Clang C++20 Feature: Declarations and where to find them (P1787)

[cor3ntin]

Paper link: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html

This fixes a number of DRs, I think a first order of business would be to understand what changes are actually needed in clang

[llvmbot]

@llvm/issue-subscribers-c-20

[frederick-vs-ja]

I'm adding DR status in Clang's implementation staus page (#68846). Do you think we should specially list P1787R6 (which is effectively a DR) in that page or track resolved issues in the page for CWG issues?

[AaronBallman]

CC @Endilll who has also been working on conformance tests in this area.

Personally, I think it's more helpful to track the individual resolved issues than it is to track status of the omnibus paper fixing them; I expect the paper's status will be Partial for approximately... ever.

[Endilll]

Here is status for tests (checkbox) and availability (in parentheses) in the order P1787R6 describes them. N/A are marked as having tests, since no work is due for them. I'm also listing only defect reports, omitting unresolved Core issues.

Definitions

• (N/A) CWG554 Definition of “declarative region” and “scope”
• (N/A) CWG2165 Namespaces, declarative regions, and translation units
• (Yes) CWG405 Unqualified function name lookup
• (Yes) CWG36 using-declarations in multiple-declaration contexts
• (No) CWG418 Imperfect wording on error on multiple default arguments on a called function
• (No) CWG1960 Visibility of entity named in class-scope using-declaration
• (N/A) CWG1291 Looking up a conversion-type-id
• (No) CWG2396 Lookup of names in complex _conversion-type-id_s
• (Yes) CWG600 Does access control apply to members or to names?
• (Yes) CWG360 Using-declaration that reduces access
• (Yes) CWG952 Insufficient description of “naming class”

Lookup completeness

• (Yes) CWG191 Name lookup does not handle complex nesting
• (N/A) CWG1200 Lookup rules for template parameters
• (N/A) CWG536 Problems in the description of id-expressions
• (Clang 11) CWG399 Destructor lookup redux
• (N/A) CWG562 qualified-ids in non-expression contexts
• (No) CWG2370 friend declarations of namespace-scope functions
• (Yes) CWG1822 Lookup of parameter names in lambda-expressions
• (Clang 3.8) CWG1894 _typedef-name_s and _using-declaration_s
• (Clang 3.8) CWG2199 Typedefs and tags
• (Yes) CWG607 Lookup of _mem-initializer-id_s
• (Clang 3.3) CWG1837 Use of this in friend and local class declarations
• (Clang 3.4) CWG2007 Argument-dependent lookup for operator=
• (Yes) CWG255 Placement deallocation functions and lookup ambiguity

Complete-class context

• (N/A) CWG2331 Redundancy in description of class scope
• (N/A) CWG2009 Unclear specification of class scope

Declaration matching

• (Yes) CWG1821 Qualified redeclarations in a class member-specification
• CWG1820 Qualified typedef names
• (No) CWG386 Friend declaration of name brought in by using-declaration
• CWG1839 Lookup of block-scope extern declarations
• (Clang 3.4) CWG1818 Visibility and inherited language linkage
• CWG2058 More errors from internal-linkage namespaces
• (Clang 2.7) CWG1900 Do friend declarations count as “previous declarations”?
• (Clang 2.7) CWG1477 Definition of a friend outside its namespace
• (Partial) CWG138 Friend declaration name lookup
• (Clang 3.3) CWG563 Linkage specification for objects
• (Clang 2.8) CWG110 Can template functions and classes be declared in the same scope?
• (Clang 2.7) CWG1898 Use of “equivalent” in overload resolution
• (Partial) CWG1884 Unclear requirements for same-named external-linkage entities
• (No) CWG279 Correspondence of "names for linkage purposes"
• (Partial) CWG338 Enumerator name with linkage used as class name in other translation unit
• CWG1252 Overloading member function templates based on dependent return type
• CWG2062 Class template redeclaration requirements
• CWG1896 Repeated alias templates
• CWG1729 Matching declarations and definitions of variable templates

Modules

No defect reports in this section.

Parsing

• CWG1828 nested-name-specifier ambiguity
• CWG1771 Restricted lookup in nested-name-specifier
• CWG1835 Dependent member lookup before <
• CWG1908 Dual destructor lookup and _template-id_s
• CWG1841 < following template injected-class-name
• CWG1616 Disambiguation parsing and template parameters

Templates

• CWG1936 Dependent _qualified-id_s
• CWG1500 Name lookup of dependent conversion function
• CWG2413 typename in _conversion-function-id_s
• CWG1028 Dependent names in non-defining declarations
• CWG1907 _using-declaration_s and default arguments
• CWG2065 Current instantiation of a partial specialization
• CWG1829 Dependent unnamed types
• CWG1478 template keyword for dependent template template arguments
• CWG2070 using-declaration with dependent nested-name-specifier
• CWG852 using-declarations and dependent base classes
• (Yes) CWG2213 Forward declaration of partial specializations
• CWG271 Explicit instantiation and template argument deduction

[Endilll]

While there are some more currently under review, I put this paper on hold, until I have more time and a wording expert willing to spend their time to dive into this. As if definitions and lookup completeness sections weren't complicated enough, it gets even worse in subsequent sections.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: cor3ntin (cor3ntin)

Paper link: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html

This fixes a number of DRs, I think a first order of business would be to understand what changes are actually needed in clang

[Endilll]

I got back to this recently with #106117 and #107019