[libcxx-commits] [libcxx] [libc++][format] Fixes nested concept evaluation. (PR #85548)
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Sat Mar 16 12:11:27 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-libcxx
Author: Mark de Wever (mordante)
<details>
<summary>Changes</summary>
Before the __formattable concept depended on itself in a contrieved example. By using the underlying concept directly the cycle is broken.
Fixes https://github.com/llvm/llvm-project/issues/81590
---
Full diff: https://github.com/llvm/llvm-project/pull/85548.diff
2 Files Affected:
- (modified) libcxx/include/__format/format_arg_store.h (+1-2)
- (added) libcxx/test/std/utilities/format/format.functions/bug_81590.compile.pass.cpp (+35)
``````````diff
diff --git a/libcxx/include/__format/format_arg_store.h b/libcxx/include/__format/format_arg_store.h
index 066cd369eb8918..23a599e9957599 100644
--- a/libcxx/include/__format/format_arg_store.h
+++ b/libcxx/include/__format/format_arg_store.h
@@ -151,7 +151,7 @@ consteval __arg_t __determine_arg_t() {
// The overload for not formattable types allows triggering the static
// assertion below.
template <class _Context, class _Tp>
- requires(!__formattable<_Tp, typename _Context::char_type>)
+ requires(!__formattable_with<_Tp, _Context>)
consteval __arg_t __determine_arg_t() {
return __arg_t::__none;
}
@@ -165,7 +165,6 @@ _LIBCPP_HIDE_FROM_ABI basic_format_arg<_Context> __create_format_arg(_Tp& __valu
using _Dp = remove_const_t<_Tp>;
constexpr __arg_t __arg = __determine_arg_t<_Context, _Dp>();
static_assert(__arg != __arg_t::__none, "the supplied type is not formattable");
-
static_assert(__formattable_with<_Tp, _Context>);
// Not all types can be used to directly initialize the
diff --git a/libcxx/test/std/utilities/format/format.functions/bug_81590.compile.pass.cpp b/libcxx/test/std/utilities/format/format.functions/bug_81590.compile.pass.cpp
new file mode 100644
index 00000000000000..0969d3bc21dfad
--- /dev/null
+++ b/libcxx/test/std/utilities/format/format.functions/bug_81590.compile.pass.cpp
@@ -0,0 +1,35 @@
+//===----------------------------------------------------------------------===//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// UNSUPPORTED: c++03, c++11, c++14, c++17
+// UNSUPPORTED: GCC-ALWAYS_INLINE-FIXME
+
+// The sample code is based on the bug report
+// https://github.com/llvm/llvm-project/issues/81590
+//
+// Tests whether this formatter does not fail to compile due to nested concept
+// evaluation.
+
+#include <format>
+#include <variant>
+
+struct X : std::variant<X*> {
+ X* p = nullptr;
+ constexpr const std::variant<X*>& decay() const noexcept { return *this; }
+};
+
+template <>
+struct std::formatter<X, char> : std::formatter<std::string, char> {
+ static constexpr auto format(const X& x, auto ctx) {
+ if (!x.p)
+ return ctx.out();
+ auto m = [&](const X* t) { return std::format_to(ctx.out(), "{}", *t); };
+ return std::visit(m, x.decay());
+ }
+};
+
+void bug_81590() { (void)std::format("{}", X{}); }
``````````
</details>
https://github.com/llvm/llvm-project/pull/85548
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