c++: Add testcases from some Issaquah DRs

The following patch adds testcases for 5 DRs.  In the DR2475, DR2530 and
CWG2691 my understanding is we already implement the desired behavior,
in DR2478 partially (I've added 2 dg-bogus there, I think we inherit
rather than overwrite DECL_DECLARED_CONSTINIT_P for explicit specialization
somewhere, still far better than clang++) and DR2673 on the other side the
DR was to codify the clang++ behavior rather than GCC.

Not 100% sure if it is better to commit the 2 with dg-bogus or just wait
until the actual fixes are implemented.  BTW, I've noticed
register_specialization does:
              FOR_EACH_CLONE (clone, fn)
                {
                  DECL_DECLARED_INLINE_P (clone)
                    = DECL_DECLARED_INLINE_P (fn);
                  DECL_SOURCE_LOCATION (clone)
                    = DECL_SOURCE_LOCATION (fn);
                  DECL_DELETED_FN (clone)
                    = DECL_DELETED_FN (fn);
                }
but not e.g. constexpr/consteval, have tried to cover that in a testcase
but haven't managed to do so.

2023-02-15  Jakub Jelinek  <jakub@redhat.com>

	* g++.dg/DRs/dr2475.C: New test.
	* g++.dg/DRs/dr2478.C: New test.
	* g++.dg/DRs/dr2530.C: New test.
	* g++.dg/DRs/dr2673.C: New test.
	* c-c++-common/cpp/delimited-escape-seq-8.c: New test.

gcc/testsuite/c-c++-common/cpp/delimited-escape-seq-8.c

@@ -0,0 +1,20 @@
+// CWG 2691 - hexadecimal-escape-sequence is too greedy
+/* { dg-do run } */
+/* { dg-require-effective-target wchar } */
+/* { dg-options "-std=gnu99 -Wno-c++-compat" { target c } } */
+/* { dg-options "-std=c++23" { target c++ } } */
+
+#ifndef __cplusplus
+#include <wchar.h>
+typedef __CHAR16_TYPE__ char16_t;
+typedef __CHAR32_TYPE__ char32_t;
+#endif
+
+const char32_t *a = U"\x{20}ab";
+
+int
+main ()
+{
+  if (a[0] != U'\x20' || a[1] != U'a' || a[2] != U'b' || a[3] != U'\0')
+    __builtin_abort ();
+}

gcc/testsuite/g++.dg/DRs/dr2475.C

@@ -0,0 +1,6 @@
+// DR 2475 - Object declarations of type cv void
+// { dg-do compile }
+
+int f(), x;
+extern void g(),
+  y;                   // { dg-error "variable or field 'y' declared void" }

gcc/testsuite/g++.dg/DRs/dr2478.C

@@ -0,0 +1,74 @@
+// DR 2478 - Properties of explicit specializations of implicitly-instantiated class templates
+// { dg-do compile { target c++20 } }
+
+template <typename T>
+struct S {
+  int foo () { return 0; }
+  constexpr int bar () { return 0; }
+  int baz () { return 0; }
+  consteval int qux () { return 0; }
+  constexpr S () {}
+  static constinit T x;
+  static T y;
+};
+
+template <typename T>
+T S<T>::x = S<T> ().foo ();	// { dg-error "'constinit' variable 'S<char>::x' does not have a constant initializer" }
+				// { dg-error "call to non-'constexpr' function" "" { target *-*-* } .-1 }
+
+template <typename T>
+T S<T>::y = S<T> ().foo ();
+
+template <>
+constexpr int
+S<int>::foo ()
+{
+  return 0;
+}
+
+template <>
+int
+S<int>::bar ()
+{
+  return 0;
+}
+
+template <>
+consteval int
+S<char>::baz ()
+{
+  return 0;
+}
+
+template <>
+int
+S<char>::qux ()
+{
+  return 0;
+}
+
+template <>
+long S<long>::x = S<long> ().foo ();	// { dg-bogus "'constinit' variable 'S<long int>::x' does not have a constant initializer" "" { xfail *-*-* } }
+					// { dg-bogus "call to non-'constexpr' function" "" { xfail *-*-* } .-1 }
+
+template <>
+constinit long S<long>::y = S<long> ().foo ();	// { dg-error "'constinit' variable 'S<long int>::y' does not have a constant initializer" }
+						// { dg-error "call to non-'constexpr' function" "" { target *-*-* } .-1 }
+
+constinit auto a = S<char> ().foo ();	// { dg-error "'constinit' variable 'a' does not have a constant initializer" }
+					// { dg-error "call to non-'constexpr' function" "" { target *-*-* } .-1 }
+constinit auto b = S<char> ().bar ();
+constinit auto c = S<int> ().foo ();
+constinit auto d = S<int> ().bar ();	// { dg-error "'constinit' variable 'd' does not have a constant initializer" }
+					// { dg-error "call to non-'constexpr' function" "" { target *-*-* } .-1 }
+constinit auto e = S<char> ().baz ();
+constinit auto f = S<char> ().qux ();	// { dg-error "'constinit' variable 'f' does not have a constant initializer" }
+					// { dg-error "call to non-'constexpr' function" "" { target *-*-* } .-1 }
+constinit auto g = S<int> ().baz ();	// { dg-error "'constinit' variable 'g' does not have a constant initializer" }
+					// { dg-error "call to non-'constexpr' function" "" { target *-*-* } .-1 }
+constinit auto h = S<int> ().qux ();
+auto i = S<char>::x;
+auto j = S<int>::x;
+auto k = S<long>::x;
+auto l = S<char>::y;
+auto m = S<int>::y;

gcc/testsuite/g++.dg/DRs/dr2530.C

@@ -0,0 +1,5 @@
+// DR 2530 - Multiple definitions of enumerators
+// { dg-do compile }
+
+enum E { e, e };		// { dg-error "redefinition of 'e'" }
+enum F { f = 0, f = 0 };	// { dg-error "redefinition of 'f'" }

gcc/testsuite/g++.dg/DRs/dr2673.C

@@ -0,0 +1,24 @@
+// DR 2673 - User-declared spaceship vs. built-in operators
+// { dg-do compile { target c++20 } }
+
+#include <compare>
+
+enum class E : int { E1, E2 };
+enum class F : int { F1, F2 };
+
+constexpr auto
+operator<=> (E lhs, E rhs)
+{
+  return (int) rhs <=> (int) lhs;
+}
+
+constexpr bool
+operator== (F lhs, F rhs)
+{
+  return (int) lhs != (int) rhs;
+}
+
+static_assert ((E::E1 <=> E::E2) == (1 <=> 0));
+static_assert (E::E1 > E::E2);		// { dg-bogus "static assertion failed" "" { xfail *-*-* } }
+static_assert (F::F1 == F::F2);
+static_assert (!(F::F1 != F::F2));	// { dg-bogus "static assertion failed" "" { xfail *-*-* } }