[Bug middle-end/64463] New: Add warning: returns_nonnull attribute on a function compared against NULL

["rurban at x-ray dot at" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64463

            Bug ID: 64463
           Summary: Add warning: returns_nonnull attribute on a function
                    compared against NULL
           Product: gcc
           Version: 4.9.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rurban@x-ray.at

feature request:

Working on gcc.gnu.org/bugzilla/show_bug.cgi?id=64459 I find it difficult to 
find all occurrences of wrong returns_nonnull attributes.

We get those errors, which are fine:
error: returns_nonnull attribute on a function not returning a pointer

But I'm missing the diagnosis of nonnull return values when they are being
wrongly used.

Such as:

__attribute__((returns_nonnull))
char *myfunc() { ...}

if (!myfunc()) { 
  ...  /* optimized away */
}

I would like to see a warning when a returns_nonnull function is compared
against a NULL value, and then actual code is removed or skipped based on that
attribute. Which is either a difficult to diagnose declaration error, or a
logical error in the usage.

warning: returns_nonnull attribute on a function compared against NULL