Similarly to PR107938, this also started with r11-557, whereby cp_finish_decl
can call check_initializer even in a template for a constexpr initializer.
Here we are rejecting
extern const Q q;
template<int>
constexpr auto p = q(0);
even though q has a constexpr operator(). It's deemed non-const by
decl_maybe_constant_var_p because even though 'q' is const it is not
of integral/enum type. I think the fix is for p_c_e to treat q(0) as
potentially-constant, as below.
Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk/12?
PR c++/107939
gcc/cp/ChangeLog:
* constexpr.cc (is_constexpr_function_object): New.
(potential_constant_expression_1): Treat an object with constexpr
operator() as potentially-constant.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/var-templ74.C: Remove dg-error.
* g++.dg/cpp1y/var-templ77.C: New test.
---
gcc/cp/constexpr.cc | 23 ++++++++++++++++++++++-
gcc/testsuite/g++.dg/cpp1y/var-templ74.C | 2 +-
gcc/testsuite/g++.dg/cpp1y/var-templ77.C | 14 ++++++++++++++
3 files changed, 37 insertions(+), 2 deletions(-)
create mode 100644 gcc/testsuite/g++.dg/cpp1y/var-templ77.C
diff --git a/gcc/cp/constexpr.cc b/gcc/cp/constexpr.cc
index acf9847a4d1..7d786f332b4 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -8929,6 +8929,24 @@ check_for_return_continue (tree *tp, int *walk_subtrees,
void *data)
return NULL_TREE;
}
+/* Return true iff TYPE is a class with constexpr operator(). */
+
+static bool
+is_constexpr_function_object (tree type)
+{
+ if (!CLASS_TYPE_P (type))
+ return false;
+
+ for (tree f = TYPE_FIELDS (type); f; f = DECL_CHAIN (f))
+ if (TREE_CODE (f) == FUNCTION_DECL
+ && DECL_OVERLOADED_OPERATOR_P (f)
+ && DECL_OVERLOADED_OPERATOR_IS (f, CALL_EXPR)
+ && DECL_DECLARED_CONSTEXPR_P (f))
+ return true;
+
+ return false;
+}
+
/* Return true if T denotes a potentially constant expression. Issue
diagnostic as appropriate under control of FLAGS. If WANT_RVAL is true,
an lvalue-rvalue conversion is implied. If NOW is true, we want to
@@ -9160,7 +9178,10 @@ potential_constant_expression_1 (tree t, bool want_rval,
bool strict, bool now,
}
else if (fun)
{
- if (RECUR (fun, rval))
+ if (VAR_P (fun)
+ && is_constexpr_function_object (TREE_TYPE (fun)))
+ /* Could be an object with constexpr operator(). */;
+ else if (RECUR (fun, rval))
/* Might end up being a constant function pointer. */;
else
return false;
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
index 4e2e800a6eb..c76a7d949ac 100644
--- a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
@@ -9,7 +9,7 @@ struct Q {
extern const Q q;
template<int>
-constexpr const Q* p = q(0); // { dg-bogus "not usable" "PR107939" { xfail
*-*-* } }
+constexpr const Q* p = q(0);
void
g ()
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
new file mode 100644
index 00000000000..b480f54b001
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
@@ -0,0 +1,14 @@
+// PR c++/107939
+// { dg-do compile { target c++14 } }
+
+struct Q {
+ struct P {
+ const Q* p;
+ };
+ int n;
+ constexpr P operator()(int) const { return {this}; }
+};
+
+extern const Q q;
+template<int>
+constexpr auto p = q(0);
base-commit: 9056d0df830c5a295d7594d517d409d10476990d
--
2.39.2
