From d426ef265a764359d16097c42d1357c19b01e218 Mon Sep 17 00:00:00 2001
From: Martin Moene <martin.moene@gmail.com>
Date: Tue, 10 Dec 2024 14:53:15 +0100
Subject: [PATCH] Mark expected as [[nodiscard]] (default), control via
 nsel_CONFIG_NO_NODISCARD (#74, thanks Quuxplusone)

---
 README.md                   |  5 +++++
 include/nonstd/expected.hpp | 21 ++++++++++++++++++---
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/README.md b/README.md
index e83392a..bf33eb8 100644
--- a/README.md
+++ b/README.md
@@ -129,6 +129,11 @@ Define this to 1 if you want to compile without exceptions. If not defined, the
 -D<b>nsel\_CONFIG\_NO\_EXCEPTIONS\_SEH</b>=0
 Define this to 1 or 0 to control the use of SEH when C++ exceptions are disabled (see above). If not defined, the header tries and detect if SEH is available if C++ exceptions have been disabled (e.g. via `-fno-exceptions` or `/kernel`). Default determined in header.
 
+#### Disable \[\[nodiscard\]\]
+
+-D<b>nsel\_CONFIG\_NO\_NODISCARD</b>=0
+Define this to 1 if you want to compile without \[\[nodiscard\]\]. Note that the default of marking `class expected` with \[\[nodiscard\]\] is not part of the C++23 standard. The rationale to use \[\[nodiscard\]\] is that unnoticed discarded expected error values may break the error handling flow.
+
 #### Enable compilation errors
 
 \-D<b>nsel\_CONFIG\_CONFIRMS\_COMPILATION\_ERRORS</b>=0  
diff --git a/include/nonstd/expected.hpp b/include/nonstd/expected.hpp
index a0385d0..54960a2 100644
--- a/include/nonstd/expected.hpp
+++ b/include/nonstd/expected.hpp
@@ -89,6 +89,14 @@
 # define nsel_CONFIG_WIN32_LEAN_AND_MEAN  0
 #endif
 
+// Control marking class expected with [[nodiscard]]]:
+
+#if !defined(nsel_CONFIG_NO_NODISCARD)
+# define nsel_CONFIG_NO_NODISCARD  0
+#else
+# define nsel_CONFIG_NO_NODISCARD  1
+#endif
+
 // Control presence of C++ exception handling (try and auto discover):
 
 #ifndef nsel_CONFIG_NO_EXCEPTIONS
@@ -429,6 +437,7 @@ nsel_DISABLE_MSVC_WARNINGS( 26409 )
 // Presence of C++17 language features:
 
 #define nsel_HAVE_DEPRECATED  nsel_CPP17_000
+#define nsel_HAVE_NODISCARD   nsel_CPP17_000
 
 // C++ feature usage:
 
@@ -438,6 +447,12 @@ nsel_DISABLE_MSVC_WARNINGS( 26409 )
 # define nsel_deprecated(msg) /*[[deprecated]]*/
 #endif
 
+#if nsel_HAVE_NODISCARD && !nsel_CONFIG_NO_NODISCARD
+# define nsel_NODISCARD  [[nodiscard]]
+#else
+# define nsel_NODISCARD  /*[[nodiscard]]*/
+#endif
+
 //
 // expected:
 //
@@ -459,7 +474,7 @@ namespace std11 {
     }
 
     template< class T >
-    const T * addressof( const T && ) = delete;    
+    const T * addressof( const T && ) = delete;
 #endif
 } // namespace std11
 
@@ -1824,10 +1839,10 @@ namespace expected_lite {
 
 #if nsel_P0323R <= 2
 template< typename T, typename E = std::exception_ptr >
-class expected
+class nsel_NODISCARD expected
 #else
 template< typename T, typename E >
-class expected
+class nsel_NODISCARD expected
 #endif // nsel_P0323R
 {
 private: