Add __WXFUNCTION_SIG__ macro

If compiling as C++20 or using GCC or MSVC, expands to function
signature. Falls back to the function name (__func__) otherwise.

Closes #24554.

include/wx/cpp.h

@@ -114,6 +114,24 @@
     #define __WXFUNCTION__ __func__
 #endif /* __WXFUNCTION__ already defined */
 
+/*
+    Expands to the current function's full signature, if available.
+
+    Falls back to the function name (i.e., __func__) if not available.
+ */
+#ifndef __WXFUNCTION_SIG__
+    #if __cplusplus >= 202002L
+        #include <source_location>
+        // actually displays the signature, not just the name
+        #define __WXFUNCTION_SIG__ std::source_location::current().function_name()
+    #elif defined(__VISUALC__)
+        #define __WXFUNCTION_SIG__ __FUNCSIG__
+    #elif defined(__GNUG__)
+        #define __WXFUNCTION_SIG__ __PRETTY_FUNCTION__
+    #else
+        #define __WXFUNCTION_SIG__ __func__
+    #endif
+#endif /* __WXFUNCTION_SIG__ already defined */
 
 /*
    wxCALL_FOR_EACH(what, ...) calls the macro from its first argument, what(pos, x),

interface/wx/cpp.h

@@ -52,9 +52,32 @@
 
     Please use the standard macro instead.
 
+    @see __WXFUNCTION_SIG__
+
     @header{wx/cpp.h}
 */
 #define __WXFUNCTION__ __func__
 
+/**
+    Expands to the current function's full signature, if available.
+
+    Falls back to the function name (i.e., @c \__func__) if not available.
+
+    As an example, if you have a class named `Calculator` with a
+    `double Add(double, double) const` member function,
+    `__WXFUNCTION_SIG__` may expand to the following:
+
+    `double Calculator::Add(double, double) const`
+
+    While `__func__` will simply expand to:
+
+    `Add`
+
+    @header{wx/cpp.h}
+
+    @since 3.3.0
+*/
+#define __WXFUNCTION_SIG__
+
 ///@}