[PATCH] c++, v2: Implement CWG2635 - Constrained structured bindings

[Jakub Jelinek <jakub@redhat.com>]

On Sat, Nov 12, 2022 at 12:23:56PM +0100, Jakub Jelinek wrote:
> The following patch implements CWG2635.
> 
> So far tested on
> GXX_TESTSUITE_STDS=98,11,14,17,20,2b make check-g++ RUNTESTFLAGS="dg.exp=decomp*"
> ok for trunk if it passes full bootstrap/regtest and it is voted in?

Here is another version of the patch that passed bootstrap/regtest, the only
change are tweaks to 2 further testcases.

2022-11-13  Jakub Jelinek  <jakub@redhat.com>

	* decl.cc (grokdeclarator): Implement
	CWG2635 - Constrained structured bindings.  Diagnose constrained
	auto type.

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

--- gcc/cp/decl.cc.jj	2022-11-11 17:14:33.103869977 +0100
+++ gcc/cp/decl.cc	2022-11-12 12:13:52.217239729 +0100
@@ -12660,7 +12660,8 @@ grokdeclarator (const cp_declarator *dec
 	  gcc_unreachable ();
 	}
       if (TREE_CODE (type) != TEMPLATE_TYPE_PARM
-	  || TYPE_IDENTIFIER (type) != auto_identifier)
+	  || TYPE_IDENTIFIER (type) != auto_identifier
+	  || PLACEHOLDER_TYPE_CONSTRAINTS_INFO (type))
 	{
 	  if (type != error_mark_node)
 	    {
--- gcc/testsuite/g++.dg/cpp2a/decomp5.C.jj	2022-11-12 12:17:21.024392082 +0100
+++ gcc/testsuite/g++.dg/cpp2a/decomp5.C	2022-11-12 12:20:00.700214521 +0100
@@ -0,0 +1,20 @@
+// 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 type 'auto \\\[requires ::C<<placeholder>, >\\\]'" }
+		      // { dg-message "type must be cv-qualified 'auto' or reference to cv-qualified 'auto'" "" { target *-*-* } .-1 }
--- gcc/testsuite/g++.dg/cpp2a/concepts-placeholder7.C.jj	2021-04-06 23:49:08.288882716 +0200
+++ gcc/testsuite/g++.dg/cpp2a/concepts-placeholder7.C	2022-11-13 12:17:38.570047510 +0100
@@ -7,16 +7,16 @@ 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 type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
+  C1 auto [c] = y;	// { dg-error "structured binding declaration cannot have type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
 }
 
 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 type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
+  C1 auto [c] = y;	// { dg-error "structured binding declaration cannot have type 'auto \\\[requires ::C1<<placeholder>, >\\\]'" }
 }
 template void g<int>();
 
@@ -27,6 +27,6 @@ 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 type 'const auto \\\[requires ::C2<<placeholder>, >\\\]'" }
 }
 template void h<int>();
--- gcc/testsuite/g++.dg/cpp2a/concepts-placeholder8.C.jj	2021-04-06 23:49:08.288882716 +0200
+++ gcc/testsuite/g++.dg/cpp2a/concepts-placeholder8.C	2022-11-13 12:19:17.034684881 +0100
@@ -5,6 +5,6 @@ template <class T> concept is_const = __
 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 type 'const auto \\\[requires ::is_const<<placeholder>, >\\\]'" }
+  const is_const auto [c] = y;		// { dg-error "structured binding declaration cannot have type 'const auto \\\[requires ::is_const<<placeholder>, >\\\]'" }
 }


	Jakub