clang vs gcc - CTAD of struct deriving from template parameter-CodePudding

Consider the following code:

template <typename B>
struct D : B { };

D d{[]{ }};

gcc 12.x accepts it and deduces d to be D</* type of lambda */> as expected.
clang 14.x rejects it with the following error:

<source>:4:3: error: no viable constructor 
              or deduction guide for deduction of template arguments of 'D'
D d{[]{ }};
  ^

<source>:2:8: note: candidate template ignored: 
              could not match 'D<B>' against '(lambda at <source>:4:5)'
struct D : B { };
       ^

<source>:2:8: note: candidate function template not viable: 
              requires 0 arguments, but 1 was provided

live example on godbolt.org

Which compiler is behaving correctly here?

CodePudding user response：

In the code snippet, no deduction guide has been provided. P1816 added deduction guides for aggregate class templates in C 20, by requiring that an aggregate deduction candidate is generated.

The code is valid, but Clang just doesn't support P1816 yet.

Adding a deduction guide allows this to compile in Clang as well.

template <typename B> D(B) -> D<B>;