PR c++/86740, ICE with constexpr if and nested generic lambdas.

When we partially instantiate the constexpr if, we walk through its body to
see what it uses from the enclosing local_specializations.  That walk was
overlooking the use of 'count' in the captures of the innermost lambda,
because we weren't walking into the capture list.

	* tree.c (cp_walk_subtrees): Handle LAMBDA_EXPR.

From-SVN: r268046

gcc/cp/ChangeLog

+2019-01-17  Jason Merrill  <jason@redhat.com>
+
+	PR c++/86740, ICE with constexpr if and nested generic lambdas.
+	* tree.c (cp_walk_subtrees): Handle LAMBDA_EXPR.
+
 2019-01-17  Paolo Carlini  <paolo.carlini@oracle.com>
 
 	* decl.c (grokdeclarator): Use typespec_loc in error messages

gcc/cp/tree.c

@@ -4933,6 +4933,14 @@ cp_walk_subtrees (tree *tp, int *walk_subtrees_p, walk_tree_fn func,
 	}
       break;
 
+    case LAMBDA_EXPR:
+      /* Don't walk into the body of the lambda, but the capture initializers
+	 are part of the enclosing context.  */
+      for (tree cap = LAMBDA_EXPR_CAPTURE_LIST (*tp); cap;
+	   cap = TREE_CHAIN (cap))
+	WALK_SUBTREE (TREE_VALUE (cap));
+      break;
+
     default:
       return NULL_TREE;
     }

gcc/testsuite/g++.dg/cpp1z/constexpr-if25.C

+// PR c++/86740
+// { dg-do compile { target c++17 } }
+
+struct Constant
+{
+  static constexpr int value = 0;
+};
+template<typename F>
+void invokeWithConstant(F &&f)
+{
+  f(Constant{});
+}
+int foo()
+{
+  int count = 0;
+  invokeWithConstant
+    ([&] (auto id1)
+     {
+       invokeWithConstant
+	 ([&] (auto id2)
+	  {
+	    if constexpr (id1.value == 0  &&  id2.value == 0)
+	      [&] { count = 1; } ();
+	  });
+     });
+  return count;
+}