c++: Disallow decomposition of lambda bases [PR90321]

Decomposition of lambda closure types is not allowed by
[dcl.struct.bind] p6, since members of a closure have no name.

r244909 made this an error, but missed the case where a lambda is used
as a base.  This patch moves the check to find_decomp_class_base to
handle this case.

As a drive-by improvement, we also slightly improve the diagnostics to
indicate why a base class was being inspected.  Ideally the diagnostic
would point directly at the relevant base, but there doesn't seem to be
an easy way to get this location just from the binfo so I don't worry
about that here.

	PR c++/90321

gcc/cp/ChangeLog:

	* decl.cc (find_decomp_class_base): Check for decomposing a
	lambda closure type.  Report base class chains if needed.
	(cp_finish_decomp): Remove no-longer-needed check.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/decomp62.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Marek Polacek <polacek@redhat.com>

2 files changed, 25 insertions, 6 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 4ba6e37..a1b9957 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -9405,6 +9405,14 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 static tree
 find_decomp_class_base (location_t loc, tree type, tree ret)
 {
+  if (LAMBDA_TYPE_P (type))
+    {
+      auto_diagnostic_group d;
+      error_at (loc, "cannot decompose lambda closure type %qT", type);
+      inform (location_of (type), "lambda declared here");
+      return error_mark_node;
+    }
+
   bool member_seen = false;
   for (tree field = TYPE_FIELDS (type); field; field = DECL_CHAIN (field))
     if (TREE_CODE (field) != FIELD_DECL
@@ -9447,9 +9455,13 @@ find_decomp_class_base (location_t loc, tree type, tree ret)
   for (binfo = TYPE_BINFO (type), i = 0;
        BINFO_BASE_ITERATE (binfo, i, base_binfo); i++)
     {
+      auto_diagnostic_group d;
       tree t = find_decomp_class_base (loc, TREE_TYPE (base_binfo), ret);
       if (t == error_mark_node)
-	return error_mark_node;
+	{
+	  inform (location_of (type), "in base class of %qT", type);
+	  return error_mark_node;
+	}
       if (t != NULL_TREE && t != ret)
 	{
 	  if (ret == type)
@@ -9905,11 +9917,6 @@ cp_finish_decomp (tree decl, cp_decomp *decomp, bool test_p)
       error_at (loc, "cannot decompose non-array non-class type %qT", type);
       goto error_out;
     }
-  else if (LAMBDA_TYPE_P (type))
-    {
-      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))
diff --git a/gcc/testsuite/g++.dg/cpp1z/decomp62.C b/gcc/testsuite/g++.dg/cpp1z/decomp62.C
new file mode 100644
index 0000000..b0ce105
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1z/decomp62.C
@@ -0,0 +1,12 @@
+// PR c++/90321
+// { dg-do compile { target c++17 } }
+
+template<class F> struct hack : F { };
+template<class F> hack(F) -> hack<F>;
+
+int main()
+{
+    auto f = [x = 1, y = 2]() { };
+    auto [a, b] = hack { f };  // { dg-error "cannot decompose lambda closure type" }
+    return b;
+}