[Bug c++/124070] New: [reflection] injected decl not detected in default argument

[mpolacek at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124070

            Bug ID: 124070
           Summary: [reflection] injected decl not detected in default
                    argument
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mpolacek at gcc dot gnu.org
  Target Milestone: ---

Dan K. points out that we emit no diagnostic for:

```
#include <meta>

struct S;
template <std::meta::info I = define_aggregate(^^S, {})>
constexpr void f() {}

consteval {
  f<>();
}
```

but since the template argument is a separate definition from the consteval
block, [expr.const]/9.30 says it shouldn't be core constant:

-- an expression that would produce an injected declaration (see below), unless
E is the corresponding expression of a consteval-block-declaration