Disable checkConstraintClosed by default

Checking that constraints are closed caused cyclic reference exceptions in
DottyBackedInterface. What's worrying is that these were seemingly not checked
by the checkin tests. Or maybe there is some dependcy on compilation order that triggers
the erros only in my setup.

Author: odersky

src/dotty/tools/dotc/config/Config.scala

@@ -32,8 +32,16 @@ object Config {
    */
   final val checkConstraintsPropagated = false
 
-  /** Check that constraints of globally committable typer states are closed */
-  final val checkConstraintsClosed = true
+  /** Check that constraints of globally committable typer states are closed.
+   *  NOTE: When enabled, the check can cause CyclicReference errors because
+   *  it traverses all elements of a type. Such failures were observed when
+   *  compiling all of dotty together (source seems to be in GenBCode which
+   *  accesses javac's settings.)
+   *
+   *  It is recommended to turn this option on only when chasing down
+   *  a PolyParam instantiation error. See comment in Types.TypeVar.instantiate.
+   */
+  final val debugCheckConstraintsClosed = false
 
   /** Check that no type appearing as the info of a SymDenotation contains
    *  skolem types.

src/dotty/tools/dotc/core/TyperState.scala

@@ -87,7 +87,7 @@ extends TyperState(r) {
 
   override def constraint = myConstraint
   override def constraint_=(c: Constraint)(implicit ctx: Context) = {
-    if (Config.checkConstraintsClosed && isGlobalCommittable) c.checkClosed()
+    if (Config.debugCheckConstraintsClosed && isGlobalCommittable) c.checkClosed()
     myConstraint = c
   }
 

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

@@ -2460,6 +2460,8 @@ object Types {
 
       if (ctx.typerState.isGlobalCommittable)
         assert(!inst.isInstanceOf[PolyParam], i"bad inst $this := $inst, constr = ${ctx.typerState.constraint}")
+          // If this fails, you might want to turn on Config.debugCheckConstraintsClosed
+          // to help find the root of the problem.
 
       instantiateWith(inst)
     }