[gcc r9-8323] c++: Allow parm of empty class type in constexpr.

[Jason Merrill]

https://gcc.gnu.org/g:b4e53e9b3c963b1c0fe9637618dec8042764f599

commit r9-8323-gb4e53e9b3c963b1c0fe9637618dec8042764f599
Author: Jason Merrill <jason@redhat.com>
Date:   Mon Mar 2 14:42:47 2020 -0500

    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.
    
    gcc/cp/ChangeLog
    2020-03-02  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.

Diff:
---
 gcc/cp/ChangeLog                               |  8 ++++++++
 gcc/cp/constexpr.c                             | 19 +++++++++++++++----
 gcc/testsuite/g++.dg/cpp0x/constexpr-empty14.C | 10 ++++++++++
 gcc/testsuite/g++.dg/cpp1z/constexpr-if12.C    |  3 ++-
 4 files changed, 35 insertions(+), 5 deletions(-)

diff --git a/gcc/cp/ChangeLog b/gcc/cp/ChangeLog
index 5a90824..7692a8f 100644
--- a/gcc/cp/ChangeLog
+++ b/gcc/cp/ChangeLog
@@ -1,5 +1,13 @@
 2020-03-02  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.
+
+2020-03-02  Jason Merrill  <jason@redhat.com>
+
 	PR c++/88256
 	* cp-gimplify.c (predeclare_vla): New.
 	(cp_genericize_r) [NOP_EXPR]: Call it.
diff --git a/gcc/cp/constexpr.c b/gcc/cp/constexpr.c
index bc2476f..2006ad9 100644
--- a/gcc/cp/constexpr.c
+++ b/gcc/cp/constexpr.c
@@ -5996,8 +5996,13 @@ 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;
@@ -6238,10 +6243,7 @@ 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
@@ -6250,6 +6252,15 @@ 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 0000000..ca4f9a5
--- /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 f21a989..005aa80 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;