diff --git a/compiler/src/dotty/tools/dotc/core/Denotations.scala b/compiler/src/dotty/tools/dotc/core/Denotations.scala
index 06b21bab6443..b34416a095db 100644
--- a/compiler/src/dotty/tools/dotc/core/Denotations.scala
+++ b/compiler/src/dotty/tools/dotc/core/Denotations.scala
@@ -610,16 +610,13 @@ object Denotations {
      */
     def signature(sourceLanguage: SourceLanguage)(using Context): Signature =
       if (isType) Signature.NotAMethod // don't force info if this is a type denotation
-      else info match {
+      else info match
         case info: MethodOrPoly =>
           try info.signature(sourceLanguage)
-          catch { // !!! DEBUG
-            case scala.util.control.NonFatal(ex) =>
-              report.echo(s"cannot take signature of $info")
-              throw ex
-          }
+          catch case ex: Exception =>
+            if ctx.debug then report.echo(s"cannot take signature of $info")
+            throw ex
         case _ => Signature.NotAMethod
-      }
 
     def derivedSingleDenotation(symbol: Symbol, info: Type, pre: Type = this.prefix, isRefinedMethod: Boolean = this.isRefinedMethod)(using Context): SingleDenotation =
       if ((symbol eq this.symbol) && (info eq this.info) && (pre eq this.prefix) && (isRefinedMethod == this.isRefinedMethod)) this
diff --git a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
index f7c7bfa2f8a1..ad32af71fc0d 100644
--- a/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
+++ b/compiler/src/dotty/tools/dotc/typer/RefChecks.scala
@@ -102,18 +102,15 @@ object RefChecks {
     withMode(Mode.CheckBoundsOrSelfType) {
       val cinfo = cls.classInfo
 
-      def checkSelfConforms(other: ClassSymbol, category: String, relation: String) =
+      def checkSelfConforms(other: ClassSymbol) =
         val otherSelf = other.declaredSelfTypeAsSeenFrom(cls.thisType)
         if otherSelf.exists then
           if !(cinfo.selfType <:< otherSelf) then
-            report.error(DoesNotConformToSelfType(category, cinfo.selfType, cls, otherSelf, relation, other),
+            report.error(DoesNotConformToSelfType("illegal inheritance", cinfo.selfType, cls, otherSelf, "parent", other),
               cls.srcPos)
 
       for psym <- parents do
-        checkSelfConforms(psym.asClass, "illegal inheritance", "parent")
-      for reqd <- cls.asClass.givenSelfType.classSymbols do
-        if reqd != cls then
-          checkSelfConforms(reqd, "missing requirement", "required")
+        checkSelfConforms(psym.asClass)
     }
   end checkSelfAgainstParents
 
diff --git a/tests/neg/i16407.check b/tests/neg/i16407.check
new file mode 100644
index 000000000000..5c6bd19ca8c1
--- /dev/null
+++ b/tests/neg/i16407.check
@@ -0,0 +1,12 @@
+-- Error: tests/neg/i16407.scala:2:2 -----------------------------------------------------------------------------------
+2 |  f(g()) // error // error
+  |  ^
+  |  cannot resolve reference to type (X.this : Y & X).A
+  |  the classfile defining the type might be missing from the classpath
+  |  or the self type of (X.this : Y & X) might not contain all transitive dependencies
+-- Error: tests/neg/i16407.scala:2:4 -----------------------------------------------------------------------------------
+2 |  f(g()) // error // error
+  |    ^
+  |    cannot resolve reference to type (X.this : Y & X).A
+  |    the classfile defining the type might be missing from the classpath
+  |    or the self type of (X.this : Y & X) might not contain all transitive dependencies
diff --git a/tests/neg/i16407.scala b/tests/neg/i16407.scala
new file mode 100644
index 000000000000..ff7192390eef
--- /dev/null
+++ b/tests/neg/i16407.scala
@@ -0,0 +1,11 @@
+trait X { self: Y =>
+  f(g()) // error // error
+}
+trait Y { self: Z =>
+  type B = A
+  def f(a: B): Unit = ()
+  def g(): A = ???
+}
+trait Z {
+  type A
+}
diff --git a/tests/neg/selfInheritance.scala b/tests/neg/selfInheritance.scala
index 073316de008c..e8eb2bab5624 100644
--- a/tests/neg/selfInheritance.scala
+++ b/tests/neg/selfInheritance.scala
@@ -26,7 +26,3 @@ object Test {
   object M extends C // error: illegal inheritance: self type Test.M.type of object M$ does not conform to self type B of parent class C
 
 }
-
-trait X { self: Y => } // error: missing requirement: self type Y & X of trait X does not conform to self type Z of required trait Y
-trait Y { self: Z => }
-trait Z
diff --git a/tests/pos/i16407.scala b/tests/pos/i16407.scala
new file mode 100644
index 000000000000..8529f81a29af
--- /dev/null
+++ b/tests/pos/i16407.scala
@@ -0,0 +1,7 @@
+trait X { //missing requirement: self type Z[?] & X of trait X does not conform to self type Z[X.this.A] of required trait Z
+  self: Z[_] =>
+}
+
+trait Z[A] extends X {
+  self: Z[A] => // comment this to compile successfully
+}
\ No newline at end of file
diff --git a/tests/pos/open-selftype.scala b/tests/pos/open-selftype.scala
new file mode 100644
index 000000000000..e834f405ec7f
--- /dev/null
+++ b/tests/pos/open-selftype.scala
@@ -0,0 +1,8 @@
+
+trait X { self: Y => } // error: missing requirement: self type Y & X of trait X does not conform to self type Z of required trait Y
+trait Y { self: Z => }
+trait Z
+
+package squants:
+  trait Quantity[A <: Quantity[A]] { self: A => }
+  trait TimeDerivative[A <: Quantity[A]] { self: Quantity[_] => }
\ No newline at end of file