c++: placeholder type constraint in structured binding [PR99899]

In this PR, we're crashing because the constraint handling inside
do_auto_deduction doesn't expect to see an adc_decomp_type context.
This patch fixes this by treating adc_decomp_type like adc_variable_type
or adc_return_type during placeholder type constraint checking.

Meanwhile, I noticed we weren't checking constraints at all when binding
an array via a structured binding, since do_auto_deduction would exit
early and bypass the constraint check.  This patch fixes this by
replacing the early exit with an appropriate setup of the 'targs'
vector.

gcc/cp/ChangeLog:

	PR c++/99899
	* pt.c (do_auto_deduction): Don't exit early when deducing the
	array type of a structured binding.  Also handle adc_decomp_type
	during constraint checking.

gcc/testsuite/ChangeLog:

	PR c++/99899
	* g++.dg/cpp2a/concepts-placeholder7.C: New test.
	* g++.dg/cpp2a/concepts-placeholder8.C: New test.

3 files changed, 54 insertions, 8 deletions

diff --git a/gcc/cp/pt.c b/gcc/cp/pt.c
index 41bc633..a08d08d 100644
--- a/gcc/cp/pt.c
+++ b/gcc/cp/pt.c
@@ -29455,8 +29455,6 @@ do_auto_deduction (tree type, tree init, tree auto_node,
                    tsubst_flags_t complain, auto_deduction_context context,
 		   tree outer_targs, int flags)
 {
-  tree targs;
-
   if (init == error_mark_node)
     return error_mark_node;
 
@@ -29520,14 +29518,17 @@ do_auto_deduction (tree type, tree init, tree auto_node,
   else
     init = resolve_nondeduced_context (init, complain);
 
+  tree targs;
   if (context == adc_decomp_type
       && auto_node == type
       && init != error_mark_node
       && TREE_CODE (TREE_TYPE (init)) == ARRAY_TYPE)
-    /* [dcl.decomp]/1 - if decomposition declaration has no ref-qualifiers
-       and initializer has array type, deduce cv-qualified array type.  */
-    return cp_build_qualified_type_real (TREE_TYPE (init), TYPE_QUALS (type),
-					 complain);
+    {
+      /* [dcl.struct.bind]/1 - if decomposition declaration has no ref-qualifiers
+	 and initializer has array type, deduce cv-qualified array type.  */
+      targs = make_tree_vec (1);
+      TREE_VEC_ELT (targs, 0) = TREE_TYPE (init);
+    }
   else if (AUTO_IS_DECLTYPE (auto_node))
     {
       tree stripped_init = tree_strip_any_location_wrapper (init);
@@ -29613,7 +29614,8 @@ do_auto_deduction (tree type, tree init, tree auto_node,
       if (processing_template_decl)
 	{
 	  gcc_checking_assert (context == adc_variable_type
-			       || context == adc_return_type);
+			       || context == adc_return_type
+			       || context == adc_decomp_type);
 	  gcc_checking_assert (!type_dependent_expression_p (init));
 	  /* If the constraint is dependent, we need to wait until
 	     instantiation time to resolve the placeholder.  */
@@ -29621,7 +29623,9 @@ do_auto_deduction (tree type, tree init, tree auto_node,
 	    return type;
 	}
 
-      if ((context == adc_return_type || context == adc_variable_type)
+      if ((context == adc_return_type
+	   || context == adc_variable_type
+	   || context == adc_decomp_type)
 	  && current_function_decl
 	  && DECL_TEMPLATE_INFO (current_function_decl))
 	outer_targs = DECL_TI_ARGS (current_function_decl);
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder7.C b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder7.C
new file mode 100644
index 0000000..93219ff
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder7.C
@@ -0,0 +1,32 @@
+// PR c++/99899
+// { dg-do compile { target c++20 } }
+
+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>();
diff --git a/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder8.C b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder8.C
new file mode 100644
index 0000000..e786592
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/concepts-placeholder8.C
@@ -0,0 +1,10 @@
+// { dg-do compile { target c++20 } }
+
+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;
+}