Joseph, while looking into implementing enhancement your request
pr81824 I noticed that GCC silently accepts incompatible alias
declarations (pr81854) so as sort of a proof-concept for the
former I enhanced the checking already done for other kinds of
incompatibilities to also detect those mentioned in the latter
bug.  Attached is this patch, tested on x85_64-linux.

Jonathan, the patch requires suppressing the warning in libstdc++
compatibility symbol definitions in compatibility.cc.  I couldn't
find a way to do it without the suppression but I'd be happy to
try again if you have an idea for how.

As an example, the patch lets GCC detect mistakes like:

    size_t __attribute__ ((ifunc ("bar_resolver")))
    bar (void*, const void*, size_t);

    void* fast_bar (void *d, const void *s, size_t n) { ... }
    void* slow_bar (void *d, const void *s, size_t n) { ... }

    void* bar_resolver (void)
    {
       return fast ? &fast_bar : &slow_bar;
    }

By complaining that the ifunc resolver should return a function
pointer it makes the programmer change the declaration of the
resolver to one of:

    __typeof__ (bar)* bar_resolver (void) { ... }

or

    __typeof__ (fast_bar)* bar_resolver (void) { ... }

which then triggers either -Wincompatible-pointer-types or
-Wattributes, respectively.  (I used the latter warning in
my patch but maybe the former would be more appropriate).

Martin

PS A documentation-only patch to update the description of
attribute ifunc was posted separately here:
    https://gcc.gnu.org/ml/gcc-patches/2017-08/msg01095.html

PPS To make use of this checking (and compile without the new
warnings) Glibc needs the following patch:

diff --git a/include/libc-symbols.h b/include/libc-symbols.h
index fe3ab81..5413e56 100644
--- a/include/libc-symbols.h
+++ b/include/libc-symbols.h
@@ -790,7 +790,8 @@ for linking")

  /* Helper / base  macros for indirect function symbols.  */
  #define __ifunc_resolver(type_name, name, expr, arg, init, classifier) \
-  classifier inhibit_stack_protector void *name##_ifunc (arg) 
                        \
+  classifier inhibit_stack_protector                                   \
+  __typeof (type_name) *name##_ifunc (arg)                             \
    {                                                                    \
      init ();                                                           \
      __typeof (type_name) *res = expr;                                  \