Class type non-type template parameter initialization does not compile

user_4858640 0
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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?

c++20 language-lawyer gcc c++
2 Answers

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.

user_83728530

It's a C++ grammar thing. Things used for template arguments must be either type-ids, id-names, or constant-expressions.

And a braced-init-list is not an expression of any kind. They can only grammatically be present in a small number of places, specifically those used to initialize an object or variable.

In the past, it wasn't really relevant, since there wasn't much reason to use a braced-init-list to initialize the few valid NTTPs. Obviously that's changed, so this is something of an oversight. But this is what the C++20 standard says.

user_7340690
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