9a88aa0e2b
According to [temp.expl.spec] p2:
> The declaration in an _explicit-specialization_ shall not be an
_export-declaration_. An explicit specialization shall not use a
_storage-class-specifier_ other than `thread_local`.
Clang partially implements this, but a number of issues exist:
1. We don't diagnose class scope explicit specializations of variable
templates with _storage-class-specifiers_, e.g.
```
struct A
{
template<typename T>
static constexpr int x = 0;
template<>
static constexpr int x<void> = 1; // ill-formed, but clang accepts
};
````
2. We incorrectly reject class scope explicit specializations of
variable templates when `static` is not used, e.g.
```
struct A
{
template<typename T>
static constexpr int x = 0;
template<>
constexpr int x<void> = 1; // error: non-static data member cannot be
constexpr; did you intend to make it static?
};
````
3. We don't diagnose dependent class scope explicit specializations of
function templates with storage class specifiers, e.g.
```
template<typename T>
struct A
{
template<typename U>
static void f();
template<>
static void f<int>(); // ill-formed, but clang accepts
};
````
This patch addresses these issues as follows:
- # 1 is fixed by issuing a diagnostic when an explicit
specialization of a variable template has storage class specifier
- # 2 is fixed by considering any non-function declaration with any
template parameter lists at class scope to be a static data member. This
also allows for better error recovery (it's more likely the user
intended to declare a variable template than a "field template").
- # 3 is fixed by checking whether a function template explicit
specialization has a storage class specifier even when the primary
template is not yet known.
One thing to note is that it would be far simpler to diagnose this when
parsing the _decl-specifier-seq_, but such an implementation would
necessitate a refactor of `ParsedTemplateInfo` which I believe to be
outside the scope of this patch.
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