[clang] Add tests for CWG issues about language linkage

[Endilll]

This patch covers Core issues about language linkage during declaration matching resolved in P1787R6, namely CWG563 and CWG1818.

CWG563 "Linkage specification for objects"

P1787R6:

CWG563 is resolved by simplifications that follow its suggestions.

Wording ([dcl.link]/5):

In a linkage-specification, the specified language linkage applies to the function types of all function declarators and to all functions and variables whose names have external linkage.

Now the wording clearly says that linkage-specification applies to variables with external linkage.

CWG1818 "Visibility and inherited language linkage"

P1787R6:

CWG386, CWG1839, CWG1818, CWG2058, CWG1900, and Richard’s observation in “are non-type names ignored in a class-head-name or enum-head-name?” are resolved by describing the limited lookup that occurs for a declarator-id, including the changes in Richard’s proposed resolution for CWG1839 (which also resolves CWG1818 and what of CWG2058 was not resolved along with CWG2059) and rejecting the example from CWG1477.

Wording ([dcl.link]/6):

A redeclaration of an entity without a linkage specification inherits the language linkage of the entity and (if applicable) its type..

Answer to the question in the example is extern "C", and not linkage mismatch. Further analysis of the example is provided as inline comments in the test itself. Note that https://eel.is/c++draft/dcl.link#7 does NOT apply in this example, as it's focused squarely at declarations that are already known to have C language linkage, and declarations of variables in the global scope.

[llvmbot]

@llvm/pr-subscribers-clang

Author: Vlad Serebrennikov (Endilll)

Changes

This patch covers Core issues about language linkage during declaration matching resolved in P1787R6, namely CWG563 and CWG1818.

CWG563 "Linkage specification for objects"

P1787R6:
> CWG563 is resolved by simplifications that follow its suggestions.

Wording ([[dcl.link]/5](https://eel.is/c++draft/dcl.link#5)):
> In a linkage-specification, the specified language linkage applies to the function types of all function declarators and to all functions and variables whose names have external linkage[.](https://eel.is/c++draft/dcl.link#5.sentence-5)

Now the wording clearly says that linkage-specification applies to variables with external linkage.

CWG1818 "Visibility and inherited language linkage"

P1787R6:
> [CWG386](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#386), [CWG1839](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1839), [CWG1818](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1818), [CWG2058](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#2058), [CWG1900](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1900), and Richard’s observation in “are non-type names ignored in a class-head-name or enum-head-name?” are resolved by describing the limited lookup that occurs for a declarator-id, including the changes in Richard’s proposed resolution for CWG1839 (which also resolves CWG1818 and what of CWG2058 was not resolved along with CWG2059) and rejecting the example from [CWG1477](http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_defects.html#1477).

Wording ([[dcl.link]/6](https://eel.is/c++draft/dcl.link#6)):
> A redeclaration of an entity without a linkage specification inherits the language linkage of the entity and (if applicable) its type[.](https://eel.is/c++draft/dcl.link#6.sentence-2).

Answer to the question in the example is extern "C", and not linkage mismatch. Further analysis of the example is provided as inline comments in the test itself. Note that https://eel.is/c++draft/dcl.link#7 does NOT apply in this example, as it's focused squarely at declarations that are already known to have C language linkage, and declarations of variables in the global scope.

---

Full diff: https://github.com/llvm/llvm-project/pull/107019.diff

5 Files Affected:

• (added) clang/test/CXX/drs/cwg1818.cpp (+34)
• (modified) clang/test/CXX/drs/cwg18xx.cpp (+2)
• (added) clang/test/CXX/drs/cwg563.cpp (+16)
• (modified) clang/test/CXX/drs/cwg5xx.cpp (+1)
• (modified) clang/www/cxx_dr_status.html (+2-2)

