Fix TermRefWithSignature#newLikeThis

The signature might need to be updated if the symbol has a different
type as seen from the new prefix.

Author: odersky

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

@@ -49,6 +49,13 @@ case class Signature(paramsSig: List[TypeName], resSig: TypeName) {
     loop(this.paramsSig, that.paramsSig)
   }
 
+  /** Is this siganture consistent with that signature?
+   *  This is the case if the signatures have consistent parameters
+   *  and result types.
+   */
+  final def consistentWith(that: Signature): Boolean =
+    consistentParams(that) && consistent(resSig, that.resSig)
+
   /** The degree to which this signature matches `that`.
    *  If parameter names are consistent and result types names match (i.e. they are the same
    *  or one is a wildcard), the result is `FullMatch`.

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

@@ -1867,8 +1867,15 @@ object Types {
       }
       else candidate
 
-    override def newLikeThis(prefix: Type)(implicit ctx: Context): TermRef =
-      fixDenot(TermRef.withSig(prefix, name, sig), prefix)
+    override def newLikeThis(prefix: Type)(implicit ctx: Context): TermRef = {
+      // If symbol exists, the new signature is the symbol's signature as seen
+      // from the new prefix, modulo consistency
+      val symSig =
+        if (symbol.exists) symbol.info.asSeenFrom(prefix, symbol.owner).signature
+        else sig
+      val newSig = if (sig.consistentWith(symSig)) sig else symSig
+      fixDenot(TermRef.withSig(prefix, name, newSig), prefix)
+    }
 
     override def shadowed(implicit ctx: Context): NamedType =
       fixDenot(TermRef.withSig(prefix, name.derived(ShadowedName), sig), prefix)