C++: Add tests for experimental cpp/guarded-free query

cpp/ql/test/experimental/query-tests/Best Practices/GuardedFree/GuardedFree.expected

@@ -0,0 +1,10 @@
+| test.cpp:5:7:5:7 | x | unnecessary NULL check before call to $@ | test.cpp:6:5:6:8 | call to free | free |
+| test.cpp:23:7:23:7 | x | unnecessary NULL check before call to $@ | test.cpp:26:5:26:8 | call to free | free |
+| test.cpp:31:7:31:8 | ! ... | unnecessary NULL check before call to $@ | test.cpp:35:3:35:6 | call to free | free |
+| test.cpp:31:7:31:24 | ... \|\| ... | unnecessary NULL check before call to $@ | test.cpp:35:3:35:6 | call to free | free |
+| test.cpp:31:8:31:8 | x | unnecessary NULL check before call to $@ | test.cpp:35:3:35:6 | call to free | free |
+| test.cpp:94:12:94:12 | x | unnecessary NULL check before call to $@ | test.cpp:94:3:94:13 | call to free | free |
+| test.cpp:98:7:98:8 | ! ... | unnecessary NULL check before call to $@ | test.cpp:101:3:101:6 | call to free | free |
+| test.cpp:98:8:98:8 | x | unnecessary NULL check before call to $@ | test.cpp:101:3:101:6 | call to free | free |
+| test.cpp:106:7:106:18 | ... != ... | unnecessary NULL check before call to $@ | test.cpp:107:5:107:8 | call to free | free |
+| test.cpp:113:7:113:18 | ... != ... | unnecessary NULL check before call to $@ | test.cpp:114:17:114:20 | call to free | free |

cpp/ql/test/experimental/query-tests/Best Practices/GuardedFree/GuardedFree.qlref

@@ -0,0 +1 @@
+experimental/Best Practices/GuardedFree.ql

cpp/ql/test/experimental/query-tests/Best Practices/GuardedFree/test.cpp

@@ -0,0 +1,115 @@
+extern "C" void free(void *ptr);
+extern "C" int strcmp(const char *s1, const char *s2);
+
+void test0(int *x) {
+  if (x) // BAD
+    free(x);
+}
+
+void test1(int *x) {
+  if (x) { // BAD
+    free(x);
+  }
+}
+
+void test2(int *x) {
+  if (x) { // GOOD: x is being accessed in the body of the if
+    *x = 42;
+    free(x);
+  }
+}
+
+void test3(int *x, bool b) {
+  if (x) { // GOOD [FALSE POSITIVE]: x is being accessed in the body of the if
+    if (b)
+      *x = 42;
+    free(x);
+  }
+}
+
+bool test4(char *x, char *y) {
+  if (!x || strcmp(x, y)) { // GOOD [FALSE POSITIVE]: x is being accessed in the guard and return value depends on x
+    free(x);
+    return true;
+  }
+  free(x);
+  return false;
+}
+
+void test5(char *x) {
+  if (x)
+    *x = 42;
+  if (x) { // BAD
+    free(x);
+  }
+}
+
+void test6(char *x) {
+  *x = 42;
+  if (x) { // BAD
+    free(x);
+  }
+}
+
+void test7(char *x) {
+  if (x || x) { // BAD [NOT DETECTED]
+    free(x);
+  }
+}
+
+bool test8(char *x) {
+  if (x) { // GOOD: return value depends on x
+    free(x);
+    return true;
+  }
+  return false;
+}
+
+#ifdef FOO
+#define my_free(x) free(x - 1)
+#else
+#define my_free(x) free(x)
+#endif
+
+void test9(char *x) {
+  if (x) { // GOOD: macro may make free behave unexpectedly when compiled differently
+    my_free(x);
+  }
+}
+
+void test10(char *x) {
+  if (x) { // GOOD: #ifdef may make free behave unexpectedly when compiled differently
+#ifdef FOO
+    free(x - 1);
+#else
+    free(x);
+#endif
+  }
+}
+
+#define TRY_FREE(x) \
+  if (x) free(x);
+
+void test11(char *x) {
+  TRY_FREE(x) // BAD
+}
+
+bool test12(char *x) {
+  if (!x) // GOOD [FALSE POSITIVE]: return value depends on x
+    return false;
+
+  free(x);
+  return true;
+}
+
+void test13(char *x) {
+  if (x != nullptr) // BAD
+    free(x);
+}
+
+void inspect(char *x);
+
+void test14(char *x) {
+  if (x != nullptr) // GOOD [FALSE POSITIVE]: x might be accessed in the first operand of the comma operator
+    inspect(x), free(x);
+}