Address incomplete constexpr function behavior

A call to a `constexpr` function is insufficient to determine that the
return value is compile time computed. We need to also validate that
its arguments are compile time computed.

Author: rvermeulen

cpp/autosar/src/rules/A7-1-2/VariableMissingConstexpr.ql

@@ -33,6 +33,15 @@ predicate isTypeZeroInitializable(Type t) {
   t.getUnderlyingType() instanceof ArrayType
 }
 
+predicate isCompileTimeEvaluated(Call call) {
+  call.getTarget().isConstexpr() and
+  forall(Expr arg | arg = call.getAnArgument() |
+    DataFlow::localExprFlow(any(Literal l), arg)
+    or
+    DataFlow::localExprFlow(any(Call c | isCompileTimeEvaluated(call)), arg)
+  )
+}
+
 from Variable v
 where
   not isExcluded(v, ConstPackage::variableMissingConstexprQuery()) and
@@ -46,7 +55,7 @@ where
   (
     v.getInitializer().getExpr().isConstant()
     or
-    v.getInitializer().getExpr().(Call).getTarget().isConstexpr()
+    any(Call call | isCompileTimeEvaluated(call)) = v.getInitializer().getExpr()
     or
     isZeroInitializable(v)
     or

cpp/autosar/test/rules/A7-1-2/VariableMissingConstexpr.expected

@@ -10,3 +10,4 @@
 | test.cpp:55:7:55:8 | m2 | Variable m2 could be marked 'constexpr'. |
 | test.cpp:130:7:130:8 | m1 | Variable m1 could be marked 'constexpr'. |
 | test.cpp:141:7:141:8 | m1 | Variable m1 could be marked 'constexpr'. |
+| test.cpp:215:7:215:7 | x | Variable x could be marked 'constexpr'. |

cpp/autosar/test/rules/A7-1-2/test.cpp

@@ -204,4 +204,14 @@ class ExcludedCases {
 
   void operator=(ExcludedCases &) {}  // COMPLIANT
   void operator=(ExcludedCases &&) {} // COMPLIANT
-};
+};
+
+
+constexpr int add(int x, int y) {
+  return x + y;
+}
+
+void fp_reported_in_466(int p) {
+  int x = add(1,2); // NON_COMPLIANT
+  int y = add(1,p); // COMPLIANT
+}