Add reference to issue with union types

Author: smarter

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

@@ -76,7 +76,8 @@ case class Signature(paramsSig: List[ParamSig], resSig: TypeName) {
     else if (!this.paramsSig.hasSameLengthAs(that.paramsSig)) that
     else {
       val mapped = Signature(
-          // DOTTY: we shouldn't have to explicitly pass a type argument to `update`
+          // DOTTY: we shouldn't have to explicitly pass a type argument to `update`,
+          // see https://github.com/lampepfl/dotty/issues/4867
           this.paramsSig.zipWithConserve(that.paramsSig)(update[ParamSig]),
           update(this.resSig, that.resSig))
       if (mapped == this) this else mapped

compiler/src/dotty/tools/dotc/core/tasty/TastyUnpickler.scala

@@ -66,7 +66,8 @@ class TastyUnpickler(reader: TastyReader) {
       case SIGNED =>
         val original = readName()
         val result = readName().toTypeName
-        // DOTTY: we shouldn't have to give an explicit type to paramsSig
+        // DOTTY: we shouldn't have to give an explicit type to paramsSig,
+        // see https://github.com/lampepfl/dotty/issues/4867
         val paramsSig: List[Signature.ParamSig] = until(end)(readParamSig())
         val sig = Signature(paramsSig, result)
         SignedName(original, sig)