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

[Jakub Jelinek via Gcc-patches]

Hi!

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?

2022-11-12  Jakub Jelinek  <ja...@redhat.com>

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

        * g++.dg/cpp2a/decomp5.C: New test.

--- 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 }

        Jakub