Add comment to Compiler.scala about behaviour of Constructors

Author: DarkDimius

src/dotty/tools/dotc/Compiler.scala

@@ -71,7 +71,7 @@ class Compiler {
            new LinkScala2ImplClasses,
            new NonLocalReturns,
            new CapturedVars, // capturedVars has a transformUnit: no phases should introduce local mutable vars here
-           new Constructors,
+           new Constructors, // constructors changes decls in transformTemplate, no InfoTransformers should be added after it
            new FunctionalInterfaces,
            new GetClass), // getClass transformation should be applied to specialized methods
       List(new LambdaLift,   // in this mini-phase block scopes are incorrect. No phases that rely on scopes should be here

src/dotty/tools/dotc/transform/Constructors.scala

@@ -253,6 +253,8 @@ class Constructors extends MiniPhaseTransform with SymTransformer { thisTransfor
       cls.copy(
         info = clsInfo.derivedClassInfo(
           decls = clsInfo.decls.filteredScope(!dropped.contains(_))))
+
+      // TODO: this happens to work only because Constructors is the last phase in group
     }
 
     val (superCalls, followConstrStats) = constrStats.toList match {