From 8fda2c274ac66d60c1dfc1349e9efb4e8c2a3580 Mon Sep 17 00:00:00 2001
From: Jason Merrill <jason@redhat.com>
Date: Mon, 3 Feb 2020 11:11:55 -0500
Subject: [PATCH] c++: Allow parm of empty class type in constexpr.

Since copying a class object is defined in terms of the copy constructor,
copying an empty class is OK even if it would otherwise not be usable in a
constant expression.  Relatedly, using a parameter as an lvalue is no more
problematic than a local variable, and calling a member function uses the
object as an lvalue.

	PR c++/91953
	* constexpr.c (potential_constant_expression_1) [PARM_DECL]: Allow
	empty class type.
	[COMPONENT_REF]: A member function reference doesn't use the object
	as an rvalue.
---
 gcc/cp/ChangeLog                              |  8 ++++++++
 gcc/cp/constexpr.c                            | 19 +++++++++++++++----
 .../g++.dg/cpp0x/constexpr-empty14.C          | 10 ++++++++++
 gcc/testsuite/g++.dg/cpp1z/constexpr-if12.C   |  3 ++-
 4 files changed, 35 insertions(+), 5 deletions(-)
 create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-empty14.C

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 2429b1f1642..a8af79c77a7 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,3 +1,11 @@
+2020-02-03  Jason Merrill  <jason@redhat.com>
+
+	PR c++/91953
+	* constexpr.c (potential_constant_expression_1) [PARM_DECL]: Allow
+	empty class type.
+	[COMPONENT_REF]: A member function reference doesn't use the object
+	as an rvalue.
+
 2020-02-03  Iain Sandoe  <iain@sandoe.co.uk>
 
 	PR c++/93458
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index 577022e9b9a..a39ba413d68 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -7013,8 +7013,13 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
       return true;
 
     case PARM_DECL:
-      if (now)
+      if (now && want_rval)
 	{
+	  tree type = TREE_TYPE (t);
+	  if (dependent_type_p (type)
+	      || is_really_empty_class (type, /*ignore_vptr*/false))
+	    /* An empty class has no data to read.  */
+	    return true;
 	  if (flags & tf_error)
 	    error ("%qE is not a constant expression", t);
 	  return false;
@@ -7270,10 +7275,7 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 #endif
       return RECUR (t, any);
 
-    case REALPART_EXPR:
-    case IMAGPART_EXPR:
     case COMPONENT_REF:
-    case BIT_FIELD_REF:
     case ARROW_EXPR:
     case OFFSET_REF:
       /* -- a class member access unless its postfix-expression is
@@ -7282,6 +7284,15 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
 	 postfix-expression being a potential constant expression.  */
       if (type_unknown_p (t))
 	return true;
+      if (is_overloaded_fn (t))
+	/* In a template, a COMPONENT_REF of a function expresses ob.fn(),
+	   which uses ob as an lvalue.  */
+	want_rval = false;
+      gcc_fallthrough ();
+
+    case REALPART_EXPR:
+    case IMAGPART_EXPR:
+    case BIT_FIELD_REF:
       return RECUR (TREE_OPERAND (t, 0), want_rval);
 
     case EXPR_PACK_EXPANSION:
diff --git a/gcc/testsuite/g++.dg/cpp0x/constexpr-empty14.C b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty14.C
new file mode 100644
index 00000000000..ca4f9a55e5f
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp0x/constexpr-empty14.C
@@ -0,0 +1,10 @@
+// PR c++/91953
+// { dg-do compile { target c++11 } }
+
+struct S {};
+
+template <class T> void
+foo (S s)
+{
+  constexpr S x = s;
+}
diff --git a/gcc/testsuite/g++.dg/cpp1z/constexpr-if12.C b/gcc/testsuite/g++.dg/cpp1z/constexpr-if12.C
index f21a9896ff2..005aa80fc09 100644
--- a/gcc/testsuite/g++.dg/cpp1z/constexpr-if12.C
+++ b/gcc/testsuite/g++.dg/cpp1z/constexpr-if12.C
@@ -2,12 +2,13 @@
 // { dg-do compile { target c++17 } }
 
 struct T {
+  int i;
   constexpr auto foo() { return false; }
 };
 
 template <class MustBeTemplate>
 constexpr auto bf(T t) {
-    if constexpr(t.foo()) {	// { dg-error "constant expression" }
+    if constexpr(t.foo()) {
         return false;
     }
     return true;
-- 
2.30.2