C++: Unknown template literals are constant

Author: jbj

cpp/ql/src/semmle/code/cpp/exprs/Expr.qll

@@ -131,6 +131,8 @@ class Expr extends StmtParent, @expr {
     valuebind(_, underlyingElement(this))
     or
     addressConstantExpression(this)
+    or
+    constantTemplateLiteral(this)
   }
 
   /**
@@ -1119,3 +1121,17 @@ private predicate isStandardPlacementNewAllocator(Function operatorNew) {
 
 // Pulled out for performance. See QL-796.
 private predicate hasNoConversions(Expr e) { not e.hasConversion() }
+
+/**
+ * Holds if `e` is a literal of unknown value in a template, or a cast thereof.
+ * We assume that such literals are constant.
+ */
+private predicate constantTemplateLiteral(Expr e) {
+  // Unknown literals in uninstantiated templates could be enum constant
+  // accesses or pointer-to-member literals.
+  e instanceof Literal and
+  e.isFromUninstantiatedTemplate(_) and
+  not exists(e.getValue())
+  or
+  constantTemplateLiteral(e.(Cast).getExpr())
+}

cpp/ql/test/library-tests/syntax-zoo/tellDifferent.expected

@@ -1,6 +1,3 @@
 | staticlocals__staticlocals_f2 | file://:0:0:0:0 | call to C | staticlocals.cpp:30:1:30:1 | return ... | Standard edge, only from QL |  |
 | staticlocals__staticlocals_f2 | file://:0:0:0:0 | initializer for c | file://:0:0:0:0 | call to C | Standard edge, only from QL |  |
 | staticlocals__staticlocals_f2 | staticlocals.cpp:29:5:29:17 | declaration | file://:0:0:0:0 | initializer for c | Standard edge, only from QL |  |
-| staticlocals__staticlocals_f3 | staticlocals.cpp:39:3:39:34 | declaration | staticlocals.cpp:39:18:39:33 | initializer for i | Standard edge, only from QL | uninstantiated |
-| staticlocals__staticlocals_f3 | staticlocals.cpp:39:18:39:33 | Unknown literal | staticlocals.cpp:40:1:40:1 | return ... | Standard edge, only from QL | uninstantiated |
-| staticlocals__staticlocals_f3 | staticlocals.cpp:39:18:39:33 | initializer for i | staticlocals.cpp:39:18:39:33 | Unknown literal | Standard edge, only from QL | uninstantiated |