Add support for @deprecatedInheritance

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

@@ -1006,6 +1006,7 @@ class Definitions {
   @tu lazy val ProvisionalSuperClassAnnot: ClassSymbol = requiredClass("scala.annotation.internal.ProvisionalSuperClass")
   @tu lazy val DeprecatedAnnot: ClassSymbol = requiredClass("scala.deprecated")
   @tu lazy val DeprecatedOverridingAnnot: ClassSymbol = requiredClass("scala.deprecatedOverriding")
+  @tu lazy val deprecatedInheritance: ClassSymbol = requiredClass("scala.deprecatedInheritance")
   @tu lazy val ImplicitAmbiguousAnnot: ClassSymbol = requiredClass("scala.annotation.implicitAmbiguous")
   @tu lazy val ImplicitNotFoundAnnot: ClassSymbol = requiredClass("scala.annotation.implicitNotFound")
   @tu lazy val InlineParamAnnot: ClassSymbol = requiredClass("scala.annotation.internal.InlineParam")

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

@@ -114,6 +114,16 @@ object RefChecks {
     }
   end checkSelfAgainstParents
 
+  /** warn if `@deprecatedInheritance` is present */
+  def warnDeprecatedInheritance(cls: ClassSymbol, parentTrees: List[Tree])(using Context): Unit =
+    val psyms = cls.parentSyms
+    for (psym, pos) <- psyms.zip(parentTrees.map(_.srcPos))
+        annot <- psym.getAnnotation(defn.deprecatedInheritance)
+    do
+      val msg = annot.argumentConstantString(0).getOrElse("")
+      val since = annot.argumentConstantString(1).getOrElse("")
+      report.deprecationWarning(em"inheritance from $psym is deprecated (since: $since): $msg", pos)
+
   /** Check that self type of this class conforms to self types of parents
    *  and required classes. Also check that only `enum` constructs extend
    *  `java.lang.Enum` and no user-written class extends ContextFunctionN.
@@ -123,6 +133,8 @@ object RefChecks {
       val psyms = cls.asClass.parentSyms
       checkSelfAgainstParents(cls.asClass, psyms)
 
+      warnDeprecatedInheritance(cls.asClass, parentTrees)
+
       def isClassExtendingJavaEnum =
         !cls.isOneOf(Enum | Trait) && psyms.contains(defn.JavaEnumClass)
 

tests/warn/i19002.check

@@ -0,0 +1,20 @@
+-- Deprecation Warning: tests/warn/i19002.scala:5:20 -------------------------------------------------------------------
+5 |class TBar1 extends TFoo // warn
+  |                    ^^^^
+  |                    inheritance from trait TFoo is deprecated (since: FooLib 12.0): this class will be made final
+-- Deprecation Warning: tests/warn/i19002.scala:6:20 -------------------------------------------------------------------
+6 |trait TBar2 extends TFoo // warn
+  |                    ^^^^
+  |                    inheritance from trait TFoo is deprecated (since: FooLib 12.0): this class will be made final
+-- Deprecation Warning: tests/warn/i19002.scala:10:20 ------------------------------------------------------------------
+10 |class CBar1 extends CFoo // warn
+   |                    ^^^^
+   |                   inheritance from class CFoo is deprecated (since: FooLib 11.0): this class will be made final
+-- Deprecation Warning: tests/warn/i19002.scala:14:20 ------------------------------------------------------------------
+14 |class ABar1 extends AFoo // warn
+   |                    ^^^^
+   |                   inheritance from class AFoo is deprecated (since: FooLib 10.0): this class will be made final
+-- Deprecation Warning: tests/warn/i19002.scala:15:20 ------------------------------------------------------------------
+15 |trait ABar2 extends AFoo // warn
+   |                    ^^^^
+   |                   inheritance from class AFoo is deprecated (since: FooLib 10.0): this class will be made final

tests/warn/i19002.scala

@@ -0,0 +1,15 @@
+//> using options -deprecation
+
+@deprecatedInheritance("this class will be made final", "FooLib 12.0")
+trait TFoo
+class TBar1 extends TFoo // warn
+trait TBar2 extends TFoo // warn
+
+@deprecatedInheritance("this class will be made final", "FooLib 11.0")
+class CFoo
+class CBar1 extends CFoo // warn
+
+@deprecatedInheritance("this class will be made final", "FooLib 10.0")
+abstract class AFoo
+class ABar1 extends AFoo // warn
+trait ABar2 extends AFoo // warn