Make sure signatures are computed before erasure

Some symbols change their signatures after erasure,
e.g. getters with Unit type or constructors taking an outer parameter.
Since signatures are assumed to be stable for a complete run,
we need to always compute signatures for NamedTypes before ersure.

Author: odersky

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

@@ -1897,29 +1897,31 @@ object Types {
     }
 
     /** The signature of the last known denotation, or if there is none, the
-     *  signature of the symbol
+     *  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 = {
+    protected def computeSignature(implicit ctx: Context): Signature =
       val lastd = lastDenotation
-      if (lastd != null) lastd.signature
+      val isErased = ctx.erasedTypes
+      if lastd != null && !isErased then lastd.signature
+      else if isErased then computeSignature(using ctx.withPhase(ctx.erasurePhase))
       else symbol.asSeenFrom(prefix).signature
-    }
 
     /** The signature of the current denotation if it is known without forcing.
      *  Otherwise the signature of the current symbol if it is known without forcing.
      *  Otherwise NotAMethod.
      */
     private def currentSignature(implicit ctx: Context): Signature =
-      if (ctx.runId == mySignatureRunId) mySignature
-      else {
+      if ctx.runId == mySignatureRunId then mySignature
+      else
         val lastd = lastDenotation
-        if (lastd != null) lastd.signature
-        else {
+        val isErased = ctx.erasedTypes
+        if lastd != null && !isErased then lastd.signature
+        else if isErased then currentSignature(using ctx.withPhase(ctx.erasurePhase))
+        else
           val sym = currentSymbol
-          if (sym.exists) sym.asSeenFrom(prefix).signature
+          if sym.exists then sym.asSeenFrom(prefix).signature
           else Signature.NotAMethod
-        }
-      }
 
     final def symbol(implicit ctx: Context): Symbol =
       // We can rely on checkedPeriod (unlike in the definition of `denot` below)