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

[Jason Merrill via Gcc-patches]

On 11/13/22 01:50, Jakub Jelinek wrote:

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  <ja...@redhat.com>

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

Let's make the constrained auto case a pedwarn instead of an error.

        * 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