c++: Implement CWG2635 - Constrained structured bindings

The following patch implements CWG2635.

2022-11-18  Jakub Jelinek  <jakub@redhat.com>

	* decl.cc (grokdeclarator): Implement
	CWG2635 - Constrained structured bindings.  Emit a pedwarn on
	constrained auto type.  Add auto_diagnostic_group for error_at
	and inform for non-auto type on structured bindings declaration.

	* g++.dg/cpp2a/decomp5.C: New test.
	* g++.dg/cpp2a/decomp6.C: New test.
	* g++.dg/cpp2a/decomp7.C: New test.
	* g++.dg/cpp2a/concepts-placeholder7.C: Adjust expected diagnostics.
	* g++.dg/cpp2a/concepts-placeholder8.C: Likewise.
	* g++.dg/cpp2a/concepts-placeholder9.C: New test.
	* g++.dg/cpp2a/concepts-placeholder10.C: New test.

gcc/cp/decl.cc

@@ -12664,6 +12664,7 @@ grokdeclarator (const cp_declarator *declarator,
 	{
 	  if (type != error_mark_node)
 	    {
+	      auto_diagnostic_group d;
 	      error_at (loc, "structured binding declaration cannot have "
 			"type %qT", type);
 	      inform (loc,
@@ -12673,6 +12674,10 @@ grokdeclarator (const cp_declarator *declarator,
 	  type = build_qualified_type (make_auto (), type_quals);
 	  declspecs->type = type;
 	}
+      else if (PLACEHOLDER_TYPE_CONSTRAINTS_INFO (type))
+	pedwarn (loc, OPT_Wpedantic,
+		 "structured binding declaration cannot have constrained "
+		 "%<auto%> type %qT", type);
       inlinep = 0;
       typedef_p = 0;
       constexpr_p = 0;

gcc/testsuite/g++.dg/cpp2a/concepts-placeholder10.C

@@ -0,0 +1,11 @@
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wno-pedantic" }
+
+template <class T> concept is_const = __is_same(T, const T);
+
+void f() {
+  int x[] = {1,2};
+  const int y[] = {3};
+  const is_const auto [a,b] = x; // { dg-error "constraints" }
+  const is_const auto [c] = y;
+}

gcc/testsuite/g++.dg/cpp2a/concepts-placeholder7.C

@@ -7,16 +7,18 @@ template <class>
 void f() {
   int x[] = {1,2};
   int y[] = {3};
-  C1 auto [a,b] = x;
-  C1 auto [c] = y; // { dg-error "constraints" }
+  C1 auto [a,b] = x;	// { dg-error "structured binding declaration cannot have constrained 'auto' type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
+  C1 auto [c] = y;	// { dg-error "structured binding declaration cannot have constrained 'auto' type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
+			// { dg-error "constraints" "" { target *-*-* } .-1 }
 }
 
 template <class T>
 void g() {
   T x[] = {1,2};
   T y[] = {3};
-  C1 auto [a,b] = x;
-  C1 auto [c] = y; // { dg-error "constraints" }
+  C1 auto [a,b] = x;	// { dg-error "structured binding declaration cannot have constrained 'auto' type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
+  C1 auto [c] = y;	// { dg-error "structured binding declaration cannot have constrained 'auto' type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
+			// { dg-error "constraints" "" { target *-*-* } .-1 }
 }
 template void g<int>();
 
@@ -27,6 +29,6 @@ struct S { int a, b; } s;
 
 template <class T>
 void h() {
-  const C2<T> auto& [a, b] = s;
+  const C2<T> auto& [a, b] = s;	// { dg-error "structured binding declaration cannot have constrained 'auto' type 'const auto \\\[requires ::C2<<placeholder>, >\\\]'" }
 }
 template void h<int>();

