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.

5 files changed, 49 insertions, 9 deletions

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 0cc971d..1489453 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,9 @@
+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
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 128f760..3da1609 100644
--- a/gcc/cp/constexpr.c
+++ b/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)))
diff --git a/gcc/testsuite/ChangeLog b/gcc/testsuite/ChangeLog
index aa5d41e..4f46c38 100644
--- a/gcc/testsuite/ChangeLog
+++ b/gcc/testsuite/ChangeLog
@@ -1,3 +1,9 @@
+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
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-init16.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-init16.C
new file mode 100644
index 0000000..16db297
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-init16.C
@@ -0,0 +1,15 @@
+// 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{};
diff --git a/gcc/testsuite/g++.dg/cpp2a/constexpr-init17.C b/gcc/testsuite/g++.dg/cpp2a/constexpr-init17.C
new file mode 100644
index 0000000..34d9fe5
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp2a/constexpr-init17.C
@@ -0,0 +1,15 @@
+// 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{};