Mark bad_variant_access as [[nodiscard]] (default), control via varia…

…nt_CONFIG_NO_NODISCARD (nonstd-lite-project issue 74)

Add configuration option `variant_CONFIG_NO_NODISCARD`.

martinmoene/nonstd-lite-project#74

README.md

@@ -212,14 +212,19 @@ Define this macro to override the auto-detection of the supported C++ standard,
 
 At default, *variant lite* uses `std::variant` if it is available and lets you use it via namespace `nonstd`. You can however override this default and explicitly request to use `std::variant` or variant lite's `nonstd::variant` as `nonstd::variant` via the following macros.
 
-\-D<b>variant\_CONFIG\_SELECT\_VARIANT</b>=variant_VARIANT_DEFAULT  
+-D<b>variant\_CONFIG\_SELECT\_VARIANT</b>=variant_VARIANT_DEFAULT  
 Define this to `variant_VARIANT_STD` to select `std::variant` as `nonstd::variant`. Define this to `variant_VARIANT_NONSTD` to select `nonstd::variant` as `nonstd::variant`. Default is undefined, which has the same effect as defining to `variant_VARIANT_DEFAULT`.
 
 #### Disable exceptions
 
 -D<b>variant_CONFIG_NO_EXCEPTIONS</b>=0  
 Define this to 1 if you want to compile without exceptions. If not defined, the header tries and detect if exceptions have been disabled (e.g. via `-fno-exceptions`). Default is undefined.
 
+#### Disable \[\[nodiscard\]\]
+
+-D<b>variant\_CONFIG\_NO\_NODISCARD</b>=0
+Define this to 1 if you want to compile without \[\[nodiscard\]\]. Note that the default of marking `class bad_variant_access` with \[\[nodiscard\]\] is not part of the C++17 standard. The rationale to use \[\[nodiscard\]\] is that unnoticed discarded access error values may break the error handling flow.
+
 #### Presence of `variant_size_V()` simulation macro
 
 \-D<b>variant\_CONFIG\_OMIT\_VARIANT\_SIZE\_V\_MACRO</b>=0  

include/nonstd/variant.hpp

@@ -47,6 +47,14 @@
 # define variant_CONFIG_OMIT_VARIANT_ALTERNATIVE_T_MACRO  0
 #endif
 
+// Control marking class bad_variant_access with [[nodiscard]]]:
+
+#if !defined(variant_CONFIG_NO_NODISCARD)
+# define variant_CONFIG_NO_NODISCARD  0
+#else
+# define variant_CONFIG_NO_NODISCARD  1
+#endif
+
 // Control presence of exception handling (try and auto discover):
 
 #ifndef variant_CONFIG_NO_EXCEPTIONS
@@ -364,7 +372,7 @@ namespace nonstd {
 
 // Presence of C++17 language features:
 
-// no flag
+#define variant_HAVE_NODISCARD          variant_CPP17_000
 
 // Presence of C++ library features:
 
@@ -404,6 +412,12 @@ namespace nonstd {
 # define variant_nullptr NULL
 #endif
 
+#if variant_HAVE_NODISCARD && !variant_CONFIG_NO_NODISCARD
+# define variant_nodiscard [[nodiscard]]
+#else
+# define variant_nodiscard /*[[nodiscard]]*/
+#endif
+
 #if variant_HAVE_OVERRIDE
 # define variant_override override
 #else
@@ -1266,7 +1280,7 @@ static const std::size_t variant_npos = static_cast<std::size_t>( -1 );
 
 // 19.7.11 Class bad_variant_access
 
-class bad_variant_access : public std::exception
+class variant_nodiscard bad_variant_access : public std::exception
 {
 public:
 #if variant_CPP11_OR_GREATER