[clang] Add test for CWG2486 (noexcept and function pointer conversion) (#107131)

[CWG2486](https://cplusplus.github.io/CWG/issues/2486.html) "Call to
`noexcept` function via `noexcept(false)` pointer/lvalue" allows
`noexcept` functions to be called via `noexcept(false)` pointers or
values. There appears to be no implementation divergence whatsoever:
https://godbolt.org/z/3afTfeEM8. That said, in C++14 and earlier we do
not issue all the diagnostics we issue in C++17 and newer, so I'm
specifying the status of the issue accordingly.

Author: Endilll

clang/test/CXX/drs/cwg24xx.cpp

@@ -1,15 +1,11 @@
-// RUN: %clang_cc1 -std=c++98 -pedantic-errors %s -verify=expected
-// RUN: %clang_cc1 -std=c++11 -pedantic-errors %s -verify=expected
-// RUN: %clang_cc1 -std=c++14 -pedantic-errors %s -verify=expected
+// RUN: %clang_cc1 -std=c++98 -pedantic-errors %s -verify=expected,cxx98-14
+// RUN: %clang_cc1 -std=c++11 -pedantic-errors %s -verify=expected,cxx98-14
+// RUN: %clang_cc1 -std=c++14 -pedantic-errors %s -verify=expected,cxx98-14
 // RUN: %clang_cc1 -std=c++17 -pedantic-errors %s -verify=expected,since-cxx17
 // RUN: %clang_cc1 -std=c++20 -pedantic-errors %s -verify=expected,since-cxx17
 // RUN: %clang_cc1 -std=c++23 -pedantic-errors %s -verify=expected,since-cxx17
 // RUN: %clang_cc1 -std=c++2c -pedantic-errors %s -verify=expected,since-cxx17
 
-#if __cplusplus <= 201402L
-// expected-no-diagnostics
-#endif
-
 namespace cwg2406 { // cwg2406: 5
 #if __cplusplus >= 201703L
 void fallthrough(int n) {
@@ -186,3 +182,36 @@ namespace cwg2445 { // cwg2445: 19
   }
 #endif
 }
+
+namespace cwg2486 { // cwg2486: 4 c++17
+struct C {
+  void fn() throw();
+};
+
+static void call(C& c, void (C::*f)()) {
+  (c.*f)();
+}
+
+static void callNE(C& c, void (C::*f)() throw()) {
+// cxx98-14-warning@-1 {{mangled name of 'callNE' will change in C++17 due to non-throwing exception specification in function signature}}
+  (c.*f)();
+}
+
+void ref() {
+  C c;
+  call(c, &C::fn); // <= implicit cast removes noexcept
+  callNE(c, &C::fn);
+}
+
+void (*p)();
+void (*pp)() throw() = p;
+// since-cxx17-error@-1 {{cannot initialize a variable of type 'void (*)() throw()' with an lvalue of type 'void (*)()': different exception specifications}}
+
+struct S {
+  typedef void (*p)();
+  operator p(); // #cwg2486-conv
+};
+void (*q)() throw() = S();
+// since-cxx17-error@-1 {{no viable conversion from 'S' to 'void (*)() throw()'}}
+//   since-cxx17-note@#cwg2486-conv {{candidate function}}
+} // namespace cwg2486

clang/www/cxx_dr_status.html

@@ -14751,7 +14751,7 @@ <h2 id="cxxdr">C++ defect report implementation status</h2>
     <td><a href="https://cplusplus.github.io/CWG/issues/2486.html">2486</a></td>
     <td>CD6</td>
     <td>Call to <TT>noexcept</TT> function via <TT>noexcept(false)</TT> pointer/lvalue</td>
-    <td class="unknown" align="center">Unknown</td>
+    <td class="full" align="center">Clang 4 (C++17 onwards)</td>
   </tr>
   <tr class="open" id="2487">
     <td><a href="https://cplusplus.github.io/CWG/issues/2487.html">2487</a></td>