Clang rejects braced-init-list as default non-type template parameter

[PiliLatiesa]

Bugzilla Link	50634
Version	12.0
OS	Linux
CC	@DougGregor,@zygoloid

Extended Description

Clang rejects this code:

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:

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

which is accepted by G++.

[llvmbot]

@llvm/issue-subscribers-c-20

Author: None (PiliLatiesa)

| | | | --- | --- | | Bugzilla Link | [50634](https://llvm.org/bz50634) | | Version | 12.0 | | OS | Linux | | CC | @DougGregor,@zygoloid |

Extended Description

Clang rejects this code:

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:

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

which is accepted by G++.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (PiliLatiesa)

| | | | --- | --- | | Bugzilla Link | [50634](https://llvm.org/bz50634) | | Version | 12.0 | | OS | Linux | | CC | @DougGregor,@zygoloid |

Extended Description

Clang rejects this code:

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:

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

which is accepted by G++.