Mark bad_any_cast as [[nodiscard]] (default), control via any_CONFIG_NO_NODISCARD (nonstd-lite-project issue 74)

Add configuration option `any_CONFIG_NO_NODISCARD`.

nonstd-lite/nonstd-lite#74

Author: martinmoene

README.md

@@ -145,6 +145,11 @@ Define this to `any_ANY_STD` to select `std::any` as `nonstd::any`. Define this
 -D<b>any_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>any\_CONFIG\_NO\_NODISCARD</b>=0
+Define this to 1 if you want to compile without \[\[nodiscard\]\]. Note that the default of marking `class bad_any_cast` and function `any_cast()` with \[\[nodiscard\]\] is not part of the C++17 standard. The rationale to use \[\[nodiscard\]\] is that unnoticed discarded error values may break the error handling flow.
+
 ## Reported to work with
 
 The table below mentions the compiler versions *any lite* is reported to work with.

include/nonstd/any.hpp

@@ -44,6 +44,14 @@
 # define any_CONFIG_SELECT_ANY  ( any_HAVE_STD_ANY ? any_ANY_STD : any_ANY_NONSTD )
 #endif
 
+// Control marking class bad_any_cast and function any_cast with [[nodiscard]]]:
+
+#if !defined(any_CONFIG_NO_NODISCARD)
+# define any_CONFIG_NO_NODISCARD  0
+#else
+# define any_CONFIG_NO_NODISCARD  1
+#endif
+
 // Control presence of exception handling (try and auto discover):
 
 #ifndef any_CONFIG_NO_EXCEPTIONS
@@ -312,7 +320,7 @@ namespace nonstd {
 # define any_nullptr NULL
 #endif
 
-#if any_HAVE_NODISCARD
+#if any_HAVE_NODISCARD && !any_CONFIG_NO_NODISCARD
 # define any_nodiscard [[nodiscard]]
 #else
 # define any_nodiscard /*[[nodiscard]]*/
@@ -415,7 +423,7 @@ namespace detail {
 
 #if ! any_CONFIG_NO_EXCEPTIONS
 
-class bad_any_cast : public std::bad_cast
+class any_nodiscard bad_any_cast : public std::bad_cast
 {
 public:
 #if any_CPP11_OR_GREATER