Fix scala#4863: support @compileTimeOnly

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

@@ -38,6 +38,10 @@ object Annotations {
 
     def sameAnnotation(that: Annotation)(implicit ctx: Context): Boolean =
       symbol == that.symbol && tree.sameTree(that.tree)
+
+    def stringArg(index: Int)(implicit ctx: Context)  = argumentConstant(index) map (_.stringValue)
+    def intArg(index: Int)(implicit ctx: Context)     = argumentConstant(index) map (_.intValue)
+    def booleanArg(index: Int)(implicit ctx: Context) = argumentConstant(index) map (_.booleanValue)
   }
 
   case class ConcreteAnnotation(t: Tree) extends Annotation {

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

@@ -855,6 +855,8 @@ class Definitions {
   def TransientParamAnnot(implicit ctx: Context): ClassSymbol = TransientParamAnnotType.symbol.asClass
   lazy val SwitchAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.switch")
   def SwitchAnnot(implicit ctx: Context): ClassSymbol = SwitchAnnotType.symbol.asClass
+  lazy val CompileTimeOnlyAnnotType: TypeRef = ctx.requiredClassRef("scala.annotation.compileTimeOnly")
+  def CompileTimeOnlyAnnot(implicit ctx: Context): ClassSymbol = CompileTimeOnlyAnnotType.symbol.asClass
   lazy val ThrowsAnnotType: TypeRef = ctx.requiredClassRef("scala.throws")
   def ThrowsAnnot(implicit ctx: Context): ClassSymbol = ThrowsAnnotType.symbol.asClass
   lazy val TransientAnnotType: TypeRef                = ctx.requiredClassRef("scala.transient")

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

@@ -837,6 +837,17 @@ object SymDenotations {
       this.exists && (test(symbol) || allOverriddenSymbols.exists(test))
     }
 
+    /** Should reference to the symbol disappear after typer?
+     *
+     *  Used in inlining and macros to enforce invariants.
+     */
+    def isCompileTimeOnly(implicit ctx: Context): Boolean =
+      hasAnnotation(defn.CompileTimeOnlyAnnot)
+
+    /** Get the message associated with @compileTimeOnly annotation */
+    def compileTimeOnlyMessage(implicit ctx: Context): Option[String]  =
+      getAnnotation(defn.CompileTimeOnlyAnnot) flatMap (_ stringArg 0)
+
     // ------ access to related symbols ---------------------------------
 
     /* Modules and module classes are represented as follows:

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

@@ -833,16 +833,15 @@ object RefChecks {
         case _ =>
       }
     }
-    /*  (Not enabled yet)
-       *  See an explanation of compileTimeOnly in its scaladoc at scala.annotation.compileTimeOnly.
-       *
-      if (sym.isCompileTimeOnly) {
-        def defaultMsg =
-          sm"""Reference to ${sym.fullLocationString} should not have survived past type checking,
-              |it should have been processed and eliminated during expansion of an enclosing macro."""
-        // The getOrElse part should never happen, it's just here as a backstop.
-        ctx.error(sym.compileTimeOnlyMessage getOrElse defaultMsg, pos)
-      }*/
+
+    if (sym.isCompileTimeOnly && !sym.owner.isInlineMethod) {
+      def defaultMsg: String =
+        em"""Reference to ${sym.show} should not have survived past type checking,
+            |it should have been processed and eliminated during expansion of an enclosing macro."""
+      // The getOrElse part should never happen, it's just here as a backstop.
+      val msg = sym.compileTimeOnlyMessage.getOrElse(defaultMsg)
+      ctx.error(msg, pos)
+    }
   }
 
   /** Check that a deprecated val or def does not override a

tests/neg/i4863.scala

@@ -0,0 +1,16 @@
+import scala.annotation.compileTimeOnly
+
+sealed trait Nat
+case class S(n: Nat) extends Nat
+case object Z extends Nat
+
+inline def pred(n: Nat) = inline n match {
+  case S(m) => m
+  case Z =>
+    @compileTimeOnly("n cannot be Z") val res = ???
+    res
+}
+
+class Test {
+  pred(Z) // error
+}

tests/pos/i4863.scala

@@ -0,0 +1,16 @@
+import scala.annotation.compileTimeOnly
+
+sealed trait Nat
+case class S(n: Nat) extends Nat
+case object Z extends Nat
+
+inline def pred(n: Nat) = inline n match {
+  case S(m) => m
+  case Z =>
+    @compileTimeOnly("n cannot be Z") val res = ???
+    res
+}
+
+class Test {
+  pred(S(Z))
+}