[Clang][C++23] < not parsed as the start of a template list in type-only contexts

[MitalAshok]

https://eel.is/c++draft/temp.names#3

A < is interpreted as the delimiter of a template-argument-list if it follows a name [...] that is [...] in a type-only context other than a nested-name-specifier.

This wording was added by P1787R6 for C++23

https://godbolt.org/z/GanbhKTh1

template<typename T>
struct X : T::type<void> {};

template<typename T, typename = typename T::type<void>>
int Y = sizeof(typename T::type<void>);

// CWG2806
template<typename T>
concept C = requires {
  typename T::type<void>;
};

// CWG2903
template<typename T>
struct Z : T::type<void>::type {};

Last two are valid after https://cplusplus.github.io/CWG/issues/2806.html and https://cplusplus.github.io/CWG/issues/2903.html

<source>:2:15: error: use 'template' keyword to treat 'type' as a dependent template name
    2 | struct X : T::type<void> {};
      |               ^
      |               template 
<source>:4:45: error: use 'template' keyword to treat 'type' as a dependent template name
    4 | template<typename T, typename = typename T::type<void>>
      |                                             ^
      |                                             template 
<source>:5:28: error: use 'template' keyword to treat 'type' as a dependent template name
    5 | int Y = sizeof(typename T::type<void>);
      |                            ^
      |                            template 
<source>:10:15: error: use 'template' keyword to treat 'type' as a dependent template name
   10 |   typename T::type<void>;
      |               ^
      |               template 
<source>:15:15: error: use 'template' keyword to treat 'type' as a dependent template name
   15 | struct Z : T::type<void>::type {};
      |               ^
      |               template 

This compiles with -fms-extensions.

MSVC only compiles this with /permissive (not /permissive-):

template<typename T, typename = typename T::type<void>>
int Y = sizeof(typename T::type<void>);

GCC compiles it all.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Mital Ashok (MitalAshok)

https://eel.is/c++draft/temp.names#3

> A < is interpreted as the delimiter of a template-argument-list if it follows a name [...] that is [...] in a type-only context other than a nested-name-specifier.

This wording was added by P1787R6 for C++23

https://godbolt.org/z/GanbhKTh1

template<typename T>
struct X : T::type<void> {};

template<typename T, typename = typename T::type<void>>
int Y = sizeof(typename T::type<void>);

// CWG2806
template<typename T>
concept C = requires {
  typename T::type<void>;
};

// CWG2903
template<typename T>
struct Z : T::type<void>::type {};

Last two are valid after https://cplusplus.github.io/CWG/issues/2806.html and https://cplusplus.github.io/CWG/issues/2903.html

<source>:2:15: error: use 'template' keyword to treat 'type' as a dependent template name
    2 | struct X : T::type<void> {};
      |               ^
      |               template 
<source>:4:45: error: use 'template' keyword to treat 'type' as a dependent template name
    4 | template<typename T, typename = typename T::type<void>>
      |                                             ^
      |                                             template 
<source>:5:28: error: use 'template' keyword to treat 'type' as a dependent template name
    5 | int Y = sizeof(typename T::type<void>);
      |                            ^
      |                            template 
<source>:10:15: error: use 'template' keyword to treat 'type' as a dependent template name
   10 |   typename T::type<void>;
      |               ^
      |               template 
<source>:15:15: error: use 'template' keyword to treat 'type' as a dependent template name
   15 | struct Z : T::type<void>::type {};
      |               ^
      |               template 

This compiles with -fms-extensions.

MSVC only compiles this with /permissive (not /permissive-):

template<typename T, typename = typename T::type<void>>
int Y = sizeof(typename T::type<void>);

GCC compiles it all.

[frederick-vs-ja]

https://eel.is/c++draft/temp.names#3

A < is interpreted as the delimiter of a template-argument-list if it follows a name [...] that is [...] in a type-only context other than a nested-name-specifier.

This wording was added by P1787R6 for C++23

These parts of changes seem to be the resolution of CWG1478 but touch more things than the original issue.

Edit: Is this already tracked in #54150?

[MitalAshok]

@frederick-vs-ja I don't think this is related to CWG1478 because that's specifically about when there isn't a <.

Reading P1787P6§Parsing makes it seem like type-only contexts (previously: type-id-only contexts, in Clang as ImplicitTypenameContext) were extended to include "implicit template" that doesn't seem to be mentioned in #54150.

[MitalAshok]

This might be a dup of #95447

[shafik]

Also see: #81731