c++: check completeness after auto deduction [PR80351]

Normally we check for incomplete type in start_decl, but that obviously
doesn't work for auto variables. Thanks to Pokechu22 for the analysis and
testcases:

"When cp_finish_decl calls cp_apply_type_quals_to_decl on a const auto or
constexpr auto variable, the type might not be complete the first time
(this happened when auto deduces to an initializer_list).
cp_apply_type_quals_to_decl removes the const qualifier if the type is
not complete, which is appropriate for grokdeclarator, on the assumption
that the type will be complete when called by cp_finish_decl."

	PR c++/80351

gcc/cp/ChangeLog:

	* decl.cc (cp_finish_decl): Check completeness of deduced type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-77482.C: Adjust message.
	* g++.dg/cpp1y/auto-fn27.C: Likewise.
	* g++.dg/cpp1y/lambda-generic-variadic22.C: Likewise.
	* g++.dg/cpp1z/decomp54.C: Likewise.
	* g++.dg/cpp0x/initlist-const1.C: New test.
	* g++.dg/warn/Wunused-var-37.C: New test.
	* g++.dg/warn/Wunused-var-38.C: New test.
	* g++.dg/warn/Wunused-var-39.C: New test.

1 files changed, 11 insertions, 0 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 2852093..45206c2 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -8129,6 +8129,17 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 	  TREE_TYPE (decl) = error_mark_node;
 	  return;
 	}
+      /* As in start_decl_1, complete so TREE_READONLY is set properly.  */
+      if (!processing_template_decl
+	  && !type_uses_auto (type)
+	  && !COMPLETE_TYPE_P (complete_type (type)))
+	{
+	  error_at (location_of (decl),
+		    "deduced type %qT for %qD is incomplete", type, decl);
+	  cxx_incomplete_type_inform (type);
+	  TREE_TYPE (decl) = error_mark_node;
+	  return;
+	}
       cp_apply_type_quals_to_decl (cp_type_quals (type), decl);
     }