2019-03-25  Marek Polacek  <polacek@redhat.com>
 
	PR c++/89214 - ICE when initializing aggregates with bases.
	* typeck2.c (digest_init_r): Warn about object slicing instead of
	crashing.

--- gcc/cp/typeck2.c
+++ gcc/cp/typeck2.c
@@ -1209,8 +1209,29 @@ digest_init_r (tree type, tree init, int nested, int flags,
     {
       tree elt = CONSTRUCTOR_ELT (stripped_init, 0)->value;
       if (reference_related_p (type, TREE_TYPE (elt)))
-	/* We should have fixed this in reshape_init.  */
-	gcc_unreachable ();
+	{
+	  /* In C++17, aggregates can have bases, thus participate in
+	     aggregate initialization.  In the following case:
+
+	       struct B { int c; };
+	       struct D : B { };
+	       D d{{D{{42}}}};
+
+	    there's an extra set of braces, so the D temporary initializes
+	    the first element of d, which is the B base subobject.  The base
+	    of type B is copy-initialized from the D temporary, causing
+	    object slicing.  */
+	  tree field = next_initializable_field (TYPE_FIELDS (type));
+	  if (field && DECL_FIELD_IS_BASE (field))
+	    {
+	      if (warning_at (loc, 0, "initializing a base class of type %qT "
+			      "results in object slicing", TREE_TYPE (field)))
+		inform (loc, "remove %<{ }%> around initializer");
+	    }
+	  else
+	    /* We should have fixed this in reshape_init.  */
+	    gcc_unreachable ();
+	}
     }
 
   if (BRACE_ENCLOSED_INITIALIZER_P (stripped_init)
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1z/aggr-base8.C
@@ -0,0 +1,48 @@
+// PR c++/89214
+// { dg-do compile { target c++17 } }
+
+struct A
+{
+  A (int);
+};
+
+struct BB
+{
+  A a;
+};
+
+struct B : BB
+{
+};
+
+void
+foo ()
+{
+  B b1 = {42};
+  B b2 = {{42}};
+  B b3 = {{{42}}};
+
+  B b4 = B{42};
+  B b5 = B{{42}};
+  B b6 = B{{{42}}};
+
+  B b7 = {B{42}};
+  B b8 = {B{{42}}};
+  B b9 = {B{{{42}}}};
+
+  B b10 = {{B{42}}}; // { dg-warning "initializing a base class of type .BB. results in object slicing" }
+  B b11 = {{B{{42}}}}; // { dg-warning "initializing a base class of type .BB. results in object slicing" }
+  B b12 = {{B{{{42}}}}}; // { dg-warning "initializing a base class of type .BB. results in object slicing" }
+
+  B bb1{42};
+  B bb2{{42}};
+  B bb3{{{42}}};
+
+  B bb7{B{42}};
+  B bb8{B{{42}}};
+  B bb9{B{{{42}}}};
+
+  B bb10{{B{42}}}; // { dg-warning "initializing a base class of type .BB. results in object slicing" }
+  B bb11{{B{{42}}}}; // { dg-warning "initializing a base class of type .BB. results in object slicing" }
+  B bb12{{B{{{42}}}}}; // { dg-warning "initializing a base class of type .BB. results in object slicing" }
+}
--- /dev/null
+++ gcc/testsuite/g++.dg/cpp1z/aggr-base9.C
@@ -0,0 +1,33 @@
+// PR c++/89214
+// { dg-do compile { target c++17 } }
+
+struct B {
+  int c;
+};
+
+struct D : B { };
+
+void
+foo ()
+{
+  D d1 = {42};
+  D d2 = {{42}};
+  
+  D d4 = D{42};
+  D d5 = D{{42}};
+ 
+  D d7 = {D{42}};
+  D d8 = {D{{42}}};
+
+  D d10 = {{D{42}}}; // { dg-warning "initializing a base class of type .B. results in object slicing" }
+  D d11 = {{D{{42}}}}; // { dg-warning "initializing a base class of type .B. results in object slicing" }
+
+  D dd1{42};
+  D dd2{{42}};
+  
+  D dd7{D{42}};
+  D dd8{D{{42}}};
+
+  D dd10{{D{42}}}; // { dg-warning "initializing a base class of type .B. results in object slicing" }
+  D dd11{{D{{42}}}}; // { dg-warning "initializing a base class of type .B. results in object slicing" }
+}