Fix C++20 compatibility in testcases

Examples/test-suite/cpp11_raw_string_literals.i

@@ -1,7 +1,7 @@
 /* This module tests whether SWIG correctly parses:
-   -    ordinary strings (char_t)
+   -    ordinary strings (char)
    - L  wide strings (wchar_t)
-   - u8 unicode8 strings (char_t)
+   - u8 unicode8 strings (char / char8_t since C++20)
    - u  unicode16 strings (char16_t)
    - U  unicode32 strings (char32_t)
 
@@ -49,7 +49,8 @@ struct URStruct {
 
 // New string literals
 wstring         aa =  L"Wide string";
-const char     *bb = u8"UTF-8 string";
+// u8"" is const char8_t[N] in C++20; const char[N] from C++11 until then.
+const char     *bb = reinterpret_cast<const char*>(u8"UTF-8 string");
 const char16_t *cc =  u"UTF-16 string";
 const char32_t *dd =  U"UTF-32 string";
 // New char literals
@@ -62,7 +63,7 @@ char32_t char32_t_char = U'b';
 const char      *xx =        ")I'm an \"ascii\" \\ string.";
 const char      *ee =   R"XXX()I'm an "ascii" \ string.)XXX";
 wstring          ff =  LR"XXX(I'm a "raw wide" \ string.)XXX";
-const char      *gg = u8R"XXX(I'm a "raw UTF-8" \ string.)XXX";
+const char      *gg = reinterpret_cast<const char*>(u8R"XXX(I'm a "raw UTF-8" \ string.)XXX");
 const char16_t  *hh =  uR"XXX(I'm a "raw UTF-16" \ string.)XXX";
 const char32_t  *ii =  UR"XXX(I'm a "raw UTF-32" \ string.)XXX";
 %}

Examples/test-suite/cpp11_userdefined_literals.i

@@ -30,6 +30,9 @@ OutputType operator "" _mySuffixFloat(long double) { return OutputType(30); }
 
 // Cooked string literals
 OutputType operator "" _mySuffix1(const char * string_values, size_t num_chars) { return OutputType(100); }
+#ifdef __cpp_lib_char8_t // For C++20
+OutputType operator "" _mySuffix1(const char8_t * string_values, size_t num_chars) { return OutputType(100); }
+#endif
 OutputType operator "" _mySuffix2(const wchar_t * string_values, size_t num_chars) { return OutputType(200); }
 OutputType operator "" _mySuffix3(const char16_t * string_values, size_t num_chars) { return OutputType(300); }
 OutputType operator "" _mySuffix4(const char32_t * string_values, size_t num_chars) { return OutputType(400); }

Examples/test-suite/cpp17_u8_char_literals.i

@@ -8,7 +8,8 @@
 #endif
 %}
 
-// UTF-8 character literals will (apparently) have type char8_t in C++20.
+// UTF-8 character literals are type `char` in C++17 but `char8_t` in C++20,
+// but the latter can be assigned to `char`.
 char a = u8'a';
 char u = u8'u';
 char u8 = u8'8';

Examples/test-suite/template_construct.i

@@ -7,7 +7,13 @@ template<class T>
 class Foo {
     T y;
 public:
+#ifdef SWIG
     Foo<T>(T x) : y(x) { }
+#else
+    // Modern compilers reject this, so feed the compiler the corrected
+    // version.
+    Foo(T x) : y(x) { }
+#endif
 };
 
 %}

Examples/test-suite/valuewrapper_opaque.i

@@ -14,7 +14,7 @@ class C;
 %{
 template<typename T> class TemplateClass {
 public:
-TemplateClass<T>(T a) {}
+TemplateClass(T a) {}
 };
 
 struct B