[clang] [clang] Add tests for CWG issues about friend declaration matching (PR #106117)
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Mon Aug 26 11:39:27 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang
Author: Vlad Serebrennikov (Endilll)
<details>
<summary>Changes</summary>
This patch covers CWG issues regarding declaration matching when `friend` declarations are involved: [CWG138](https://cplusplus.github.io/CWG/issues/138.html), [CWG386](https://cplusplus.github.io/CWG/issues/386.html), and [CWG1477](https://cplusplus.github.io/CWG/issues/1477.html). Atypical for our CWG tests, the ones in this patch are quite extensively commented in-line, explaining the mechanics. PR description focuses on high-level concerns and references.
[CWG138](https://cplusplus.github.io/CWG/issues/138.html) "Friend declaration name lookup"
-----------
[P1787R6](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html):
> [CWG138](https://cplusplus.github.io/CWG/issues/138.html) is resolved according to [N1229](http://wg21.link/n1229), except that using-directives that nominate nested namespaces are considered.
I find it hard to pin down the scope of this issue, so I'm relying on three examples from the filing to define it. Because of that, it's also hard to pinpoint exact wording changes that resolve it. Relevant references are: [[dcl.meaning.general]/2](http://eel.is/c++draft/dcl.meaning#general-2), [[namespace.udecl]/10](https://eel.is/c++draft/namespace.udecl#<!-- -->10), [[dcl.type.elab]/3](https://eel.is/c++draft/dcl.type.elab#<!-- -->3), [[basic.lookup.elab]/1](https://eel.is/c++draft/basic.lookup.elab#<!-- -->1).
[CWG386](https://cplusplus.github.io/CWG/issues/386.html) "Friend declaration of name brought in by _using-declaration_"
-----------
[P1787R6](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html):
> [CWG386](https://cplusplus.github.io/CWG/issues/386.html), [CWG1839](https://cplusplus.github.io/CWG/issues/1839.html), [CWG1818](https://cplusplus.github.io/CWG/issues/1818.html), [CWG2058](https://cplusplus.github.io/CWG/issues/2058.html), [CWG1900](https://cplusplus.github.io/CWG/issues/1900.html), and Richard’s observation in [“are non-type names ignored in a class-head-name or enum-head-name?”](http://lists.isocpp.org/core/2017/01/1604.php) are resolved by describing the limited lookup that occurs for a declarator-id, including the changes in Richard’s [proposed resolution for CWG1839](http://wiki.edg.com/pub/Wg21cologne2019/CoreWorkingGroup/cwg1839.html) (which also resolves CWG1818 and what of CWG2058 was not resolved along with CWG2059) and rejecting the example from [CWG1477](https://cplusplus.github.io/CWG/issues/1477.html).
Wording ([[dcl.meaning.general]/2](http://eel.is/c++draft/dcl.meaning#general-2)):
> — If the [id-expression](http://eel.is/c++draft/expr.prim.id.general#nt:id-expression) E in the [declarator-id](http://eel.is/c++draft/dcl.decl.general#nt:declarator-id) of the [declarator](http://eel.is/c++draft/dcl.decl.general#nt:declarator) is a [qualified-id](http://eel.is/c++draft/expr.prim.id.qual#nt:qualified-id) or a [template-id](http://eel.is/c++draft/temp.names#nt:template-id):
> — [...]
> — The [declarator](http://eel.is/c++draft/dcl.decl.general#nt:declarator) shall correspond to one or more declarations found by the lookup; they shall all have the same target scope, and the target scope of the [declarator](http://eel.is/c++draft/dcl.decl.general#nt:declarator) is that scope[.](http://eel.is/c++draft/dcl.meaning#general-2.2.2.sentence-1)
This issue focuses on interaction of `friend` declarations with template-id and qualified-id with using-declarations. The short answer is that terminal name in such declarations undergo lookup, and using-declarations do what they usually do helping that lookup. Target scope of such friend declaration is the target scope of lookup result, so no conflicts arise with the using-declarations.
[CWG1477](https://cplusplus.github.io/CWG/issues/1477.html) "Definition of a `friend` outside its namespace"
-----------
[P1787R6](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2020/p1787r6.html):
> [...] and rejecting the example from [CWG1477](https://cplusplus.github.io/CWG/issues/1477.html).
