Add Error type

Author: MaximeKjaer

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

@@ -902,7 +902,7 @@ class Definitions {
   final def isCompiletimeAppliedType(sym: Symbol)(implicit ctx: Context): Boolean = {
     def isOpsPackageObjectAppliedType: Boolean =
       sym.owner == CompiletimeOpsPackageObject.moduleClass && Set(
-        tpnme.Equals, tpnme.NotEquals,
+        tpnme.Error, tpnme.Equals, tpnme.NotEquals,
         tpnme.Plus, tpnme.Minus, tpnme.Times, tpnme.Div, tpnme.Mod,
         tpnme.Lt, tpnme.Gt, tpnme.Ge, tpnme.Le,
         tpnme.Abs, tpnme.Negate, tpnme.Min, tpnme.Max, tpnme.ToString,

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

@@ -210,6 +210,7 @@ object StdNames {
     final val Abs: N       = "Abs"
     final val And: N       = "&&"
     final val Div: N       = "/"
+    final val Error: N     = "Error"
     final val Equals: N    = "=="
     final val Ge: N        = ">="
     final val Gt: N        = ">"

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

@@ -3619,6 +3619,11 @@ object Types {
           case _ => None
         }
 
+        def stringValue(tp: Type): Option[String] = tp match {
+          case ConstantType(Constant(n: String)) => Some(n)
+          case _ => None
+        }
+
         def natValue(tp: Type): Option[Int] = intValue(tp).filter(n => n >= 0 && n < Int.MaxValue)
 
         def constantFold1[T](extractor: Type => Option[T], op: T => Any): Option[Type] =
@@ -3637,6 +3642,7 @@ object Types {
             case tpnme.Negate => constantFold1(intValue, x => -x)
             case tpnme.Not => constantFold1(boolValue, x => !x)
             case tpnme.ToString => constantFold1(intValue, _.toString)
+            case tpnme.Error => constantFold1(stringValue, msg => throw new TypeError(msg))
             case _ => None
           } else if (args.length == 2) tycon.symbol.name match {
             case tpnme.Equals => constantFold2(constValue, _ == _)

library/src/scala/compiletime/ops/package.scala

@@ -3,6 +3,7 @@ package scala.compiletime
 import scala.annotation.infix
 
 package object ops {
+  type Error[Msg <: String] <: Any
   @infix type ==[X <: AnyVal, Y <: AnyVal] <: Boolean
   @infix type !=[X <: AnyVal, Y <: AnyVal] <: Boolean
 

tests/playground/Errors.scala

@@ -0,0 +1,23 @@
+import scala.compiletime.ops._
+
+object Test {
+  type Require[Cond <: Boolean, Msg <: String] = Cond match {
+    case true => Any
+    case false => Error[Msg]
+  }
+
+  opaque type Positive = Int
+
+  object Positive {
+    def apply[T <: Int](value: T)(given Require[T > 0, "The value provided isn't positive"]): Positive = value
+  }
+
+  val x: Positive = Positive[1](1)
+
+  /*
+   |no implicit argument of type Test.Require[Int(-1) > Int(0), String("The value provided isn\'t positive"),
+   |  scala.compiletime.ops.ToString[Int(-1)]
+   |] was found for parameter x$2 of method apply in object Positive
+   */
+  val y: Positive = Positive[-1](-1)
+}