XPath: Remove the use of fallthrough switch cases

We were previously relying on non-standard comment detection that is
supported by gcc/clang to avoid warnings about implicit fallthrough.

This can be solved using attributes but using them requires a lot of
compiler-specific detection logic because not all versions of gcc/clang
support them.

We don't *really* need to rely on fallthrough here - the type conversion
block can be located *after* the AST type switch instead, which means
that any AST type that has type ambiguity can fall back to that in the
future.

Fixes #331.

src/pugixml.cpp

@@ -10415,12 +10415,15 @@ PUGI__NS_BEGIN
 				if (_rettype == xpath_type_boolean)
 					return _data.variable->get_boolean();
 
-				// variable needs to be converted to the correct type, this is handled by the default statement below
+				// variable needs to be converted to the correct type, this is handled by the fallthrough block below
+				break;
 			}
 
-			// fallthrough
 			default:
-			{
+				;
+			}
+
+			// none of the ast types that return the value directly matched, we need to perform type conversion
 			switch (_rettype)
 			{
 			case xpath_type_number:
@@ -10445,8 +10448,6 @@ PUGI__NS_BEGIN
 				return false;
 			}
 		}
-			}
-		}
 
 		double eval_number(const xpath_context& c, const xpath_stack& stack)
 		{
@@ -10552,12 +10553,15 @@ PUGI__NS_BEGIN
 				if (_rettype == xpath_type_number)
 					return _data.variable->get_number();
 
-				// variable needs to be converted to the correct type, this is handled by the default statement below
+				// variable needs to be converted to the correct type, this is handled by the fallthrough block below
+				break;
 			}
 
-			// fallthrough
 			default:
-			{
+				;
+			}
+
+			// none of the ast types that return the value directly matched, we need to perform type conversion
 			switch (_rettype)
 			{
 			case xpath_type_boolean:
@@ -10581,9 +10585,6 @@ PUGI__NS_BEGIN
 				assert(false && "Wrong expression for return type number"); // unreachable
 				return 0;
 			}
-
-			}
-			}
 		}
 
 		xpath_string eval_string_concat(const xpath_context& c, const xpath_stack& stack)
@@ -10838,12 +10839,15 @@ PUGI__NS_BEGIN
 				if (_rettype == xpath_type_string)
 					return xpath_string::from_const(_data.variable->get_string());
 
-				// variable needs to be converted to the correct type, this is handled by the default statement below
+				// variable needs to be converted to the correct type, this is handled by the fallthrough block below
+				break;
 			}
 
-			// fallthrough
 			default:
-			{
+				;
+			}
+
+			// none of the ast types that return the value directly matched, we need to perform type conversion
 			switch (_rettype)
 			{
 			case xpath_type_boolean:
@@ -10867,8 +10871,6 @@ PUGI__NS_BEGIN
 				return xpath_string();
 			}
 		}
-			}
-		}
 
 		xpath_node_set_raw eval_node_set(const xpath_context& c, const xpath_stack& stack, nodeset_eval_t eval)
 		{
@@ -10989,15 +10991,18 @@ PUGI__NS_BEGIN
 					return ns;
 				}
 
-				// variable needs to be converted to the correct type, this is handled by the default statement below
+				// variable needs to be converted to the correct type, this is handled by the fallthrough block below
+				break;
 			}
 
-			// fallthrough
 			default:
+				;
+			}
+
+			// none of the ast types that return the value directly matched, but conversions to node set are invalid
 			assert(false && "Wrong expression for return type node set"); // unreachable
 			return xpath_node_set_raw();
 		}
-		}
 
 		void optimize(xpath_allocator* alloc)
 		{