using vs. typedef - is there a subtle, lesser known difference?

Question

Background

Everybody agrees that

using  = ;

is equivalent to

typedef          


        

Answer 1

It looks like the standard is unclear on this.

On one hand,

[dcl.typedef] A typedef-name can also be introduced by an alias-declaration. [...] Such a typedef-name has the same semantics as if it were introduced by the typedef specifier.

On the other hand, the standard clearly separates the notions of typedef declaration and alias-declaration (the latter term is a grammar production name, so it is italicised and hyphenated; the former is not). In some contexts it talks about "a typedef declaration or alias-declaration", making them equivalent in these contexts; and sometimes it talks solely about "a typedef declaration". In particular, whenever the standard talks about linkage and typedef declarations, it only talks about typedef declarations and does not mention alias-declaration. This includes the key passage

[dcl.typedef] If the typedef declaration defines an unnamed class (or enum), the first typedef-name declared by the declaration to be that class type (or enum type) is used to denote the class type (or enum type) for linkage purposes only.

Note the standard insists on the first typedef-name being used for linkage. This means that in

typedef struct { int x; } A, B;

only A is used for linkage, and B is not. Nothing in the standard indicates that a name introduced by alias-declaration should behave like A and not like B.

It is my opinion that the standard is insufficiently clear in this area. If the intent is to make only typedef declaration work for linkage, then it would be appropriate to state explicitly in [dcl.typedef] that alias-declaration does not. If the intent is to make alias-declaration work for linkage, this should be stated explicitly too, as is done in other contexts.

Answer 2

This looks to me like a bug in GCC.

Note that [decl.typedef] does not say that an alias declaration is a typedef declaration

You're right, [dcl.dcl]p9 gives a definition of the term typedef declaration which excludes alias-declarations. However, [dcl.typedef] does explicitly say, as you quoted in your question:

2 A typedef-name can also be introduced by an alias-declaration. The identifier following the using keyword becomes a typedef-name and the optional attribute-specifier-seq following the identifier appertains to that typedef-name. It has the same semantics as if it were introduced by the typedef specifier. [...]

"The same semantics" doesn't leave any doubt. Under GCC's interpretation, typedef and using have different semantics, therefore the only reasonable conclusion is that GCC's interpretation is wrong. Any rules applying to typedef declarations must be interpreted as applying to alias-declarations as well.