preprocessor directives in a macro argument list is UB

[BobSteagall]

The use of preprocessor directives within the macro argument list in lines 206-211 of layout_stride.hpp is undefined behavior. Please consider applying the following patch, which wraps the entirety of the MDSPAN_FUNCTION_REQUIRES macro invocation inside the #if/#else clauses:

--- layout_stride.hpp-orig      2020-04-23 14:59:54.000000000 -0400
+++ layout_stride.hpp   2020-04-23 15:02:54.000000000 -0400
@@ -198,19 +198,27 @@

   // TODO @proposal-bug @proposal-extension layout stride needs this constructor
   // clang-format off
+#if defined(__INTEL_COMPILER)
   MDSPAN_FUNCTION_REQUIRES(
     (MDSPAN_INLINE_FUNCTION constexpr explicit),
     layout_stride_impl, (std::experimental::extents<Exts...> const& e), noexcept,
     /* requires */ (
       // remember that this also covers the zero strides case because an && fold on an empty pack is true
-#if defined(__INTEL_COMPILER)
       // Work-around for an ICE. layout_stride won't properly SFINAE with ICC, but oh well
       true
+    )
+  )
 #else
+  MDSPAN_FUNCTION_REQUIRES(
+    (MDSPAN_INLINE_FUNCTION constexpr explicit),
+    layout_stride_impl, (std::experimental::extents<Exts...> const& e), noexcept,
+    /* requires */ (
+      // remember that this also covers the zero strides case because an && fold on an empty pack is true
       _MDSPAN_FOLD_AND(Strides != dynamic_extent /* && ... */)
-#endif
     )
-  ) : __base_t(__base_t{__member_pair_t(e, __strides_storage_t())})
+  )
+#endif
+  : __base_t(__base_t{__member_pair_t(e, __strides_storage_t())})
   { }
   // clang-format on

@@ -323,4 +331,4 @@
 };

 } // end namespace experimental
-} // end namespace std
\ No newline at end of file
+} // end namespace std

[dhollman]

Yeah, I didn't realize this. This is now fixed by #21. Hope this helps!