Why are typedef templates illegal?

Question

From a practical point of view, I understand that both typedef and test are somewhat "superfluous" and need to be removed if we want the following code to compile:

template< typename type_t >
typedef struct tagTest
{
    int a;
} test;

However, I thought that the set of typedef declarations was a subset of the set of declarations. They just happened to have that specific decl-specifier. That was my rationalization for

typedef struct tagTest
{
    int a;
} test;

introducing the identifier test and declaring the structure tagTest. If that interpretation is correct, then the following paragraph from the standard should allow template typedef's (although not with the meaning given by the keyword using).

The declaration in a template-declaration shall — (1.1) declare or define a function, a class, or a variable, or — (1.2) define a member function, a member class, a member enumeration, or a static data member of a class template or of a class nested within a class template, or — (1.3) define a member template of a class or class template, or — (1.4) be an alias-declaration.

I cannot see error in my reasoning, yet the conclusion is illegal.

What are the relevant parts of the standard that solve the above conundrum?

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UPDATE Part of the above reasoning uses the fact that typedef struct declares a structure. The typedef specifier, as far as I understand it, implies that any variables declared are really types. That is, the typedef upgrades test from a mere variable to a type that is equivalent to the declared tagTest. That is why the following code compiles (albeit with a warning).

typedef struct tagTest
{
    int a;
};
tagTest t;

One of the answers takes care of the superfluous test. But, it is possible to use typedef without a declarator because "Init-declarator-list is optional when declaring a named class/struct/union or a named enumeration"

Answer 1

Template typedefs weren't allowed pre-C++11 and with C++11 template aliases were introduced to address those issues. Cfr. C++ template typedefs and wikipedia.

Since, as you noted, the standard doesn't allow typedef to be in there, the code is invalid

alias-declaration:

 using identifier attribute-specifier-seqopt= type-id ;

typedef declarations are not alias declarations.

Furthermore you can't have a declarator if you're declaring a class template, it is explicitly forbidden by the standard

[temp]/p3

In a template-declaration, explicit specialization, or explicit instantiation the init-declarator-list in the declaration shall contain at most one declarator. When such a declaration is used to declare a class template, no declarator is permitted.

so not even the following will compile

template< typename type_t >
struct tagTest
{
    int a;
} test;

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Edit:

It is nowhere specified that

typedef struct S { };

should be an error, thus both gcc and clang accept it with a warning. I assume Clang counts on [temp]/3 to issue an error in case typedef was being used with a template while gcc rejects this code immediately

template<typename T>
typedef struct S { };

cfr. clang bug 22249

Answer 2

C does not support templates and the

typedef struct tagX {
} X;

syntax in C++ is vestigial C, there to allow continued support for C headers etc, not for use in actual C++.

The C++ syntax for the above is

struct X {};

(YMMV on brace placement)

Answer 3

Independently from what the typedef defines, that is a typedef declaration, which is not listed in those cases:

• [member] function
• [member] class
• variable
• member enumeration
• static data member of a class template/of a class nested within a class template
• member template of a class or class template
• alias declaration

And just to be clear typedef declarations are not alias declarations. Alias declaration, as specified by the grammar at §7 of the standard are:

alias-declaration:

using identifier attribute-specifier-seqopt= type-id ;

Not to mention that if this was possible, then template using declaration would not be nearly as "cool" as they are today, and there would be little to no sense to have both.