https://bugs.llvm.org/show_bug.cgi?id=50634

            Bug ID: 50634
           Summary: Clang rejects braced-init-list as default non-type
                    template parameter
           Product: clang
           Version: 12.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: C++
          Assignee: [email protected]
          Reporter: [email protected]
                CC: [email protected], [email protected],
                    [email protected], [email protected],
                    [email protected]

Clang rejects this code:

```c++
template<bool = {}>
int f() {}
```

with:
> error: expected expression

However I believe that the C++ grammar permits that.

[temp.param]:
> The syntax for template-parameters is:
> template-parameter:
>       type-parameter
>       parameter-declaration

[dcl.fct]:

> parameter-declaration:
>       attribute-specifier-seq_{opt} decl-specifier-seq declarator
>       attribute-specifier-seq_{opt} decl-specifier-seq declarator = 
> initializer-clause
>       attribute-specifier-seq_{opt} decl-specifier-seq 
> abstract-declarator_{opt} 
>       attribute-specifier-seq_{opt} decl-specifier-seq abstract-declarator
_{opt} = initializer-clause

[dcl.init]:
> initializer-clause:
>       assignment-expression
>       braced-init-list

My motivating usecase is:

```c++
template<std::true_type = {}>
int f() {}
```

which is accepted by G++.

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