Rewrite switch statements to avoid compiler warnings.

We have a problem with switch statements: Some compiler versions warn
when a switch doesn't handle all options, so we need them to be
complete. But if they are complete, there is never a case where the
code falls out of the switch, so the return at the end of the function
is never called (which some compilers don't like), but if there is no
return at the end of the function other compiler versions complain. The
only way to handle this is to pick one of the cases, make it into a
default case that drops out of the switch and then do the return
outside the switch.

include/osmium/io/detail/string_util.hpp

@@ -181,7 +181,7 @@ namespace osmium {
 
             inline void append_xml_encoded_string(std::string& out, const char* data) {
                 for (; *data != '\0'; ++data) {
-                    switch(*data) {
+                    switch (*data) {
                         case '&':  out += "&";  break;
                         case '\"': out += """; break;
                         case '\'': out += "'"; break;

include/osmium/io/file_compression.hpp

@@ -45,20 +45,17 @@ namespace osmium {
             bzip2 = 2
         };
 
-// avoid g++ false positive
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wreturn-type"
         inline const char* as_string(file_compression compression) {
             switch (compression) {
-                case file_compression::none:
-                    return "none";
                 case file_compression::gzip:
                     return "gzip";
                 case file_compression::bzip2:
                     return "bzip2";
+                default: // file_compression::none:
+                    break;
             }
+            return "none";
         }
-#pragma GCC diagnostic pop
 
         template <typename TChar, typename TTraits>
         inline std::basic_ostream<TChar, TTraits>& operator<<(std::basic_ostream<TChar, TTraits>& out, const file_compression compression) {

include/osmium/io/file_format.hpp

@@ -54,13 +54,8 @@ namespace osmium {
             yes = 1
         };
 
-// avoid g++ false positive
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wreturn-type"
         inline const char* as_string(file_format format) {
             switch (format) {
-                case file_format::unknown:
-                    return "unknown";
                 case file_format::xml:
                     return "XML";
                 case file_format::pbf:
@@ -73,9 +68,11 @@ namespace osmium {
                     return "O5M";
                 case file_format::debug:
                     return "DEBUG";
+                default: // file_format::unknown
+                    break;
             }
+            return "unknown";
         }
-#pragma GCC diagnostic pop
 
         template <typename TChar, typename TTraits>
         inline std::basic_ostream<TChar, TTraits>& operator<<(std::basic_ostream<TChar, TTraits>& out, const file_format format) {

include/osmium/osm/item_type.hpp

@@ -87,8 +87,6 @@ namespace osmium {
 
     inline item_type char_to_item_type(const char c) noexcept {
         switch (c) {
-            case 'X':
-                return item_type::undefined;
             case 'n':
                 return item_type::node;
             case 'w':
@@ -113,18 +111,14 @@ namespace osmium {
                 return item_type::inner_ring;
             case 'D':
                 return item_type::changeset_discussion;
-            default:
-                return item_type::undefined;
+            default: // 'X'
+                break;
         }
+        return item_type::undefined;
     }
 
-// avoid g++ false positive
-#pragma GCC diagnostic push
-#pragma GCC diagnostic ignored "-Wreturn-type"
     inline char item_type_to_char(const item_type type) noexcept {
         switch (type) {
-            case item_type::undefined:
-                return 'X';
             case item_type::node:
                 return 'n';
             case item_type::way:
@@ -149,13 +143,14 @@ namespace osmium {
                 return 'I';
             case item_type::changeset_discussion:
                 return 'D';
+            default: // item_type::undefined
+                break;
         }
+        return 'X';
     }
 
     inline const char* item_type_to_name(const item_type type) noexcept {
         switch (type) {
-            case item_type::undefined:
-                return "undefined";
             case item_type::node:
                 return "node";
             case item_type::way:
@@ -180,9 +175,11 @@ namespace osmium {
                 return "inner_ring";
             case item_type::changeset_discussion:
                 return "changeset_discussion";
+            default: // item_type::undefined
+                break;
         }
+        return "undefined";
     }
-#pragma GCC diagnostic pop
 
     template <typename TChar, typename TTraits>
     inline std::basic_ostream<TChar, TTraits>& operator<<(std::basic_ostream<TChar, TTraits>& out, const item_type item_type) {

include/osmium/util/memory_mapping.hpp

@@ -644,9 +644,10 @@ inline DWORD osmium::util::MemoryMapping::get_protection() const noexcept {
             return PAGE_READONLY;
         case mapping_mode::write_private:
             return PAGE_WRITECOPY;
-        case mapping_mode::write_shared:
-            return PAGE_READWRITE;
+        default: // mapping_mode::write_shared
+            break;
     }
+    return PAGE_READWRITE;
 }
 
 inline DWORD osmium::util::MemoryMapping::get_flags() const noexcept {
@@ -655,9 +656,10 @@ inline DWORD osmium::util::MemoryMapping::get_flags() const noexcept {
             return FILE_MAP_READ;
         case mapping_mode::write_private:
             return FILE_MAP_COPY;
-        case mapping_mode::write_shared:
-            return FILE_MAP_WRITE;
+        default: // mapping_mode::write_shared
+            break;
     }
+    return FILE_MAP_WRITE;
 }
 
 inline HANDLE osmium::util::MemoryMapping::get_handle() const noexcept {