Reset typerState when reducing a match type

It is not enough to reset the constraint only, since type variables might have
gotten instantiated during matchtype reduction.

With that change we now test in -Ycheck that constraints are empty after each phase.

Author: odersky

compiler/src/dotty/tools/dotc/Run.scala

@@ -212,6 +212,7 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
             Stats.record(s"total trees at end of $phase", ast.Trees.ntrees)
             for (unit <- units)
               Stats.record(s"retained typed trees at end of $phase", unit.tpdTree.treeSize)
+            ctx.typerState.gc()
           }
 
       profiler.finished()

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -2876,12 +2876,8 @@ class TrackingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
       // obviously sound, but quite restrictive. With the current formulation,
       // we need to be careful that `provablyEmpty` covers all the conditions
       // used to conclude disjointness in `provablyDisjoint`.
-      if (provablyEmpty(scrut))
-        NoType
-      else
-        val savedConstraint = constraint
-        try recur(cases)
-        finally constraint = savedConstraint // caseLambda additions are dropped
+      if (provablyEmpty(scrut)) NoType
+      else recur(cases)
     }
   }
 }

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

@@ -4597,10 +4597,16 @@ object Types {
         myReduced =
           trace(i"reduce match type $this $hashCode", matchTypes, show = true) {
             def matchCases(cmp: TrackingTypeComparer): Type =
+              val saved = ctx.typerState.snapshot()
               try cmp.matchCases(scrutinee.normalized, cases)
               catch case ex: Throwable =>
                 handleRecursive("reduce type ", i"$scrutinee match ...", ex)
-              finally updateReductionContext(cmp.footprint)
+              finally
+                updateReductionContext(cmp.footprint)
+                ctx.typerState.resetTo(saved)
+                  // this drops caseLambdas in constraint and undoes any typevar
+                  // instantiations during matchtype reduction
+
             TypeComparer.tracked(matchCases)
           }
       myReduced

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

@@ -128,6 +128,8 @@ class TreeChecker extends Phase with SymTransformer {
     report.echo(s"checking ${ctx.compilationUnit} after phase ${fusedPhase}")(using ctx)
 
     inContext(ctx) {
+      assert(ctx.typerState.constraint.domainLambdas.isEmpty,
+        i"non-empty constraint at end of $fusedPhase: ${ctx.typerState.constraint}, ownedVars = ${ctx.typerState.ownedVars.toList}%, %")
       assertSelectWrapsNew(ctx.compilationUnit.tpdTree)
     }