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.
Tested x86_64-pc-linux-gnu, applying to trunk.
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/constexpr.c | 19 +++++++++++++++----
.../g++.dg/cpp0x/constexpr-empty14.C | 10 ++++++++++
gcc/testsuite/g++.dg/cpp1z/constexpr-if12.C | 3 ++-
3 files changed, 27 insertions(+), 5 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp0x/constexpr-empty14.C
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;
base-commit: 19e43cbce353b63a05c3b7c39d83a2e32c9f911f
--
2.18.1
