c++: Fix ICE with constexpr init and [[no_unique_address]] [PR93803]

Here we crash when constexpr-initializing a class member of empty class
type with [[no_unique_address]].  Without the attribute we would have
a ctor (that initializes bar) of the form

  { .D.2173 = { .x = {} } }

but with the attribute reduced_constant_expression_p gets

  { .x = {} }

That means that "idx != field" is true for the latter and we see that
foo, the base class of bar, is an empty class, so we want to look at
the next initializable field (since empty class fields may not have an
initializer).  But in this case there are no more, therefore accessing
DECL_CHAIN (field) crashes.  Long story short, we need to avoid a crash
on a null field when we're initializing a class that only contains an
empty base class.

While poking into this I discovered c++/93898, but that's a different
problem.

2020-02-26  Marek Polacek  <polacek@redhat.com>

	PR c++/93803 - ICE with constexpr init and [[no_unique_address]].
	* constexpr.c (reduced_constant_expression_p): Don't crash on a null
	field.

	* g++.dg/cpp2a/constexpr-init16.C: New test.
	* g++.dg/cpp2a/constexpr-init17.C: New test.

gcc/cp/ChangeLog

+2020-02-26  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93803 - ICE with constexpr init and [[no_unique_address]].
+	* constexpr.c (reduced_constant_expression_p): Don't crash on a null
+	field.
+
 2020-02-24  Martin Sebor  <msebor@redhat.com>
 
 	PR c++/93804

gcc/cp/constexpr.c

@@ -2603,16 +2603,14 @@ reduced_constant_expression_p (tree t)
 	     element.  */
 	  if (!reduced_constant_expression_p (val))
 	    return false;
+	  /* Empty class field may or may not have an initializer.  */
+	  for (; field && idx != field;
+	       field = next_initializable_field (DECL_CHAIN (field)))
+	    if (!is_really_empty_class (TREE_TYPE (field),
+					/*ignore_vptr*/false))
+	      return false;
 	  if (field)
-	    {
-	      /* Empty class field may or may not have an initializer.  */
-	      for (; idx != field;
-		   field = next_initializable_field (DECL_CHAIN (field)))
-		if (!is_really_empty_class (TREE_TYPE (field),
-					    /*ignore_vptr*/false))
-		  return false;
-	      field = next_initializable_field (DECL_CHAIN (field));
-	    }
+	    field = next_initializable_field (DECL_CHAIN (field));
 	}
       /* There could be a non-empty field at the end.  */
       for (; field; field = next_initializable_field (DECL_CHAIN (field)))

gcc/testsuite/ChangeLog

+2020-02-26  Marek Polacek  <polacek@redhat.com>
+
+	PR c++/93803 - ICE with constexpr init and [[no_unique_address]].
+	* g++.dg/cpp2a/constexpr-init16.C: New test.
+	* g++.dg/cpp2a/constexpr-init17.C: New test.
+
 2020-02-26  Richard Sandiford  <richard.sandiford@arm.com>
 
 	PR middle-end/93843

gcc/testsuite/g++.dg/cpp2a/constexpr-init16.C

+// PR c++/93803 - ICE with constexpr init and [[no_unique_address]].
+// { dg-do compile { target c++2a } }
+
+struct empty { };
+
+struct foo {
+  [[no_unique_address]] empty x;
+  constexpr foo() : x{} { }
+};
+
+struct bar : foo {
+  using foo::foo;
+};
+
+constexpr bar a{};

gcc/testsuite/g++.dg/cpp2a/constexpr-init17.C

+// PR c++/93803 - ICE with constexpr init and [[no_unique_address]].
+// { dg-do compile { target c++2a } }
+
+struct empty { };
+
+struct foo {
+  [[no_unique_address]] empty x, x2, x3;
+  constexpr foo() : x{}, x2{} { }
+};
+
+struct bar : foo {
+  using foo::foo;
+};
+
+constexpr bar a{};