Fix #6312: Add migration warning for @compileTimeOnly

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

@@ -857,6 +857,8 @@ class Definitions {
   def TailrecAnnot(implicit ctx: Context): ClassSymbol = TailrecAnnotType.symbol.asClass
   lazy val TransientParamAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.constructorOnly")
   def TransientParamAnnot(implicit ctx: Context): ClassSymbol = TransientParamAnnotType.symbol.asClass
+  lazy val CompileTimeOnlyAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.compileTimeOnly")
+  def CompileTimeOnlyParamAnnot(implicit ctx: Context): ClassSymbol = CompileTimeOnlyAnnotType.symbol.asClass
   lazy val SwitchAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.switch")
   def SwitchAnnot(implicit ctx: Context): ClassSymbol = SwitchAnnotType.symbol.asClass
   lazy val ThrowsAnnotType: TypeRef = ctx.requiredClassRef("scala.throws")

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

@@ -401,6 +401,8 @@ object Checking {
       if (!sym.is(Deferred))
         fail(NativeMembersMayNotHaveImplementation(sym))
     }
+    if (sym.hasAnnotation(defn.CompileTimeOnlyParamAnnot))
+      ctx.migrationWarning("`@compileTimeOnly(msg)` will be replaced by `scala.compiletime.error(msg)`", sym.sourcePos)
     else if (sym.is(Deferred, butNot = Param) && !sym.isType && !sym.isSelfSym) {
       if (!sym.owner.isClass || sym.owner.is(Module) || sym.owner.isAnonymousClass)
         fail(OnlyClassesCanHaveDeclaredButUndefinedMembers(sym))

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -173,6 +173,7 @@ class CompilationTests extends ParallelTesting {
       "tests/neg-custom-args/toplevel-samesource/nested/S.scala"),
       defaultOptions) +
     compileFile("tests/neg-custom-args/i6300.scala", allowDeepSubtypes)
+    compileFile("tests/neg-custom-args/i6312.scala", defaultOptions and "-Xfatal-warnings" and "-migration")
   }.checkExpectedErrors()
 
   @Test def fuzzyAll: Unit = {

tests/neg-custom-args/i6312.scala

@@ -0,0 +1,6 @@
+class Foo {
+  inline def foo: Unit = {
+    @scala.annotation.compileTimeOnly("some message") val res = ??? // error
+    res
+  }
+}

tests/neg/i6312.scala

@@ -0,0 +1,6 @@
+class Foo {
+  inline def foo: Unit = {
+    scala.compiletime.error("some message")
+  }
+  foo // error: some message
+}

tests/pos/i6312.scala

@@ -0,0 +1,5 @@
+class Foo {
+  inline def foo: Unit = {
+    scala.compiletime.error("some message")
+  }
+}