c++: Try to complete decomp types [PR95328]

Two years ago Paolo has added the
  else if (processing_template_decl && !COMPLETE_TYPE_P (type))
    pedwarn (...);
lines into cp_finish_decomp.  For type dependent decl we punt much earlier,
but even for types which aren't type dependent COMPLETE_TYPE_P might be
false as this testcase shows, so this patch tries to complete_type first
(the reason for writing it that way is that it is then followed by another
else if and if complete_type returns error_mark_node, we shouldn't report
anything, as a bug should have been reported already.

2020-05-28  Jakub Jelinek  <jakub@redhat.com>

	PR c++/95328
	* decl.c (cp_finish_decomp): Call complete_type before checking
	COMPLETE_TYPE_P.

	* g++.dg/cpp1z/decomp53.C: New test.

2 files changed, 24 insertions, 0 deletions

diff --git a/gcc/cp/decl.c b/gcc/cp/decl.c
index b0de906..b8bd09b 100644
--- a/gcc/cp/decl.c
+++ b/gcc/cp/decl.c
@@ -8400,6 +8400,8 @@ cp_finish_decomp (tree decl, tree first, unsigned int count)
       error_at (loc, "cannot decompose lambda closure type %qT", type);
       goto error_out;
     }
+  else if (processing_template_decl && complete_type (type) == error_mark_node)
+    goto error_out;
   else if (processing_template_decl && !COMPLETE_TYPE_P (type))
     pedwarn (loc, 0, "structured binding refers to incomplete class type %qT",
 	     type);
diff --git a/gcc/testsuite/g++.dg/cpp1z/decomp53.C b/gcc/testsuite/g++.dg/cpp1z/decomp53.C
new file mode 100644
index 0000000..b34e6ac
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/decomp53.C
@@ -0,0 +1,22 @@
+// PR c++/95328
+// { dg-do compile { target c++11 } }
+// { dg-options "" }
+
+template <typename T>
+struct S
+{
+  int a, b;
+};
+
+template <typename T>
+void
+foo ()
+{
+  auto [a, b] = S<int>();	// { dg-warning "structured bindings only available with" "" { target c++14_down } }
+}
+
+int
+main ()
+{
+  foo<int> ();
+}