context): Unit =
     if exhaustivityCheckable(m.selector) then checkExhaustivity(m)
 }

library/src/scala/annotation/internal/RuntimeChecked.scala

@@ -0,0 +1,11 @@
+package scala.annotation.internal
+
+import scala.annotation.Annotation
+import scala.annotation.experimental
+
+/**An annotation marking an intention that all checks on a value can be reliably performed at runtime.
+ *
+ * The compiler will remove certain static checks except those that can't be performed at runtime.
+ */
+@experimental
+final class RuntimeChecked() extends Annotation

library/src/scala/runtime/stdLibPatches/Predef.scala

@@ -1,6 +1,7 @@
 package scala.runtime.stdLibPatches
 
 import scala.annotation.experimental
+import scala.annotation.internal.RuntimeChecked
 
 object Predef:
   import compiletime.summonFrom
@@ -80,4 +81,16 @@ object Predef:
   @experimental
   infix type is[A <: AnyKind, B <: Any{type Self <: AnyKind}] = B { type Self = A }
 
+  extension (x: Any)
+    /**Asserts that a term should be exempt from static checks that can be reliably checked at runtime.
+     * @example {{{
+     * val xs: Option[Int] = Some(1)
+     * xs.runtimeChecked match {
+     *   case is: Some[Int] => is.get
+     * } // no warning about exhaustiveness, as all patterns can be checked at runtime.
+     * }}}
+     */
+    @experimental
+    inline def runtimeChecked: x.type @RuntimeChecked = x: @RuntimeChecked
+
 end Predef

tests/neg/runtimeChecked-2.check

@@ -0,0 +1,5 @@
+-- [E030] Match case Unreachable Warning: tests/neg/runtimeChecked-2.scala:13:11 ---------------------------------------
+13 |      case is: Some[t] => ??? // unreachable
+   |           ^^^^^^^^^^^
+   |           Unreachable case
+No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/neg/runtimeChecked-2.scala

@@ -0,0 +1,16 @@
+//> using options -Werror -source:future
+
+import annotation.experimental
+
+@experimental
+object Foo {
+
+  val xs: Option[Int] = Some(1)
+
+  def test: Int =
+    xs.runtimeChecked match { // this test asserts that reachability is not avoided by runtimeChecked
+      case is: Some[t] => is.get
+      case is: Some[t] => ??? // unreachable
+    }
+}
+// nopos-error: No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/neg/runtimeChecked.check

@@ -0,0 +1,7 @@
+-- [E092] Pattern Match Unchecked Warning: tests/neg/runtimeChecked.scala:14:11 ----------------------------------------
+14 |      case is: ::[Int/* can not be checked so still err */] => is.head
+   |           ^
+   |the type test for ::[Int] cannot be checked at runtime because its type arguments can't be determined from List[Any]
+   |
+   | longer explanation available when compiling with `-explain`
+No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/neg/runtimeChecked.scala

@@ -0,0 +1,17 @@
+//> using options -Werror -source:future
+
+import annotation.experimental
+
+@experimental
+object Foo {
+
+  val xs: List[Any] = List(1: Any)
+
+  def test: Int =
+    xs.runtimeChecked match { // this test asserts that unsound type tests still require @unchecked
+      // tests/run/runtimeChecked.scala adds @unchecked to the
+      // unsound type test to avoid the warning.
+      case is: ::[Int/* can not be checked so still err */] => is.head
+    }
+}
+// nopos-error: No warnings can be incurred under -Werror (or -Xfatal-warnings)

tests/run/runtimeChecked.scala

@@ -0,0 +1,16 @@
+//> using options -Werror -source:future
+
+import annotation.experimental
+
+
+val xs: List[Any] = List(1: Any)
+
+@experimental
+@main
+def Test: Unit =
+  val head = xs.runtimeChecked match {
+    // tests/neg/runtimeChecked.scala asserts that @unchecked is
+    // still needed for unsound type tests.
+    case is: ::[Int @unchecked] => is.head
+  }
+  assert(head == 1)