gcc/testsuite/g++.dg/cpp2a/concepts-placeholder8.C

@@ -5,6 +5,7 @@ template <class T> concept is_const = __is_same(T, const T);
 void f() {
   int x[] = {1,2};
   const int y[] = {3};
-  const is_const auto [a,b] = x; // { dg-error "constraints" }
-  const is_const auto [c] = y;
+  const is_const auto [a,b] = x;	// { dg-error "structured binding declaration cannot have constrained 'auto' type 'const auto \\\[requires ::is_const<<placeholder>, >\\\]'" }
+					// { dg-error "constraints" "" { target *-*-* } .-1 }
+  const is_const auto [c] = y;		// { dg-error "structured binding declaration cannot have constrained 'auto' type 'const auto \\\[requires ::is_const<<placeholder>, >\\\]'" }
 }

gcc/testsuite/g++.dg/cpp2a/concepts-placeholder9.C

@@ -0,0 +1,33 @@
+// PR c++/99899
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wno-pedantic" }
+
+template <class T> concept C1 = sizeof(T) > sizeof(int[1]);
+
+template <class>
+void f() {
+  int x[] = {1,2};
+  int y[] = {3};
+  C1 auto [a,b] = x;
+  C1 auto [c] = y; // { dg-error "constraints" }
+}
+
+template <class T>
+void g() {
+  T x[] = {1,2};
+  T y[] = {3};
+  C1 auto [a,b] = x;
+  C1 auto [c] = y; // { dg-error "constraints" }
+}
+template void g<int>();
+
+
+template <class... Ts> concept C2 = sizeof...(Ts) > 1;
+
+struct S { int a, b; } s;
+
+template <class T>
+void h() {
+  const C2<T> auto& [a, b] = s;
+}
+template void h<int>();

gcc/testsuite/g++.dg/cpp2a/decomp5.C

@@ -0,0 +1,19 @@
+// CWG2635 - Constrained structured bindings 
+// { dg-do compile { target c++20 } }
+
+namespace std {
+  template<typename T> struct tuple_size;
+  template<int, typename> struct tuple_element;
+}
+
+struct A {
+  int i;
+  A(int x) : i(x) {}
+  template <int I> int& get() { return i; }
+};
+
+template<> struct std::tuple_size<A> { static const int value = 2; };
+template<int I> struct std::tuple_element<I,A> { using type = int; };
+
+template<class T> concept C = true;
+C auto [x, y] = A{1}; // { dg-error "structured binding declaration cannot have constrained 'auto' type 'auto \\\[requires ::C<<placeholder>, >\\\]'" }

gcc/testsuite/g++.dg/cpp2a/decomp6.C

@@ -0,0 +1,20 @@
+// CWG2635 - Constrained structured bindings 
+// { dg-do compile { target c++20 } }
+// { dg-options "-pedantic" }
+
+namespace std {
+  template<typename T> struct tuple_size;
+  template<int, typename> struct tuple_element;
+}
+
+struct A {
+  int i;
+  A(int x) : i(x) {}
+  template <int I> int& get() { return i; }
+};
+
+template<> struct std::tuple_size<A> { static const int value = 2; };
+template<int I> struct std::tuple_element<I,A> { using type = int; };
+
+template<class T> concept C = true;
+C auto [x, y] = A{1}; // { dg-warning "structured binding declaration cannot have constrained 'auto' type 'auto \\\[requires ::C<<placeholder>, >\\\]'" }

gcc/testsuite/g++.dg/cpp2a/decomp7.C

@@ -0,0 +1,20 @@
+// CWG2635 - Constrained structured bindings 
+// { dg-do compile { target c++20 } }
+// { dg-options "-Wno-pedantic" }
+
+namespace std {
+  template<typename T> struct tuple_size;
+  template<int, typename> struct tuple_element;
+}
+
+struct A {
+  int i;
+  A(int x) : i(x) {}
+  template <int I> int& get() { return i; }
+};
+
+template<> struct std::tuple_size<A> { static const int value = 2; };
+template<int I> struct std::tuple_element<I,A> { using type = int; };
+
+template<class T> concept C = true;
+C auto [x, y] = A{1}; // { dg-bogus "structured binding declaration cannot have constrained 'auto' type 'auto \\\[requires ::C<<placeholder>, >\\\]'" }