[clang] [-Wunsafe-buffer-usage] Support span creation from std::initializer_list and begin/end pairs (PR #145311)

[Ziqing Luo via cfe-commits]

================
@@ -232,3 +268,27 @@ struct HoldsStdSpanAndNotInitializedInCtor {
       : Ptr(P), Size(S)
   {}
 };
+
+namespace test_begin_end {
+  struct Object {
+    int * begin();
+    int * end();
+  };
+  void safe_cases(std::span<int> Sp, std::array<int, 10> Arr, std::string Str, std::initializer_list<Object> Il) {
+    std::span<int>{Sp.begin(), Sp.end()};
+    std::span<int>{Arr.begin(), Arr.end()};
+    std::span<char>{Str.begin(), Str.end()};
+    std::span<Object>{Il.begin(), Il.end()};
+  }
+
+  void unsafe_cases(std::span<int> Sp, std::array<int, 10> Arr, std::string Str, std::initializer_list<Object> Il,
+		    Object Obj) {
+    std::span<int>{Obj.begin(), Obj.end()}; // expected-warning {{the two-parameter std::span construction is unsafe as it can introduce mismatch between buffer size and the bound information}}
+    std::span<int>{Sp.end(), Sp.begin()};   // expected-warning {{the two-parameter std::span construction is unsafe as it can introduce mismatch between buffer size and the bound information}}
----------------
ziqingluo-90 wrote:

My understanding is that it warns about the mismatch between **the bound information of the constructed span** and the actual buffer size.  So I think the message is accurate regardless of how the span is constructed.

https://github.com/llvm/llvm-project/pull/145311