remove Signature.NoMatch from cases because it should not happen

Author: Jendrik Wenke

compiler/src/dotty/tools/dotc/reporting/diagnostic/messages.scala

@@ -2086,9 +2086,9 @@ object messages {
       val details = if (decl.isRealMethod && previousDecl.isRealMethod) {
         // compare the signatures when both symbols represent methods
         decl.asTerm.signature.matchDegree(previousDecl.asTerm.signature) match {
-          case Signature.NoMatch => ""
+          /* case Signature.NoMatch => // can't happen because decl.matches(previousDecl) is checked before reporting this error */
           case Signature.ParamMatch => "\nOverloads with equal parameter types but different return types are not allowed."
-          case Signature.FullMatch => "\nThe definitions have the same signature after erasure."
+          case _ /* Signature.FullMatch */ => "\nThe definitions have the same signature after erasure."
         }
       } else ""
       hl"${decl.showLocated} is already defined as ${previousDecl.showDcl} in line ${previousDecl.pos.line + 1}." + details

tests/neg/doubleDefinition.scala

@@ -124,3 +124,10 @@ class Test17 {
 		val foo = ""
 	}
 }
+
+// no error when overloading
+
+class Test18 {
+	def foo(a: A) = 1
+	def foo(b: B) = 1
+}