Wording ([[dcl.meaning.general]/3.4](http://eel.is/c++draft/dcl.meaning#general-3.4)):
> Otherwise, the terminal name of the [declarator-id](http://eel.is/c++draft/dcl.decl.general#nt:declarator-id) is not looked up[.](http://eel.is/c++draft/dcl.meaning#general-3.4.sentence-1)
If it is a qualified name, the [declarator](http://eel.is/c++draft/dcl.decl.general#nt:declarator) shall correspond to one or more declarations nominable in S; all the declarations shall have the same target scope and the target scope of the [declarator](http://eel.is/c++draft/dcl.decl.general#nt:declarator) is that scope[.](http://eel.is/c++draft/dcl.meaning#general-3.4.sentence-2)
This issue focuses on befriending a function in one scope, then defining it from other scope using qualified-id. Contrary to what P1787R6 says in prose, this example is accepted by the wording in that paper. In the wording quote above, note the absence of a statement like "terminal name of the declarator-id is not bound", contrary to similar statements made before that in [dcl.meaning.general] about friend declarations and template-ids.
There's also a note in [basic.scope.scope] that supports the rejection, but it's considered incorrect and expected to be removed in the future. This is tracked in https://github.com/cplusplus/draft/pull/7238.
------------------
There's also a relevant [CWG1900](https://cplusplus.github.io/CWG/issues/1900.html), but I'm not sure what to do about it. Call it a pure wording change? Input is appreciated.
---
Full diff: https://github.com/llvm/llvm-project/pull/106117.diff
3 Files Affected:
- (modified) clang/test/CXX/drs/cwg14xx.cpp (+19)
- (modified) clang/test/CXX/drs/cwg1xx.cpp (+74)
- (modified) clang/test/CXX/drs/cwg3xx.cpp (+69)
``````````diff
diff --git a/clang/test/CXX/drs/cwg14xx.cpp b/clang/test/CXX/drs/cwg14xx.cpp
index a23ac744436331..1240c416451b8f 100644
--- a/clang/test/CXX/drs/cwg14xx.cpp
+++ b/clang/test/CXX/drs/cwg14xx.cpp
@@ -603,6 +603,25 @@ namespace cwg1467 { // cwg1467: 3.7 c++11
#endif
} // cwg1467
+namespace cwg1477 { // cwg1477: 2.7
+namespace N {
+struct A {
+ // Name "f" is not bound in N,
+ // so single searches of 'f' in N won't find it,
+ // but the targets scope of this declaration is N,
+ // making it nominable in N.
+ friend int f();
+};
+}
+// Corresponds to the friend declaration,
+// because it's nominable in N,
+// and binds name 'f' in N.
+int N::f() { return 0; }
+// Name 'f' is bound in N,
+// so the search performed by qualified lookup finds it.
+int i = N::f();
+} // namespace cwg1477
+
namespace cwg1479 { // cwg1479: 3.1
#if __cplusplus >= 201103L
int operator"" _a(const char*, std::size_t = 0);
diff --git a/clang/test/CXX/drs/cwg1xx.cpp b/clang/test/CXX/drs/cwg1xx.cpp
index e7dddd1ea9278f..d6ee0844458b1d 100644
--- a/clang/test/CXX/drs/cwg1xx.cpp
+++ b/clang/test/CXX/drs/cwg1xx.cpp
@@ -568,6 +568,80 @@ namespace cwg137 { // cwg137: yes
const volatile int *cvqcv = static_cast<const volatile int*>(cvp);
}
+namespace cwg138 { // cwg138: partial
+namespace example1 {
+void foo(); // #cwg138-ex1-foo
+namespace A {
+ using example1::foo; // #cwg138-ex1-using
+ class X {
+ static const int i = 10;
+ // This friend declaration is using neither qualified-id nor template-id,
+ // so name 'foo' is not looked up, which means the using-declaration has no effect.
+ // Target scope of this declaration is A, so this is grating friendship to
+ // (hypothetical) A::foo instead of 'example1::foo' using declaration refers to.
+ // A::foo corresponds to example1::foo named by the using declaration,
+ // and since A::foo is a different entity, they potentially conflict.
+ // FIXME: This is ill-formed, but not for the reason diagnostic says.
+ friend void foo();
+ // expected-error at -1 {{cannot befriend target of using declaration}}
+ // expected-note@#cwg138-ex1-foo {{target of using declaration}}
+ // expected-note@#cwg138-ex1-using {{using declaration}}
+ };
+}
+} // namespace example1
+
+namespace example2 {
+void f();
+void g();
+class B {
+ void g();
+};
+class A : public B {
+ static const int i = 10;
+ void f();
+ // Both friend declaration are not using qualified-ids or template-ids,
+ // so 'f' and 'g' are not looked up, which means that presence of A::f
+ // and base B have no effect.
