[Clang] [Sema] C++23 consteval if not recognised as immediate function context in templated function

[MitalAshok]

Consider https://godbolt.org/z/qTexTzKrb:

consteval int f(int) { return 0; }

template<typename T>
constexpr int g(int x) {
    if consteval {
        return f(x);
    }
    return 1;
}

int h(int x) {
    return g<void>(x);
}

The invocation being inside the if consteval means it's in an immediate function context ([expr.const]p(16.3)), so it shouldn't be immediate-escalating, and g<void> should not be an immediate function, yet Clang complains that it is.

<source>:12:12: error: call to immediate function 'g<void>' is not a constant expression
   12 |     return g<void>(x);
      |            ^
<source>:6:16: note: 'g<void>' is an immediate function because its body contains a call to a consteval function 'f' and that call is not a constant expression
    6 |         return f(x);
      |                ^~~~
<source>:12:20: note: function parameter 'x' with unknown value cannot be used in a constant expression
   12 |     return g<void>(x);
      |                    ^
<source>:11:11: note: declared here
   11 | int h(int x) {
      |           ^

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Mital Ashok (MitalAshok)

Consider <https://godbolt.org/z/qTexTzKrb>:

consteval int f(int) { return 0; }

template<typename T>
constexpr int g(int x) {
    if consteval {
        return f(x);
    }
    return 1;
}

int h(int x) {
    return g<void>(x);
}

The invocation being inside the if consteval means it's in an immediate function context ([[expr.const]p(16.3)](https://eel.is/c++draft/expr.const#16.3)), so it shouldn't be immediate-escalating, and g<void> should not be an immediate function, yet Clang complains that it is.

<source>:12:12: error: call to immediate function 'g<void>' is not a constant expression
   12 |     return g<void>(x);
      |            ^
<source>:6:16: note: 'g<void>' is an immediate function because its body contains a call to a consteval function 'f' and that call is not a constant expression
    6 |         return f(x);
      |                ^~~~
<source>:12:20: note: function parameter 'x' with unknown value cannot be used in a constant expression
   12 |     return g<void>(x);
      |                    ^
<source>:11:11: note: declared here
   11 | int h(int x) {
      |           ^

[tbaederr]

CC @cor3ntin