The compilers (GCC and Clang) are giving an error when using the curly braces init syntax with designated initializers when initializing a struct template instance.

Here:

template <class T>
struct Foo
{
    const T value;
};

int main( )
{
    Foo foo { .value = 5 };        // compiles
    // Foo foo { .value = { 5 } }; // does not compile
    // Foo foo { .value { 5 } };   // does not compile
}

The error message:

<source>: In function 'int main()':
<source>:11:30: error: class template argument deduction failed:
   11 |     Foo foo { .value = { 5 } };
      |    

What’s causing this? Isn’t list-initialization allowed in this context? Or something else is the root cause?