[llvm-bugs] [Bug 95985] Most vexing parse with class template disambiguated as function type when it must be an expression that uses CTAD

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Issue	95985
Summary	Most vexing parse with class template disambiguated as function type when it must be an _expression_ that uses CTAD
Labels	new issue
Assignees
Reporter	MitalAshok

    https://godbolt.org/z/1MMhGa6b9

```c++
template<typename T>
struct X {
    constexpr X(T) {}
};

template<X<int> V> struct Z {};

template<typename T>
struct Y : Z<X(int())> {};
```

Clang complains that:

```
<source>:9:14: error: use of class template 'X' requires template arguments; argument deduction not allowed in function return type
    9 | struct Y : Z<X(int())> {};
 |              ^
<source>:2:8: note: template is declared here
    2 | struct X {
      |        ^
```

Because it incorrectly assumes that since `Parser::isCXXDeclarationSpecifier` at `X`, and `(int())` is a valid [*parameters-and-qualifiers*](https://eel.is/c++draft/dcl.decl.general#nt:parameters-and-qualifiers), it must be parsed as a type and not an _expression_.

But there is no ambiguity since `X(int())` cannot be a function type, so https://eel.is/c++draft/dcl.ambig.res doesn't apply and it should be an _expression_.

I think this is the root cause of #95598

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