Revert "fix #15836 proc arg return type auto unexpectly match proc with concr…" (#21057)

Revert "fix #15836 proc arg return type auto unexpectly match proc with concr… (#21044)"

This reverts commit 0cd9bdcf9f.

compiler/semtypes.nim

@@ -1405,7 +1405,9 @@ proc semProcTypeNode(c: PContext, n, genericParams: PNode,
             "' is only valid for macros and templates")
       # 'auto' as a return type does not imply a generic:
       elif r.kind == tyAnything:
-        discard
+        # 'p(): auto' and 'p(): untyped' are equivalent, but the rest of the
+        # compiler is hardly aware of 'auto':
+        r = newTypeS(tyUntyped, c)
       elif r.kind == tyStatic:
         # type allowed should forbid this type
         discard