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

[via cfe-commits]

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+// 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;
+}
+
+void changeVariantPtr(std::variant<int, char> *v) {
+  *v = 'c';
+}
+
+using var_t = std::variant<int, char>;
+using var_tt = var_t;
+using int_t = int;
+using char_t = char;
+
+
+//----------------------------------------------------------------------------//
+// std::get
+//----------------------------------------------------------------------------//
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DonatNagyE wrote:

Paranoia: add a case that tests `std::get()` used on an `std::pair()`.

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