diff --git a/clang/test/CXX/drs/cwg1818.cpp b/clang/test/CXX/drs/cwg1818.cpp
new file mode 100644
index 00000000000000..bf2d12696a729c
--- /dev/null
+++ b/clang/test/CXX/drs/cwg1818.cpp
@@ -0,0 +1,34 @@
+// RUN: %clang_cc1 -std=c++98 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++11 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++14 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++17 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++20 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++23 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++2c %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+
+// expected-no-diagnostics
+
+namespace cwg1818 { // cwg1818: 3.4
+extern "C" void f() {
+  // This declaration binds name 'g' in the scope of function 'f',
+  // but its target scope corresponds to namespace 'cwg1818' (_N4988_.[dcl.meaning]/3.5).
+  // Linkage specification of 'f' applies to 'g' per _N4988_.[dcl.link]/5.
+  void g();
+}
+// Target scope of this declaration is naturally the one
+// that corresponds to namespace 'cwg1818',
+// which makes it declare the same entity
+// as the previous declaration per _N4988_.[basic.link]/8,
+// turning it into a redeclaration per _N4988_.[basic.def]/1.
+// Then _N4988_.[dcl.link]/6 applies, making it inherit
+// the (С) language linkage of the previous declaration.
+void g();
+} // namespace cwg1818
+
+// Check that the former 'g' has C language linkage,
+// then that the latter 'g' is considered to be a redeclaration of it,
+// which would make the latter 'g' inherit C language linkage from the former 'g'.
+
+// CHECK: LinkageSpecDecl [[LINKAGE_DECL:0x[0-9a-f]+]] {{.*}} C
+// CHECK: FunctionDecl [[FIRST_DECL:0x[0-9a-f]+]] parent [[LINKAGE_DECL]] {{.*}} g 'void ()'
+// CHECK: FunctionDecl {{.*}} prev [[FIRST_DECL]] {{.*}} g 'void ()'
diff --git a/clang/test/CXX/drs/cwg18xx.cpp b/clang/test/CXX/drs/cwg18xx.cpp
index adfdb738e81c9e..61b7faa96a9fbb 100644
--- a/clang/test/CXX/drs/cwg18xx.cpp
+++ b/clang/test/CXX/drs/cwg18xx.cpp
@@ -222,6 +222,8 @@ namespace cwg1815 { // cwg1815: no
 #endif
 }
 
+// cwg1818 is in cwg1818.cpp
+
 namespace cwg1820 { // cwg1820: 3.5
 typedef int A;
 typedef int cwg1820::A;
diff --git a/clang/test/CXX/drs/cwg563.cpp b/clang/test/CXX/drs/cwg563.cpp
new file mode 100644
index 00000000000000..d585fefc44ffce
--- /dev/null
+++ b/clang/test/CXX/drs/cwg563.cpp
@@ -0,0 +1,16 @@
+// RUN: %clang_cc1 -std=c++98 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++11 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++14 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++17 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++20 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++23 %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+// RUN: %clang_cc1 -std=c++2c %s -verify=expected -fexceptions -fcxx-exceptions -pedantic-errors -ast-dump | FileCheck %s
+
+// expected-no-diagnostics
+
+namespace cwg563 { // cwg563: 3.3
+extern "C" int a;
+} // namespace cwg563
+
+// CHECK:      LinkageSpecDecl {{.*}} C
+// CHECK-NEXT: `-VarDecl {{.*}} a 'int'
diff --git a/clang/test/CXX/drs/cwg5xx.cpp b/clang/test/CXX/drs/cwg5xx.cpp
index 6a0bb7a1966693..ed0c7159dfc889 100644
--- a/clang/test/CXX/drs/cwg5xx.cpp
+++ b/clang/test/CXX/drs/cwg5xx.cpp
@@ -799,6 +799,7 @@ namespace cwg561 { // cwg561: yes
 }
 
 // cwg562: na
+// cwg563 is in cwg563.cpp
 
 namespace cwg564 { // cwg564: yes
   extern "C++" void f(int);
diff --git a/clang/www/cxx_dr_status.html b/clang/www/cxx_dr_status.html
index 4f4d8d0a97d43f..cd02132c38f628 100755
--- a/clang/www/cxx_dr_status.html
+++ b/clang/www/cxx_dr_status.html
@@ -3431,7 +3431,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/563.html">563</a></td>
     <td>CD6</td>
     <td>Linkage specification for objects</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Clang 3.3</td>
   </tr>
   <tr id="564">
     <td><a href="https://cplusplus.github.io/CWG/issues/564.html">564</a></td>
@@ -10735,7 +10735,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/1818.html">1818</a></td>
     <td>CD6</td>
     <td>Visibility and inherited language linkage</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Clang 3.4</td>
   </tr>
   <tr id="1819">
     <td><a href="https://cplusplus.github.io/CWG/issues/1819.html">1819</a></td>

[cor3ntin]

LGTM

[AaronBallman]

LGTM