C++: Add a skeleton class for requires expressions

cpp/ql/lib/cpp.qll

@@ -17,6 +17,7 @@ import semmle.code.cpp.File
 import semmle.code.cpp.Linkage
 import semmle.code.cpp.Location
 import semmle.code.cpp.Compilation
+import semmle.code.cpp.Concept
 import semmle.code.cpp.Element
 import semmle.code.cpp.Namespace
 import semmle.code.cpp.Specifier

cpp/ql/lib/semmle/code/cpp/Concept.qll

@@ -0,0 +1,14 @@
+/**
+ * Provides classes for working with C++ concepts.
+ */
+
+import semmle.code.cpp.exprs.Expr
+
+/**
+ * A C/C++ requires expression.
+ */
+class RequiresExpr extends Expr, @requires_expr {
+  override string toString() { result = "requires ..." }
+
+  override string getAPrimaryQlClass() { result = "RequiresExpr" }
+}

cpp/ql/lib/semmlecode.cpp.dbscheme

@@ -1790,6 +1790,7 @@ case @expr.kind of
 | 387 = @istriviallyrelocatable
 | 388 = @datasizeof
 | 389 = @c11_generic
+| 390 = @requires_expr
 ;
 
 @var_args_expr = @vastartexpr

cpp/ql/lib/upgrades/7ff6a6e53dbcff09d1b9b758b594bc6d17366863/upgrade.properties

@@ -0,0 +1,2 @@
+description: Add requires expressions
+compatibility: full