PR c++/88761 - ICE with reference capture of constant.

Here, we capture nf, then the use of the proxy decays to a constant during
semantic processing of +nf.  Since we saw some decay from proxy to constant,
we walk through the lambda body to see which proxies are still used, but we
weren't walking into subtrees of DECL_EXPR at all, so we missed the use of
&nf in the initializer of y, and removed the capture.  But then at
instantiation time we try to use nf, don't have a proxy anymore, and ICE.

	* lambda.c (mark_const_cap_r): Do walk subtrees of DECL_EXPR for
	non-proxy decls.

From-SVN: r268471

gcc/cp/ChangeLog

+2019-02-01  Jason Merrill  <jason@redhat.com>
+
+	PR c++/88761 - ICE with reference capture of constant.
+	* lambda.c (mark_const_cap_r): Do walk subtrees of DECL_EXPR for
+	non-proxy decls.
+
 2019-02-01  Marek Polacek  <polacek@redhat.com>
 
 	PR c++/88325 - ICE with invalid out-of-line template member definition.

gcc/cp/lambda.c

@@ -1488,8 +1488,10 @@ mark_const_cap_r (tree *t, int *walk_subtrees, void *data)
     {
       tree decl = DECL_EXPR_DECL (*t);
       if (is_constant_capture_proxy (decl))
-	var = DECL_CAPTURED_VARIABLE (decl);
-      *walk_subtrees = 0;
+	{
+	  var = DECL_CAPTURED_VARIABLE (decl);
+	  *walk_subtrees = 0;
+	}
     }
   else if (is_constant_capture_proxy (*t))
     var = DECL_CAPTURED_VARIABLE (*t);

gcc/testsuite/g++.dg/cpp1y/lambda-generic-const6.C

+// PR c++/88761
+// { dg-do compile { target c++14 } }
+
+template <class T>
+void f(T t) { t(1); }
+
+int main()
+{
+  const unsigned long nf = 10'000'000;
+
+  auto loop = [&](auto)
+  {
+    auto x = +nf;
+    auto y = &nf;
+  };
+
+  f(loop);
+}