declare toml::get_line(std::string_view, source_index) constexpr (#256)

Author: N-Dekker

include/toml++/impl/source_region.hpp

@@ -237,7 +237,7 @@ TOML_NAMESPACE_START
 	/// \returns	The specified line, excluding any possible trailing carriage return or line feed character.
 	/// \remarks Returns an empty string_view when there is no line at the specified line number, in the specified document.
 	TOML_NODISCARD
-	inline std::string_view get_line(std::string_view doc, source_index line_num) noexcept
+	constexpr std::string_view get_line(std::string_view doc, source_index line_num) noexcept
 	{
 		if (line_num == 0)
 		{

tests/user_feedback.cpp

@@ -454,7 +454,11 @@ b = []
 
 	SECTION("tomlplusplus/issues/254") // https://github.com/marzer/tomlplusplus/issues/254
 	{
-		for (const toml::source_index line_num: { 0u, 1u, 2u })
+		// Check constexpr support.
+		static_assert(toml::get_line(""sv, 1) == std::string_view{});
+		static_assert(toml::get_line("alpha = 1\nbeta = 2\n # last line # "sv, 1) == "alpha = 1"sv);
+
+		for (const toml::source_index line_num : { 0u, 1u, 2u })
 		{
 			CHECK(toml::get_line(""sv, line_num) == std::string_view{});
 		}

toml.hpp

@@ -2636,7 +2636,7 @@ TOML_NAMESPACE_START
 	};
 
 	TOML_NODISCARD
-	inline std::string_view get_line(std::string_view doc, source_index line_num) noexcept
+	constexpr std::string_view get_line(std::string_view doc, source_index line_num) noexcept
 	{
 		if (line_num == 0)
 		{