Is there a specific reason why a trailing-return-type is not a complete-class context of a class?

Question

Note that a trailing-return-type is not mentioned in [class.mem]p6

A complete-class context of a class is a

(6.1) function body,
(6

Answer 1

Because you don't want it.

 struct Test {
     auto foo() -> decltype(bar());
     auto bar() -> int;

     auto baz() -> decltype(qux());
     auto qux() -> decltype(baz());
 }; 

Now you need all sorts of rules explaining which of the above is allowed and which is not.

So why does the standard places noexcept-specifier in complete-class context? Wouldn't it allow essentially the same thing in code like this:

struct Test { 
    void foo() noexcept(noexcept(bar())); 
    void bar() noexcept(noexcept(foo())); 
};

?

It seems the standard doesn't address this well, and the compilers differ in treatment of this. Clang complains about the above code, but eats this:

struct Test { 
    void foo() noexcept(Test::b);
    static const bool b = true;
};

GCC complains about the second code too, but accepts code with the member declarations swapped. It seems it doesn't treat the noexcept specifier as complete-class context at all.