Fix #12267: Constraints should not cross phases

To enforce the invariant, we need to ensure that tvars are
instantiated at end of phases.

Author: liufengyun

bench/profiles/stdlib.yml

@@ -0,0 +1,15 @@
+charts:
+
+    - name: "scala stdlib-2.13"
+      url: https://github.com/dotty-staging/scala/commits/stdLib213-dotty-community-build
+      lines:
+        - key: stdlib213
+          label: bootstrapped
+
+scripts:
+
+    stdlib213:
+        - source $PROG_HOME/dotty/bench/scripts/stdlib213
+
+config:
+    pr_base_url: "https://github.com/lampepfl/dotty/pull/"

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

@@ -202,7 +202,16 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
           Stats.trackTime(s"$phase ms ") {
             val start = System.currentTimeMillis
             val profileBefore = profiler.beforePhase(phase)
+
+            // invariant: constraint should not cross phase boundary
+            ctx.typerState.constraint = OrderingConstraint.empty
+
             units = phase.runOn(units)
+
+            // force instantiate tvars
+            // see tests/pos/t2619b.scala
+            ctx.typerState.gc()
+
             profiler.afterPhase(phase, profileBefore)
             if (ctx.settings.Xprint.value.containsPhase(phase))
               for (unit <- units)

compiler/src/dotty/tools/dotc/util/Stats.scala

@@ -9,7 +9,7 @@ import collection.mutable
 
 @sharable object Stats {
 
-  final val enabled = true
+  final val enabled = false
 
   var monitored: Boolean = false
 

tests/pos/t2619b.scala

@@ -0,0 +1,8 @@
+abstract class AbstractModule
+
+object ModuleBE extends AbstractModule
+object ModuleBF extends AbstractModule
+
+object ModuleBM extends AbstractModule {
+    def ms: List[AbstractModule] = List(ModuleBE) ::: List(ModuleBF)
+}