c++: Fix ICE with unexpanded parameter pack [PR99844]

In explicit17.C, we weren't detecting an unexpanded parameter pack in
explicit(bool), so we crashed on a TEMPLATE_PARM_INDEX in constexpr.

I noticed the same is true for noexcept(), but only since my patch to
implement delayed parsing of noexcept.  Previously, we would detect the
unexpanded pack in push_template_decl but now the noexcept expression
has not yet been parsed, so we need to do it a bit later.

gcc/cp/ChangeLog:

	PR c++/99844
	* decl.c (build_explicit_specifier): Call
	check_for_bare_parameter_packs.
	* except.c (build_noexcept_spec): Likewise.

gcc/testsuite/ChangeLog:

	PR c++/99844
	* g++.dg/cpp2a/explicit16.C: Use c++20.
	* g++.dg/cpp0x/noexcept66.C: New test.
	* g++.dg/cpp2a/explicit17.C: New test.

gcc/cp/decl.c

@@ -17991,6 +17991,9 @@ require_deduced_type (tree decl, tsubst_flags_t complain)
 tree
 build_explicit_specifier (tree expr, tsubst_flags_t complain)
 {
+  if (check_for_bare_parameter_packs (expr))
+    return error_mark_node;
+
   if (instantiation_dependent_expression_p (expr))
     /* Wait for instantiation, tsubst_function_decl will handle it.  */
     return expr;

gcc/cp/except.c

@@ -1228,6 +1228,8 @@ type_throw_all_p (const_tree type)
 tree
 build_noexcept_spec (tree expr, tsubst_flags_t complain)
 {
+  if (check_for_bare_parameter_packs (expr))
+    return error_mark_node;
   if (TREE_CODE (expr) != DEFERRED_NOEXCEPT
       && !value_dependent_expression_p (expr))
     {

gcc/testsuite/g++.dg/cpp0x/noexcept66.C

@@ -0,0 +1,13 @@
+// PR c++/99844
+// { dg-do compile { target c++11 } }
+
+template <bool... B>
+struct S {
+ void fn() noexcept(B); // { dg-error "parameter packs not expanded" }
+};
+
+void fn ()
+{
+  S<true> s;
+  s.fn();
+}

gcc/testsuite/g++.dg/cpp2a/explicit16.C

@@ -1,5 +1,5 @@
 // PR c++/95066 - explicit malfunction with dependent expression.
-// { dg-do compile { target c++2a } }
+// { dg-do compile { target c++20 } }
 
 template <typename T>
 struct Foo {

gcc/testsuite/g++.dg/cpp2a/explicit17.C

@@ -0,0 +1,9 @@
+// PR c++/99844
+// { dg-do compile { target c++20 } }
+
+template <bool... B>
+struct S {
+  constexpr explicit(B) S() {} // { dg-error "parameter packs not expanded" }
+};
+
+constexpr S<true> s;