From 0e601c34de26fe5b46f4c62dab2039efcb0acaed Mon Sep 17 00:00:00 2001
From: Yat Ho <lagoho7@gmail.com>
Date: Fri, 26 Jun 2026 03:21:59 +0800
Subject: [PATCH] fix: explicitly check `operator[]` in `is_contiguous` (#4825)

Prevent false positives when the container itself does not have `operator[](size_t)`, but the target type of one of the non-explicit user-defined conversion functions does.
---
 include/fmt/base.h |  7 ++++---
 test/base-test.cc  | 19 +++++++++++++++++++
 2 files changed, 23 insertions(+), 3 deletions(-)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index 5d0241c8..60d1b792 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -493,9 +493,10 @@ inline FMT_CONSTEXPR auto get_container(OutputIt it) ->
 template <typename T, typename Enable = void>
 struct is_contiguous : std::false_type {};
 template <typename T>
-struct is_contiguous<T, void_t<decltype(std::declval<T&>().data()),
-                               decltype(std::declval<T&>().size()),
-                               decltype(std::declval<T&>()[size_t()])>>
+struct is_contiguous<T,
+                     void_t<decltype(std::declval<T&>().data()),
+                            decltype(std::declval<T&>().size()),
+                            decltype(std::declval<T&>().operator[](size_t()))>>
     : std::true_type {};
 }  // namespace detail
 
diff --git a/test/base-test.cc b/test/base-test.cc
index 40cc3810..92415cbd 100644
--- a/test/base-test.cc
+++ b/test/base-test.cc
@@ -874,6 +874,24 @@ struct custom_container {
   auto operator[](size_t) -> char& { return data; }
 };
 
+struct phantom_subscript_operator_container {
+  std::vector<char> buffer;
+
+  using value_type = char;
+
+  auto size() const noexcept -> size_t { return buffer.size(); }
+
+  auto data() noexcept -> char* { return buffer.data(); }
+
+  auto data() const noexcept -> const char* { return buffer.data(); }
+
+  void resize(size_t n) { buffer.resize(n); }
+
+  void push_back(const char& value) { buffer.push_back(value); }
+
+  operator const char*() const noexcept { return data(); }
+};
+
 FMT_BEGIN_NAMESPACE
 template <> struct is_contiguous<custom_container> : std::true_type {};
 FMT_END_NAMESPACE
@@ -884,6 +902,7 @@ TEST(base_test, is_contiguous) {
   EXPECT_TRUE((fmt::is_contiguous<fmt::string_view>::value));
   EXPECT_TRUE((fmt::is_contiguous<std::vector<char>>::value));
   EXPECT_FALSE((fmt::is_contiguous<std::list<char>>::value));
+  EXPECT_FALSE((fmt::is_contiguous<phantom_subscript_operator_container>::value));
 }
 
 TEST(base_test, format_to_custom_container) {