Purity of primary constructor calls

Calls to primary constructors of NoInits classes are pure if prefix and
arguments are pure.

Author: odersky

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

@@ -399,9 +399,9 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
     case Apply(fn, args) =>
       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)
+      if (tree.tpe.isInstanceOf[ConstantType] && isKnownPureOp(tree.symbol) // A constant expression with pure arguments is pure.
+          || fn.symbol.isStable
+          || 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 Impure