SI-2807 Avoid catch all warning for Stable Id patterns

Author: retronym

src/compiler/scala/tools/nsc/typechecker/Typers.scala

@@ -4865,9 +4865,10 @@ trait Typers extends Modes with Adaptations with Tags {
 
           for (cdef <- catches1 if cdef.guard.isEmpty) {
             def warn(name: Name) = context.warning(cdef.pat.pos, s"This catches all Throwables. If this is really intended, use `case ${name.decoded} : Throwable` to clear this warning.")
+            def unbound(t: Tree) = t.symbol == null || t.symbol == NoSymbol
             cdef.pat match {
-              case Bind(name, Ident(_)) => warn(name)
-              case Ident(name)          => warn(name)
+              case Bind(name, i@Ident(_)) if unbound(i) => warn(name)
+              case i@Ident(name) if unbound(i)          => warn(name)
               case _ =>
             }
           }

test/files/neg/catch-all.check

@@ -1,10 +1,10 @@
 catch-all.scala:2: error: This catches all Throwables. If this is really intended, use `case _ : Throwable` to clear this warning.
-	try { "warn" } catch { case _ => }
-                                    ^
+  try { "warn" } catch { case _ => }
+                              ^
 catch-all.scala:4: error: This catches all Throwables. If this is really intended, use `case x : Throwable` to clear this warning.
-	try { "warn" } catch { case x => }
-                                    ^
+  try { "warn" } catch { case x => }
+                              ^
 catch-all.scala:6: error: This catches all Throwables. If this is really intended, use `case x : Throwable` to clear this warning.
-	try { "warn" } catch { case _: RuntimeException => ; case x => }
-                                                                  ^
+  try { "warn" } catch { case _: RuntimeException => ; case x => }
+                                                            ^
 three errors found

test/files/neg/catch-all.scala

@@ -1,19 +1,31 @@
 object CatchAll {
-	try { "warn" } catch { case _ => }
+  try { "warn" } catch { case _ => }
 
-	try { "warn" } catch { case x => }
+  try { "warn" } catch { case x => }
 
-	try { "warn" } catch { case _: RuntimeException => ; case x => }
+  try { "warn" } catch { case _: RuntimeException => ; case x => }
 
-	try { "okay" } catch { case _: Throwable => }
+  val t = T
 
-	try { "okay" } catch { case _: Exception => }
+  try { "okay" } catch { case T => }
 
-	try { "okay" } catch { case okay: Throwable => }
+  try { "okay" } catch { case `t` => }
 
-	try { "okay" } catch { case okay: Exception => }
+  try { "okay" } catch { case x @ T => }
 
-	try { "okay" } catch { case _ if "".isEmpty => }
+  try { "okay" } catch { case x @ `t` => }
 
-	"okay" match { case _ => "" }
+  try { "okay" } catch { case _: Throwable => }
+
+  try { "okay" } catch { case _: Exception => }
+
+  try { "okay" } catch { case okay: Throwable => }
+
+  try { "okay" } catch { case okay: Exception => }
+
+  try { "okay" } catch { case _ if "".isEmpty => }
+
+  "okay" match { case _ => "" }
 }
+
+object T extends Throwable