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c++gcclanguage-lawyerc++20

Class type non-type template parameter initialization does not compile

EDIT: this question is three years old, how can it be a duplicate of another one asked Yesterday???

I was under the impression that the following should become valid code under the new C++20 standard:

struct Foo
{
  int a, b;
};

template<Foo>
struct Bar
{};

Bar<{.a=1, .b=2}> bar;

Yet, gcc 10.2.0, with -std=c++20 set complains: could not convert ‘{1, 2}’ from ‘<brace-enclosed initializer list>’ to ‘Foo’ and Clang cannot compile this snippet either. Can someone point out why it is not well formed?

Solution

  • This template-argument

    {.a=1, .b=2}

    is not allowed according to the grammar for a template-argument which only allows the following constructs:

    template-argument:

    constant-expression

    type-id

    id-expression

    A brace-init list is not any of the above constructs, it's actually an initializer and so it cannot be used as a template-argument.

    You can be explicit about the type of the object that you use as the template-argument:

    Bar<Foo{.a=1, .b=2}> bar;

    and this will work, since this is a constant-expression.