c++: ICE with loopy var tmpl auto deduction [PR109300]

Now that we resolve non-dependent variable template-ids ahead of time,
cp_finish_decl needs to handle a new invalid situation: we can end up
trying to instantiate a variable template with deduced type before we
fully parsed its initializer.

	PR c++/109300

gcc/cp/ChangeLog:

	* decl.cc (cp_finish_decl): Diagnose ordinary auto deduction
	with no initializer, instead of asserting.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/var-templ79.C: New test.

2 files changed, 19 insertions, 1 deletions

diff --git a/gcc/cp/decl.cc b/gcc/cp/decl.cc
index 20b980f..5369714 100644
--- a/gcc/cp/decl.cc
+++ b/gcc/cp/decl.cc
@@ -8276,7 +8276,20 @@ cp_finish_decl (tree decl, tree init, bool init_const_expr_p,
 	      return;
 	    }
 
-	  gcc_assert (CLASS_PLACEHOLDER_TEMPLATE (auto_node));
+	  if (CLASS_PLACEHOLDER_TEMPLATE (auto_node))
+	    /* Class deduction with no initializer is OK.  */;
+	  else
+	    {
+	      /* Ordinary auto deduction without an initializer, a situation
+		 which grokdeclarator already detects and rejects for the most
+		 part.  But we can still get here if we're instantiating a
+		 variable template before we've fully parsed (and attached) its
+		 initializer, e.g. template<class> auto x = x<int>;  */
+	      error_at (DECL_SOURCE_LOCATION (decl),
+			"declaration of %q#D has no initializer", decl);
+	      TREE_TYPE (decl) = error_mark_node;
+	      return;
+	    }
 	}
       d_init = init;
       if (d_init)
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ79.C b/gcc/testsuite/g++.dg/cpp1y/var-templ79.C
new file mode 100644
index 0000000..3c0d276
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ79.C
@@ -0,0 +1,5 @@
+// PR c++/109300
+// { dg-do compile { target c++14 } }
+
+template<class>
+auto x = x<int>; // { dg-error "" }