[clang] Remove written template args from implicit var tpl spec (#156329)

`VarTemplateSpecializationDecl::getTemplateArgsAsWritten()` function
should return `nullptr` in the case of implicit instantiation, as its
`ClassTemplateSpecializationDecl` counterpart does, and not the
arguments written in `DeclRefExpr` referencing the specialization in the
first place. Otherwise, for such code:
```cpp
template <typename>
int VarTpl;

template <typename T>
void tplFn() {
  (void)VarTpl<T>;  // (1)
}

void fn() {
  tplFn<char>();
}
```
Clang treats the `char` argument of the `VarTpl` specialization as if it
were written in the line marked as (1), which is misleading and hardly
makes sense.

Moreover, "template args as written" are stored inside `ExplicitInfo`
field of `VarTemplateSpecializationDecl`, but it is
[documented](https://github.com/llvm/llvm-project/blob/13357e8a12c1a45364a0c4d3137b6d21ee6ac40c/clang/include/clang/AST/DeclTemplate.h#L2653)
that it is not for implicit instantiations.

Moreover, it is assumed in `TraverseVarTemplateSpecializationDecl`
method of `RecursiveASTVisitor` that `getTemplateArgsAsWritten()`
returns `nullptr` for implicit instantiations, as it is stated in the
comment
[there](https://github.com/llvm/llvm-project/blob/13357e8a12c1a45364a0c4d3137b6d21ee6ac40c/clang/include/clang/AST/RecursiveASTVisitor.h#L2196).

That said, `setTemplateArgsAsWritten` should be called only for variable
template explicit specializations (it is [already done inside
`Sema::ActOnVarTemplateSpecialization`](https://github.com/llvm/llvm-project/blob/4c916273041ff5ed7b2af20bec787ffc42871c9f/clang/lib/Sema/SemaTemplate.cpp#L4459))
and explicit instantiations (hence `true` is passed to the new
`SetWrittenArgs` parameter of `CheckVarTemplateId` function inside
`Sema::ActOnExplicitInstantiation`, but not when handling expressions
referencing a variable template specialization).
`InstantiateVariableDefinition` function just passes the arguments from
the corresponding declaration. I'm not sure about instantiating a class
template containing a variable template explicit specialization and thus
have tried to leave the logic of the first overload of
`TemplateDeclInstantiator::VisitVarTemplateSpecializationDecl` as it
was.

0 files changed, 0 insertions, 0 deletions