Parse Function arity without throwing exceptions

Using NumberFormatException in the normal control flow of the program
makes it harder to debug the compiler by stopping on all instances of
RuntimeException.

Author: smarter

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

@@ -190,10 +190,13 @@ object NameOps {
 
     /** Parsed function arity for function with some specific prefix */
     private def functionArityFor(prefix: String): Int = {
-      if (name.startsWith(prefix))
-        try name.toString.substring(prefix.length).toInt
-        catch { case _: NumberFormatException => -1 }
-      else -1
+      if (name.startsWith(prefix)) {
+        val suffix = name.toString.substring(prefix.length)
+        if (suffix.matches("\\d+"))
+          suffix.toInt
+        else
+          -1
+      } else -1
     }
 
     /** The name of the generic runtime operation corresponding to an array operation */