Fix signature computation again

Author: odersky

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

@@ -1896,31 +1896,40 @@ object Types {
       case sym: Symbol => sym.originDenotation.name
     }
 
-    /** The signature of the last known denotation, or if there is none, the
-     *  signature of the symbol. Signatures are always computed before erasure, since
-     *  some symbols change their signature at erasure.
+    /** The signature computed from the last known denotation with `sigFromDenot`,
+     *  or if there is none, the signature of the symbol. Signatures are always
+     *  computed before erasure, since some symbols change their signature at erasure.
      */
     protected def computeSignature(implicit ctx: Context): Signature =
       val lastd = lastDenotation
-      if lastd != null && lastd.validFor.firstPhaseId <= ctx.erasurePhase.id then lastd.signature
+      if lastd != null then sigFromDenot(lastd)
       else if ctx.erasedTypes then computeSignature(using ctx.withPhase(ctx.erasurePhase))
       else symbol.asSeenFrom(prefix).signature
 
-    /** The signature of the current denotation if it is known without forcing.
+    /** The signature computed from the current denotation with `sigFromDenot` if it is
+     *  known without forcing.
      *  Otherwise the signature of the current symbol if it is known without forcing.
-     *  Otherwise NotAMethod.
+     *  Otherwise NotAMethod. Signatures are always computed before erasure, since
+     *  some symbols change their signature at erasure.
      */
     private def currentSignature(implicit ctx: Context): Signature =
       if ctx.runId == mySignatureRunId then mySignature
       else
         val lastd = lastDenotation
-        if lastd != null && lastd.validFor.firstPhaseId <= ctx.erasurePhase.id then lastd.signature
+        if lastd != null then sigFromDenot(lastd)
         else if ctx.erasedTypes then currentSignature(using ctx.withPhase(ctx.erasurePhase))
         else
           val sym = currentSymbol
           if sym.exists then sym.asSeenFrom(prefix).signature
           else Signature.NotAMethod
 
+    /** The signature of a pre-erasure version of denotation `lastd`. */
+    private def sigFromDenot(lastd: Denotation)(using Context) =
+      if lastd.validFor.firstPhaseId <= ctx.erasurePhase.id then lastd.signature
+      else lastd match
+        case lastd: SingleDenotation => lastd.initial.signature
+        case _ => Signature.OverloadedSignature
+
     final def symbol(implicit ctx: Context): Symbol =
       // We can rely on checkedPeriod (unlike in the definition of `denot` below)
       // because SymDenotation#installAfter never changes the symbol