[clang] [analyzer] Add std::variant checker (PR #66481)

[via cfe-commits]

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@@ -0,0 +1,349 @@
+// RUN: %clang %s -std=c++17 -Xclang -verify --analyze \
+// RUN:   -Xclang -analyzer-checker=core \
+// RUN:   -Xclang -analyzer-checker=debug.ExprInspection \
+// RUN:   -Xclang -analyzer-checker=core,alpha.core.StdVariant
+
+#include "Inputs/system-header-simulator-cxx.h"
+
+class Foo{};
+
+void clang_analyzer_warnIfReached();
+void clang_analyzer_eval(int);
+
+//helper functions
+void changeVariantType(std::variant<int, char> &v) {
+  v = 25;
+}
+
+void changesToInt(std::variant<int, char> &v);
+void changesToInt(std::variant<int, char> *v);
+
+void cannotChangePtr(const std::variant<int, char> &v);
+void cannotChangePtr(const std::variant<int, char> *v);
+
+char getUnknownChar();
+
+void swap(std::variant<int, char> &v1, std::variant<int, char> &v2) {
+  std::variant<int, char> tmp = v1;
+  v1 = v2;
+  v2 = tmp;
+}
+
+void cantDo(const std::variant<int, char>& v) {
+  std::variant<int, char> vtmp = v;
+  vtmp = 5;
+  int a = std::get<int> (vtmp);
+  (void*) a;
----------------
DonatNagyE wrote:

Why are you casting everything to `void*` (or even `void**` in one case)?

The standard idiom for "don't warn about this unused variable" is just `(void) variable`, which works for any type (unlike casts to `void*`, which would fail for e.g. a `double`).

https://github.com/llvm/llvm-project/pull/66481