Fix wrong heuristic in codegen (#9293)

A bare return may trigger the insertion of a genericReset.

Fixes #9286

compiler/cgen.nim

@@ -771,7 +771,13 @@ proc allPathsAsgnResult(n: PNode): InitResultEnum =
       result = InitRequired
   of nkReturnStmt:
     if n.len > 0:
-      result = allPathsAsgnResult(n[0])
+      if n[0].kind == nkEmpty and result != InitSkippable:
+        # This is a bare `return` statement, if `result` was not initialized
+        # anywhere else (or if we're not sure about this) let's require it to be
+        # initialized. This avoids cases like #9286 where this heuristic lead to
+        # wrong code being generated.
+        result = InitRequired
+      else: result = allPathsAsgnResult(n[0])
   of nkIfStmt, nkIfExpr:
     var exhaustive = false
     result = InitSkippable

tests/ccgbugs/t9286.nim

@@ -0,0 +1,22 @@
+discard """
+  action: run
+"""
+
+import options
+type Foo  = ref object
+  i:      int
+
+proc next(foo: Foo): Option[Foo] =
+  try:    assert(foo.i == 0)
+  except: return      # 2º: none
+  return some(foo)    # 1º: some
+
+proc test =
+  let foo = Foo()
+  var opt = next(foo) # 1º Some
+  while isSome(opt) and foo.i < 10:
+    inc(foo.i)
+    opt = next(foo)   # 2º None
+  assert foo.i == 1, $foo.i
+
+test()