Factor out readable function signatures to avoid duplication

include/pybind11/pybind11.h

@@ -248,6 +248,64 @@ inline std::string generate_type_signature() {
 #    define PYBIND11_COMPAT_STRDUP strdup
 #endif
 
+#define PYBIND11_READABLE_FUNCTION_SIGNATURE_EXPR                                                 \
+    detail::const_name("(") + cast_in::arg_names + detail::const_name(") -> ") + cast_out::name
+
+// We factor out readable function signatures to a specific template
+// so that they don't get duplicated across different instantiations of
+// cpp_function::initialize (which is templated on more types).
+template <typename cast_in, typename cast_out>
+class ReadableFunctionSignature {
+public:
+    using sig_type = decltype(PYBIND11_READABLE_FUNCTION_SIGNATURE_EXPR);
+
+private:
+    // We have to repeat PYBIND11_READABLE_FUNCTION_SIGNATURE_EXPR in decltype()
+    // because C++11 doesn't allow functions to return `auto`. (We don't
+    // know the type because it's some variant of detail::descr<N> with
+    // unknown N.)
+    static constexpr sig_type sig() { return PYBIND11_READABLE_FUNCTION_SIGNATURE_EXPR; }
+
+public:
+    static constexpr sig_type kSig = sig();
+    // We can only stash the result of detail::descr::types() in a
+    // constexpr variable if we aren't on MSVC (see
+    // PYBIND11_DESCR_CONSTEXPR).
+#if !defined(_MSC_VER)
+    using types_type = decltype(sig_type::types());
+    static constexpr types_type kTypes = sig_type::types();
+#endif
+};
+#undef PYBIND11_READABLE_FUNCTION_SIGNATURE_EXPR
+
+// Prior to C++17, we don't have inline variables, so we have to
+// provide an out-of-line definition of the class member. We should
+// remove the following chunk of code (until the PYBIND11_WARNING_POP)
+// if/when we drop support for C++11 and C++14.
+PYBIND11_WARNING_PUSH
+#if defined(PYBIND11_CPP17)
+#    if defined(__clang_major__)                                                                  \
+        && (__clang_major__ >= 17 || (defined(__APPLE__) && __clang_major__ >= 15))
+// Even with the above gating, there's one straggler CI job that
+// claims it doesn't know what
+// -Wdeprecated-redundant-constexpr-static-def is despite being on
+// Apple Clang 15. Just suppress -Wunknown-warning-option.
+PYBIND11_WARNING_DISABLE_CLANG("-Wunknown-warning-option")
+PYBIND11_WARNING_DISABLE_CLANG("-Wdeprecated-redundant-constexpr-static-def")
+#    endif
+PYBIND11_WARNING_DISABLE_CLANG("-Wdeprecated")
+PYBIND11_WARNING_DISABLE_GCC("-Wdeprecated")
+#endif
+template <typename cast_in, typename cast_out>
+constexpr typename ReadableFunctionSignature<cast_in, cast_out>::sig_type
+    ReadableFunctionSignature<cast_in, cast_out>::kSig;
+#if !defined(_MSC_VER)
+template <typename cast_in, typename cast_out>
+constexpr typename ReadableFunctionSignature<cast_in, cast_out>::types_type
+    ReadableFunctionSignature<cast_in, cast_out>::kTypes;
+#endif
+PYBIND11_WARNING_POP
+
 PYBIND11_NAMESPACE_END(detail)
 
 /// Wraps an arbitrary C++ function/method/lambda function/.. into a callable Python object
@@ -481,9 +539,14 @@ class cpp_function : public function {
 
         /* Generate a readable signature describing the function's arguments and return
            value types */
-        static constexpr auto signature
-            = const_name("(") + cast_in::arg_names + const_name(") -> ") + cast_out::name;
-        PYBIND11_DESCR_CONSTEXPR auto types = decltype(signature)::types();
+        static constexpr const auto &signature
+            = detail::ReadableFunctionSignature<cast_in, cast_out>::kSig;
+#if !defined(_MSC_VER)
+        static constexpr const auto &types
+            = detail::ReadableFunctionSignature<cast_in, cast_out>::kTypes;
+#else
+        PYBIND11_DESCR_CONSTEXPR auto types = std::decay<decltype(signature)>::type::types();
+#endif
 
         /* Register the function with Python from generic (non-templated) code */
         // Pass on the ownership over the `unique_rec` to `initialize_generic`. `rec` stays valid.