Add configuration macro nssv_CONFIG_CONSTEXPR11_STD_SEARCH (#50, thanks @oliverlee)

Allow to avoid constexpr with std::search() and select a local implementation for it for C++14 constexpr.

Author: martinmoene

README.md

@@ -201,7 +201,16 @@ Define this to 1 to provide `std::string`– `nonstd::string_view` interoper
 At default, *string-view lite* provides operations to overload the `operator<<`. If you want to use the library without the use of standard streams, you can control this with the following macro:
 
 -D<b>nssv_CONFIG_NO_STREAM_INSERTION</b>=1  
-Define this to 1 to omit the use of standard streams. Default is undefined
+Define this to 1 to omit the use of standard streams. Default is undefined.
+
+### Avoid `constexpr` with `std::search()`
+
+At default, *string-view lite* may use constexpr with `std::search()` for `string_view::find()` when compiling for C++11 or C++14, whereas `std::search()` is only constexpr since C++20. This may occur when macro `__OPTIMIZE__` is not defined and a non-recursive implementation for search is selected. Macro `__OPTIMIZE__` is used with GCC and clang.
+
+If you encounter an error related to this, you can control this behaviour with the following macro:
+
+-D<b>nssv_CONFIG_CONSTEXPR11_STD_SEARCH</b>=0  
+Define this to 0 to omit the use constexpr with `std::search()` and substitute a local implementation using `nssv_constexpr14`. Default is 1.
 
 ### Enable compilation errors
 

include/nonstd/string_view.hpp

@@ -69,6 +69,10 @@
 # define nssv_CONFIG_NO_STREAM_INSERTION  0
 #endif
 
+#ifndef  nssv_CONFIG_CONSTEXPR11_STD_SEARCH
+# define nssv_CONFIG_CONSTEXPR11_STD_SEARCH  1
+#endif
+
 // Control presence of exception handling (try and auto discover):
 
 #ifndef nssv_CONFIG_NO_EXCEPTIONS
@@ -567,12 +571,31 @@ constexpr const CharT* search( basic_string_view<CharT, Traits> haystack, basic_
 
 // non-recursive:
 
+#if nssv_CONFIG_CONSTEXPR11_STD_SEARCH
+
 template< class CharT, class Traits = std::char_traits<CharT> >
 constexpr const CharT* search( basic_string_view<CharT, Traits> haystack, basic_string_view<CharT, Traits> needle )
 {
     return std::search( haystack.begin(), haystack.end(), needle.begin(), needle.end() );
 }
 
+#else // nssv_CONFIG_CONSTEXPR11_STD_SEARCH
+
+template< class CharT, class Traits = std::char_traits<CharT> >
+nssv_constexpr14 const CharT* search( basic_string_view<CharT, Traits> haystack, basic_string_view<CharT, Traits> needle )
+{
+    while ( needle.size() <= haystack.size() )
+    {
+        if  ( haystack.starts_with(needle) )
+        {
+            return haystack.cbegin();
+        }
+        haystack = basic_string_view<CharT, Traits>{ haystack.begin() + 1, haystack.size() - 1U };
+    }
+    return haystack.cend();
+}
+#endif // nssv_CONFIG_CONSTEXPR11_STD_SEARCH
+
 #endif // OPTIMIZE
 #endif // nssv_CPP11_OR_GREATER && ! nssv_CPP17_OR_GREATER