diff --git a/docs/contributing.md b/docs/contributing.md
index dc6bb53715..f70b1e4577 100644
--- a/docs/contributing.md
+++ b/docs/contributing.md
@@ -239,6 +239,23 @@ there is no difference is wrong, QNX and VxWorks won't compile if you
 include the header as `<cfoo>` and call the function unqualified.
 
 
+#### User-Defined Literals (UDL) for Catch2' types
+
+Due to messy standardese and ... not great ... implementation of
+`-Wreserved-identifier` in Clang, avoid declaring UDLs as
+```cpp
+Approx operator "" _a(long double);
+```
+and instead declare them as
+```cpp
+Approx operator ""_a(long double);
+```
+
+Notice that the second version does not have a space between the `""` and
+the literal suffix.
+
+
+
 ### New source file template
 
 If you are adding new source file, there is a template you should use.
diff --git a/src/catch2/catch_approx.hpp b/src/catch2/catch_approx.hpp
index 7483349477..8f0e63270e 100644
--- a/src/catch2/catch_approx.hpp
+++ b/src/catch2/catch_approx.hpp
@@ -114,8 +114,8 @@ namespace Catch {
     };
 
 namespace literals {
-    Approx operator "" _a(long double val);
-    Approx operator "" _a(unsigned long long val);
+    Approx operator ""_a(long double val);
+    Approx operator ""_a(unsigned long long val);
 } // end namespace literals
 
 template<>
diff --git a/src/catch2/internal/catch_stringref.hpp b/src/catch2/internal/catch_stringref.hpp
index fed35b16d1..17f5d07dab 100644
--- a/src/catch2/internal/catch_stringref.hpp
+++ b/src/catch2/internal/catch_stringref.hpp
@@ -98,12 +98,12 @@ namespace Catch {
     };
 
 
-    constexpr auto operator "" _sr( char const* rawChars, std::size_t size ) noexcept -> StringRef {
+    constexpr auto operator ""_sr( char const* rawChars, std::size_t size ) noexcept -> StringRef {
         return StringRef( rawChars, size );
     }
 } // namespace Catch
 
-constexpr auto operator "" _catch_sr( char const* rawChars, std::size_t size ) noexcept -> Catch::StringRef {
+constexpr auto operator ""_catch_sr( char const* rawChars, std::size_t size ) noexcept -> Catch::StringRef {
     return Catch::StringRef( rawChars, size );
 }