Fix #5350: Fix purity of local lazy vals

Author: nicolasstucki

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -400,10 +400,12 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
       def isKnownPureOp(sym: Symbol) =
         sym.owner.isPrimitiveValueClass || sym.owner == defn.StringClass
       if (tree.tpe.isInstanceOf[ConstantType] && isKnownPureOp(tree.symbol) // A constant expression with pure arguments is pure.
-          || fn.symbol.isStable
+          || (fn.symbol.isStable && !fn.symbol.is(Lazy))
           || fn.symbol.isPrimaryConstructor && fn.symbol.owner.isNoInitsClass) // TODO: include in isStable?
         minOf(exprPurity(fn), args.map(exprPurity)) `min` Pure
       else if (fn.symbol.is(Erased)) Pure
+      else if (fn.symbol.isStable /* && fn.symbol.is(Lazy) */)
+        minOf(exprPurity(fn), args.map(exprPurity)) `min` Idempotent
       else Impure
     case Typed(expr, _) =>
       exprPurity(expr)

compiler/test-resources/repl/i5350

@@ -0,0 +1,7 @@
+scala> def foo = { lazy val bar: Unit = { println("Hello") }; bar }
+def foo: Unit
+scala> foo
+Hello
+scala> { lazy val bar: Unit = { println("Hello") }; bar }
+Hello
+val bar: Unit = <lazy>

tests/run/i5350.check

@@ -0,0 +1 @@
+Hello

tests/run/i5350.scala

@@ -0,0 +1,10 @@
+object Test {
+  def foo = {
+    lazy val bar: Unit = println("Hello")
+    bar
+  }
+
+  def main(args: Array[String]): Unit = {
+    foo
+  }
+}