Attempt to fix a potentially missing case in TypeComparer

Author: abgruszecki

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

@@ -451,7 +451,11 @@ class TypeComparer(initctx: Context) extends ConstraintHandling[AbsentContext] {
             // widening in `fourthTry` before adding to the constraint.
             if (frozenConstraint) isSubType(tp1, bounds(tp2).lo)
             else isSubTypeWhenFrozen(tp1, tp2)
-          alwaysTrue || {
+          alwaysTrue ||
+          frozenConstraint && (tp1 match {
+            case tp1: TypeParamRef => constraint.isLess(tp1, tp2)
+            case _ => false
+          }) || {
             if (canConstrain(tp2) && !approx.low)
               addConstraint(tp2, tp1.widenExpr, fromBelow = true)
             else fourthTry

compiler/test/dotty/tools/dotc/typer/SubtypingInvariantTests.scala

@@ -0,0 +1,22 @@
+package dotty.tools.dotc.typer
+
+import dotty.tools.DottyTest
+import dotty.tools.dotc.core.Contexts.Context
+import dotty.tools.dotc.core.Types.TypeBounds
+import dotty.tools.dotc.typer.ProtoTypes.newTypeVar
+import org.junit.Test
+
+class SubtypingInvariantTests extends DottyTest {
+
+  @Test
+  def typeVarInvariant(): Unit = {
+    checkCompile("frontend", "class A") { (_, ctx0) =>
+      implicit val ctx: Context = ctx0
+      val a = newTypeVar(TypeBounds.empty)
+      val b = newTypeVar(TypeBounds.empty)
+      assert(a <:< b)
+      assert(a frozen_<:< b)
+    }
+  }
+
+}