sdkrystian wrote:
Another example using class templates:
```cpp
template<int N>
struct A
{
template<typename T>
struct B;
};
template<>
template<typename T>
struct A<0>::B : A<1>::B<T> { };
template<>
template<typename T>
struct A<1>::B
{
static constexpr bool x = true;
};
static_assert(A<0>::B<int>::x); // error: implicit instantiation of undefined
template 'A<1>::B<int>'
```
This currently results in a crash on trunk. With this patch applied, we
instantiate the undefined class template declared in the definition of `A` and
complain.
These differences in behavior arise from the fact that we bind names to the
most recent redeclaration of an entity when creating `Expr` nodes that refer to
that entity. In the above example, the explicit specialization of `A<1>::B` is
declared after it is referenced in the explicit specialization of `A<0>::B`,
resulting in the initial declaration being used to instantiate `B<int>`.
IMO this behavior is correct per [[temp.expl.spec]
p7](http://eel.is/c++draft/temp.expl.spec#7):
> If a template, a member template or a member of a class template is
> explicitly specialized, a declaration of that specialization shall be
> reachable from every use of that specialization that would cause an implicit
> instantiation to take place, in every translation unit in which such a use
> occurs; no diagnostic is required.
However, I would like to know what @erichkeane, @mizvekov, and @zygoloid think.
In any case, the QT example can be fixed by reordering the explicit
specialization such that each specialization is declared before being
referenced.
https://github.com/llvm/llvm-project/pull/106585
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