From e09bc034d1b4d692b409fa5af52ae34480a6f4dc Mon Sep 17 00:00:00 2001
From: Marek Polacek <polacek@redhat.com>
Date: Fri, 3 Mar 2023 11:24:24 -0500
Subject: [PATCH] c++: error with constexpr operator() [PR107939]
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.
If fun is not a function pointer, we don't know if we're using it as an
lvalue or rvalue, so with this patch we pass 'any' for want_rval. With
that, p_c_e/VAR_DECL doesn't flat out reject the underlying VAR_DECL.
PR c++/107939
gcc/cp/ChangeLog:
* constexpr.cc (potential_constant_expression_1) <case CALL_EXPR>: Pass
'any' when recursing on a VAR_DECL and not a pointer to function.
gcc/testsuite/ChangeLog:
* g++.dg/cpp1y/var-templ74.C: Remove dg-error.
* g++.dg/cpp1y/var-templ77.C: New test.
---
gcc/cp/constexpr.cc | 8 ++++--
gcc/testsuite/g++.dg/cpp1y/var-templ74.C | 2 +-
gcc/testsuite/g++.dg/cpp1y/var-templ77.C | 32 ++++++++++++++++++++++++
3 files changed, 39 insertions(+), 3 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 364695b762c6..3079561f2e88 100644
--- a/gcc/cp/constexpr.cc
+++ b/gcc/cp/constexpr.cc
@@ -9179,8 +9179,12 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
}
else if (fun)
{
- if (RECUR (fun, rval))
- /* Might end up being a constant function pointer. */;
+ if (RECUR (fun, FUNCTION_POINTER_TYPE_P (fun) ? rval : any))
+ /* Might end up being a constant function pointer. But it
+ could also be a function object with constexpr op(), so
+ we pass 'any' so that the underlying VAR_DECL is deemed
+ as potentially-constant even though it wasn't declared
+ constexpr. */;
else
return false;
}
diff --git a/gcc/testsuite/g++.dg/cpp1y/var-templ74.C b/gcc/testsuite/g++.dg/cpp1y/var-templ74.C
index 4e2e800a6ebb..c76a7d949ac9 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 000000000000..0c56d70a0342
--- /dev/null
+++ b/gcc/testsuite/g++.dg/cpp1y/var-templ77.C
@@ -0,0 +1,32 @@
+// 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);
+static_assert(p<0>.p == &q, "");
+
+constexpr int
+fn (int)
+{
+ return 42;
+}
+
+struct Sur {
+ using FN = int(int);
+ constexpr operator FN*() const { return &fn; }
+};
+
+extern const Sur sur;
+template<int>
+constexpr int aja = sur (0);
+static_assert(aja<0> == 42, "");
+static_assert(sur(1) == 42, "");
--
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