diff --git a/compiler/src/dotty/tools/dotc/core/Signature.scala b/compiler/src/dotty/tools/dotc/core/Signature.scala
index b1bc760df57e..b9856dcb8e4f 100644
--- a/compiler/src/dotty/tools/dotc/core/Signature.scala
+++ b/compiler/src/dotty/tools/dotc/core/Signature.scala
@@ -87,14 +87,15 @@ case class Signature(paramsSig: List[TypeName], resSig: TypeName) {
 
   /** Construct a signature by prepending the signature names of the given `params`
    *  to the parameter part of this signature.
+   *
+   *  Like Signature#apply, the result is only cacheable if `isUnderDefined == false`.
    */
   def prepend(params: List[Type], isJava: Boolean)(implicit ctx: Context) =
     Signature(params.map(p => sigName(p, isJava)) ++ paramsSig, resSig)
 
   /** A signature is under-defined if its paramsSig part contains at least one
    *  `tpnme.Uninstantiated`. Under-defined signatures arise when taking a signature
-   *  of a type that still contains uninstantiated type variables. They are eliminated
-   *  by `fixSignature` in `PostTyper`.
+   *  of a type that still contains uninstantiated type variables.
    */
   def isUnderDefined(implicit ctx: Context) =
     paramsSig.contains(tpnme.Uninstantiated) || resSig == tpnme.Uninstantiated
@@ -116,7 +117,12 @@ object Signature {
    */
   val OverloadedSignature = Signature(List(tpnme.OVERLOADED), EmptyTypeName)
 
-  /** The signature of a method with no parameters and result type `resultType`. */
+  /** The signature of a method with no parameters and result type `resultType`.
+   *
+   *  The resulting value is only cacheable if `isUnderDefined == false`,
+   *  otherwise the signature will change once the contained type variables have
+   *  been instantiated.
+   */
   def apply(resultType: Type, isJava: Boolean)(implicit ctx: Context): Signature = {
     assert(!resultType.isInstanceOf[ExprType])
     apply(Nil, sigName(resultType, isJava))
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index 79e4a469b00a..56c46804636b 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -1580,7 +1580,7 @@ object Types {
 
 // --- NamedTypes ------------------------------------------------------------------
 
-  abstract class NamedType extends CachedProxyType with ValueType { self =>
+  abstract class NamedType extends CachedProxyType with ValueType with SignatureCachingType { self =>
 
     type ThisType >: this.type <: NamedType
     type ThisName <: Name
@@ -1592,7 +1592,6 @@ object Types {
     assert(prefix.isValueType || (prefix eq NoPrefix), s"invalid prefix $prefix")
 
     private[this] var myName: Name = null
-    private[this] var mySig: Signature = null
     private[this] var lastDenotation: Denotation = null
     private[this] var lastSymbol: Symbol = null
     private[this] var checkedPeriod: Period = Nowhere
@@ -1601,6 +1600,7 @@ object Types {
     // Invariants:
     // (1) checkedPeriod != Nowhere  =>  lastDenotation != null
     // (2) lastDenotation != null    =>  lastSymbol != null
+    // (3) mySigRunId != NoRunId     =>  mySig != null
 
     def isType = isInstanceOf[TypeRef]
     def isTerm = isInstanceOf[TermRef]
@@ -1621,12 +1621,7 @@ object Types {
     /** The signature of the last known denotation, or if there is none, the
      *  signature of the symbol
      */
-    final override def signature(implicit ctx: Context): Signature = {
-      if (mySig == null) mySig = computeSignature
-      mySig
-    }
-
-    def computeSignature(implicit ctx: Context): Signature = {
+    protected def computeSignature(implicit ctx: Context): Signature = {
       val lastd = lastDenotation
       if (lastd != null) lastd.signature
       else symbol.asSeenFrom(prefix).signature
@@ -1637,7 +1632,7 @@ object Types {
      *  Otherwise NotAMethod.
      */
     private def currentSignature(implicit ctx: Context): Signature =
-      if (mySig != null) mySig
+      if (ctx.runId == mySignatureRunId) mySignature
       else {
         val lastd = lastDenotation
         if (lastd != null) lastd.signature
@@ -2584,13 +2579,22 @@ object Types {
   // and therefore two different poly types would never be equal.
 
   /** A trait that mixes in functionality for signature caching */
-  trait MethodicType extends TermType {
-
-    private[this] var mySignature: Signature = _
-    private[this] var mySignatureRunId: Int = NoRunId
+  trait SignatureCachingType extends TermType {
+    protected[this] var mySignature: Signature = _
+    protected[this] var mySignatureRunId: Int = NoRunId
 
     protected def computeSignature(implicit ctx: Context): Signature
 
+    final override def signature(implicit ctx: Context): Signature = {
+      if (ctx.runId != mySignatureRunId) {
+        mySignature = computeSignature
+        if (!mySignature.isUnderDefined) mySignatureRunId = ctx.runId
+      }
+      mySignature
+    }
+  }
+
+  trait MethodicType extends SignatureCachingType {
     protected def resultSignature(implicit ctx: Context) = try resultType match {
       case rtp: MethodicType => rtp.signature
       case tp =>
@@ -2602,14 +2606,6 @@ object Types {
         println(i"failure while taking result signature of $this: $resultType")
         throw ex
     }
-
-    final override def signature(implicit ctx: Context): Signature = {
-      if (ctx.runId != mySignatureRunId) {
-        mySignature = computeSignature
-        if (!mySignature.isUnderDefined) mySignatureRunId = ctx.runId
-      }
-      mySignature
-    }
   }
 
   /** A by-name parameter type of the form `=> T`, or the type of a method with no parameter list. */