C++: Add a test for constexpr if

Author: ian-semmle

cpp/ql/test/library-tests/constexpr_if/stmts.expected

@@ -0,0 +1,11 @@
+| test.cpp:3:17:13:1 | { ... } |
+| test.cpp:4:5:4:10 | declaration |
+| test.cpp:5:5:9:5 | if constexpr (...) ...  |
+| test.cpp:5:25:7:5 | { ... } |
+| test.cpp:6:9:6:14 | ExprStmt |
+| test.cpp:7:12:9:5 | { ... } |
+| test.cpp:8:9:8:14 | ExprStmt |
+| test.cpp:10:5:12:5 | if constexpr (...) ...  |
+| test.cpp:10:26:12:5 | { ... } |
+| test.cpp:11:9:11:14 | ExprStmt |
+| test.cpp:13:1:13:1 | return ... |

cpp/ql/test/library-tests/constexpr_if/stmts.ql

@@ -0,0 +1,4 @@
+import cpp
+
+from Stmt s
+select s

cpp/ql/test/library-tests/constexpr_if/test.cpp

@@ -0,0 +1,14 @@
+// semmle-extractor-options: --edg --c++20
+
+void test(void) {
+    int x;
+    if constexpr (true) {
+        x = 1;
+    } else {
+        x = 2;
+    }
+    if constexpr (false) {
+        x = 3;
+    }
+}
+