Handle non-zero length characters

Author: lcartey

c/misra/src/codingstandards/c/misra/EssentialTypes.qll

@@ -393,9 +393,11 @@ class EssentialEnumConstantAccess extends EssentialExpr, EnumConstantAccess {
 class EssentialLiteral extends EssentialExpr, Literal {
   override Type getEssentialType() {
     if this instanceof BooleanLiteral
-    then result instanceof MisraBoolType
+    then
+      // This returns a multitude of types - not sure if we really want that
+      result instanceof MisraBoolType
     else (
-      if this.(CharLiteral).getCharacter().length() = 1
+      if this instanceof CharLiteral
       then result instanceof PlainCharType
       else
         exists(Type underlyingStandardType |

c/misra/test/c/misra/EssentialTypes.expected

@@ -87,3 +87,6 @@
 | test.c:73:3:73:5 | EC4 | (unnamed enum) | (unnamed enum) | essentially Enum Type |
 | test.c:74:3:74:5 | EC5 | (unnamed enum) | (unnamed enum) | essentially Enum Type |
 | test.c:75:3:75:5 | EC6 | (unnamed enum) | (unnamed enum) | essentially Enum Type |
+| test.c:79:3:79:5 | 97 | char | char | essentially Character type |
+| test.c:80:3:80:6 | 10 | char | char | essentially Character type |
+| test.c:81:3:81:6 | 0 | char | char | essentially Character type |

c/misra/test/c/misra/test.c

@@ -73,4 +73,10 @@ void testEnums() {
   EC4; // Should be essentially enum
   EC5; // Should be essentially enum
   EC6; // Should be essentially enum
+}
+
+void testControlChar() {
+  'a';  // Essentially char
+  '\n'; // Essentially char
+  '\0'; // Essentially char
 }