Consider missing signature equivalent to non-generic one

Since for those it should be sufficient to check the descriptor anyway,
and the Scala compiler started emitting more signatures between
3.1.1 and 3.1.2

(from the functional tests it seems there were already a couple
of cases where Scala3 produced more signatures than expected, we now
ignore those as well)

References #693

core/src/main/scala/com/typesafe/tools/mima/core/Signature.scala

@@ -20,9 +20,18 @@ class Signature(private val signature: String) {
   }
 
   def matches(newer: Signature, isConstructor: Boolean): Boolean = {
-    return (signature == newer.signature) ||
-      (isConstructor && hasMatchingCtorSig(newer.signature)) ||
-      canonicalized == newer.canonicalized
+    // If the signature is identical obviously it matches
+    (signature == newer.signature) ||
+    // Consider missing signatures identical to non-generic ones.
+    // This is particularly helpful because between Scala 3.1.1 and 3.1.2 the compiler
+    // started emitting signatures for non-generic methods. Incompatibilities for those
+    // will be caught through mismatching descriptors anyway.
+    (signature == "" && newer.signature.indexOf('<') == -1) ||
+    (newer.signature == "" && signature.indexOf('<') == -1) ||
+    // Special rules for constructors
+    (isConstructor && hasMatchingCtorSig(newer.signature)) ||
+    // Also match when the signature only differs in the name of a type parameter
+    canonicalized == newer.canonicalized
   }
 
   // Special case for scala#7975