Fixes #1856: mimic Scalac sematic of recursive lazy vals.

compiler/src/dotty/tools/dotc/transform/LazyVals.scala

@@ -186,8 +186,22 @@ class LazyVals extends MiniPhaseTransform with IdentityDenotTransformer {
 
     def mkNonThreadSafeDef(target: Tree, flag: Tree, rhs: Tree)(implicit ctx: Context) = {
       val setFlag = flag.becomes(Literal(Constants.Constant(true)))
-      val setTargets = if (isWildcardArg(rhs)) Nil else target.becomes(rhs) :: Nil
-      val init = Block(setFlag :: setTargets, target.ensureApplied)
+      val flagSet = setFlag :: Nil
+
+      val init = evalOnce(rhs) { rhsRef =>
+        val targetSet = if (isWildcardArg(rhs)) flagSet else target.becomes(rhsRef) :: flagSet
+        val checkValidity =
+          tpd.If(
+            target.select(defn.Any_!=).appliedTo(tpd.defaultValue(target.tpe.widenDealias)).select(defn.Boolean_&&).
+              appliedTo(
+                target.select(defn.Any_!=).appliedTo(rhsRef)
+              ),
+            tpd.Throw(tpd.New(ctx.requiredClass("java.lang.IllegalStateException").namedType, Nil)),
+            tpd.EmptyTree
+          )
+        Block(checkValidity :: targetSet, rhsRef)
+      }
+
       If(flag.ensureApplied, target.ensureApplied, init)
     }
 

tests/run/i1856.check

@@ -0,0 +1,11 @@
+Iteration 0
+Iteration 1
+Iteration 2
+Iteration 3
+Iteration 4
+Iteration 5
+Iteration 6
+Iteration 7
+Iteration 8
+Iteration 9
+42

tests/run/i1856.scala

@@ -0,0 +1,14 @@
+object Test {
+  var count: Int = 0
+  lazy val lzy: Int = {
+    if (count < 10) {
+      println(s"Iteration $count")
+      count += 1
+      lzy
+    } else 42
+  }
+
+  def main(args: Array[String]): Unit = {
+    println(lzy)
+  }
+}

tests/run/i1856b.check

@@ -0,0 +1,22 @@
+Iteration 0
+Iteration 1
+Iteration 2
+Iteration 3
+Iteration 4
+Iteration 5
+Iteration 6
+Iteration 7
+Iteration 8
+Iteration 9
+Lzy has been read as: 42
+Lzy has been read as: 43
+Lzy has been read as: 44
+Lzy has been read as: 45
+Lzy has been read as: 46
+Lzy has been read as: 47
+Lzy has been read as: 48
+Lzy has been read as: 49
+Lzy has been read as: 50
+Lzy has been read as: 51
+52
+Lzy has been read as: 52

tests/run/i1856b.scala

@@ -0,0 +1,21 @@
+object Test {
+  var count: Int = 0
+  def getLzy = {
+   val read = lzy
+   println("Lzy has been read as: " +read)
+   read
+ }
+
+  lazy val lzy: Int = {
+    if (count < 10) {
+      println(s"Iteration $count")
+      count += 1
+      getLzy + 1
+    } else 42
+  }
+
+  def main(args: Array[String]): Unit = {
+    println(lzy)
+    getLzy
+  }
+}