c++: Fix constexpr if and braced functional cast.

While partially instantiating a generic lambda, we can encounter pack
expansions or constexpr if where we can't actually do the substitution
immediately, and instead remember a partial instantiation context
in *_EXTRA_ARGS.  This includes any local_specializations used in the
pattern or condition.  In this testcase our tree walk wasn't finding the use
of i because we weren't walking into the type of a CONSTRUCTOR.  Fixed by
moving the code for doing that from find_parameter_packs_r into
cp_walk_subtrees.

2020-02-11  Jason Merrill  <jason@redhat.com>

	PR c++/92583
	PR c++/92654
	* tree.c (cp_walk_subtrees): Walk CONSTRUCTOR types here.
	* pt.c (find_parameter_packs_r): Not here.

gcc/cp/ChangeLog

+2020-02-12  Jason Merrill  <jason@redhat.com>
+
+	PR c++/92583
+	PR c++/92654
+	* tree.c (cp_walk_subtrees): Walk CONSTRUCTOR types here.
+	* pt.c (find_parameter_packs_r): Not here.
+
 2020-02-12 Iain Sandoe <iain@sandoe.co.uk>
 
 	* coroutines.cc (build_actor_fn): Implement deallocation function

gcc/cp/pt.c

@@ -3924,9 +3924,6 @@ find_parameter_packs_r (tree *tp, int *walk_subtrees, void* data)
     case TEMPLATE_DECL:
       if (!DECL_TEMPLATE_TEMPLATE_PARM_P (t))
 	return NULL_TREE;
-      gcc_fallthrough();
-
-    case CONSTRUCTOR:
       cp_walk_tree (&TREE_TYPE (t),
 		    &find_parameter_packs_r, ppd, ppd->visited);
       return NULL_TREE;

gcc/cp/tree.c

@@ -5024,6 +5024,11 @@ cp_walk_subtrees (tree *tp, int *walk_subtrees_p, walk_tree_fn func,
       *walk_subtrees_p = 0;
       break;
 
+    case CONSTRUCTOR:
+      if (COMPOUND_LITERAL_P (*tp))
+	WALK_SUBTREE (TREE_TYPE (*tp));
+      break;
+
     case TRAIT_EXPR:
       WALK_SUBTREE (TRAIT_EXPR_TYPE1 (*tp));
       WALK_SUBTREE (TRAIT_EXPR_TYPE2 (*tp));

gcc/testsuite/g++.dg/cpp0x/nondeduced7.C

@@ -2,5 +2,5 @@
 // { dg-do compile { target c++11 } }
 
 template<typename, int> struct A;
-template<typename T> struct A<T, T{}> {}; // { dg-error "partial specialization" }
+template<typename T> struct A<T, T{}> {}; // { dg-error "partial specialization|involves template parameter" }
 A<int, 0> a;

gcc/testsuite/g++.dg/cpp1z/constexpr-if-lambda3.C

+// PR c++/92583
+// { dg-do compile { target c++17 } }
+
+template <int> struct a {
+  constexpr operator int() { return 42; }
+};
+template <typename> using b = int;
+template <typename d, d> struct e {};
+template <typename d, d g> using h = e<d, __integer_pack(g)...>;
+template <typename j, typename k, k... index> void apply(j f, e<k, index...>) {
+  (f(a<index>{}), ...);
+}
+template <auto l, typename j> void m(j f) {
+  using k = b<decltype(l)>;
+  using n = h<k, l>;
+  apply(f, n{});
+}
+template <int, int c> void o() {
+  auto p = [](auto i) {
+    if constexpr (a<i>{}) ;
+  };
+  m<c>(p);
+}
+auto q() { o<0, 1>; }