Make compile-time operations on stable arguments stable

Author: mbovel

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

@@ -165,6 +165,8 @@ object Types {
       case _: SingletonType | NoPrefix => true
       case tp: RefinedOrRecType => tp.parent.isStable
       case tp: ExprType => tp.resultType.isStable
+      case AppliedType(tycon: TypeRef, args: List[Type])
+        if defn.isCompiletimeAppliedType(tycon.symbol) && args.forall(_.isStable) => true
       case tp: AnnotatedType =>
         // NOTE UncheckedStableAnnot was originally meant to be put on fields,
         // not on types. Allowing it on types is a Scala 3 extension. See:
@@ -4242,7 +4244,7 @@ object Types {
             // final val one = 1
             // type Two = one.type + one.type
             // ```
-            case tp: TermRef => tp.underlying
+            case tp: TypeProxy if tp.underlying.isStable => tp.underlying.fixForEvaluation
             case tp => tp
           }
 

tests/neg/singleton-ops-int.scala

@@ -116,4 +116,24 @@ object Test {
 
   val t83: ToDouble[1] = 1.0
   val t84: ToDouble[2] = 2 // error
+
+  // Singletons are dereferenced
+  val t85: Int = 5
+  val t86: t85.type = t85
+  summon[t85.type + t85.type =:= t86.type + t86.type]
+
+  // Singletons are dereferenced recursively
+  val t87: t86.type = t87
+  summon[t85.type + t85.type =:= t87.type + t87.type]
+
+  // Skolems of compile-time types are dereferenced:
+  // (?1 : (Test.x : Int) * (Test.x : Int)) --> (Test.x : Int) * (Test.x : Int)
+  def mult(x: Int, y: Int): x.type * y.type = (x * y).asInstanceOf
+  val t88: t85.type * t85.type * t85.type = mult(mult(t85, t85), t85)
+
+  // Compile-time operations with singleton arguments are singletons
+  summon[t85.type + t86.type <:< Singleton]
+
+  // Compile-time operations with non-singleton arguments are not singletons  
+  summon[t85.type + Int <:< Singleton] // error
 }