+ // Both target scope of namespace 'example2', and grant friendship to
+ // example2::f and example2::g respectively.
+ friend void f();
+ friend void g();
+};
+void f() {
+ int i2 = A::i;
+}
+void g() {
+ int i3 = A::i;
+}
+} // namespace example2
+
+namespace example3 {
+struct Base {
+private:
+ static const int i = 10; // #cwg138-ex3-Base-i
+
+public:
+ struct Data;
+ // Elaborated type specifier is not the sole constituent of declaration,
+ // so 'Data' undergoes unqualified type-only lookup, which finds Base::Data.
+ friend class Data;
+
+ struct Data {
+ void f() {
+ int i2 = Base::i;
+ }
+ };
+};
+struct Data {
+ void f() {
+ int i2 = Base::i;
+ // expected-error at -1 {{'i' is a private member of 'cwg138::example3::Base'}}
+ // expected-note@#cwg138-ex3-Base-i {{declared private here}}
+ }
+};
+} // namespace example3
+} // namespace cwg138
+
namespace cwg139 { // cwg139: yes
namespace example1 {
typedef int f; // #cwg139-typedef-f
diff --git a/clang/test/CXX/drs/cwg3xx.cpp b/clang/test/CXX/drs/cwg3xx.cpp
index a10ed95941ba4a..44bf974ef66649 100644
--- a/clang/test/CXX/drs/cwg3xx.cpp
+++ b/clang/test/CXX/drs/cwg3xx.cpp
@@ -1369,6 +1369,75 @@ namespace cwg385 { // cwg385: 2.8
// expected-note@#cwg385-n {{member is declared here}}
}
+namespace cwg386 { // cwg386: no
+namespace example1 {
+namespace N1 {
+// Binds name 'f' in N1. Target scope is N1.
+template<typename T> void f( T* x ) {
+ // ... other stuff ...
+ delete x;
+}
+}
+
+namespace N2 {
+// Bind name 'f' in N2. When a single search find this declaration,
+// it's replaced with N1::f declaration.
+using N1::f;
+
+// `f<int>` is not a qualified-id, so its target scope is N2.
+// `f<int>` is a template-id, so 'f' undergoes (unqualified) lookup.
+// Search performed by unqualified lookup finds N1::f via using-declaration,
+// but this result is not considered, because it's not nominable in N2,
+// which is because its target scope is N1.
+// So unqualified lookup doesn't find anything, making this declaration ill-formed.
+template<> void f<int>( int* );
+// expected-error at -1 {{no function template matches function template specialization 'f'}}
+
+class Test {
+ ~Test() { }
+ // `f<>` is a template-id and not a template declaration,
+ // so its terminal name 'f' undergoes (unqualified) lookup.
+ // Search in N2 performed by unqualified lookup finds
+ // (single) N1::f declaration via using-declaration.
+ // N1::f is replaced with N1::f<> specialization after deduction,
+ // and this is the result of the unqualified lookup.
+ // This friend declaration correspond to the result of the lookup.
+ // All lookup results target the same scope, which is N1,
+ // so target scope of this friend declaration is also N1.
+ // FIXME: This is well-formed.
+ friend void f<>( Test* x );
+ // expected-error at -1 {{no function template matches function template specialization 'f'}}
+};
+}
+} // namespace example1
+
+namespace example2 {
+namespace N1 {
+// Binds name 'f' in N1. Target scope is N1.
+void f(); // #cwg386-ex2-N1-f
+}
+
+namespace N2 {
+// Bind name 'f' in N2. When a single search finds this declaration,
+// it's replaced with N1::f declaration.
+using N1::f; // #cwg386-ex2-using
+class A {
+ // `N2::f` is a qualified-id, so its terminal name 'f' undergoes (qualified) lookup.
+ // Search in N2 performed by qualified lookup finds N1::f via using-declaration,
+ // which is the (only) result of qualified lookup.
+ // This friend declaration corresponds to the result of the lookup.
+ // All lookup results target the same scope, which is N1,
+ // so target scope of this friend declaration is also N1.
+ // FIXME: This is well-formed.
+ friend void N2::f();
+ // expected-error at -1 {{cannot befriend target of using declaration}}
+ // expected-note@#cwg386-ex2-N1-f {{target of using declaration}}
+ // expected-note@#cwg386-ex2-using {{using declaration}}
+};
+}
+} // namespace example2
+} // namespace cwg386
+
namespace cwg387 { // cwg387: 2.8
namespace old {
template<typename T> class number {
``````````
</details>
https://github.com/llvm/llvm-project/pull/106117
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