From 8978ab09b2577c374494d55c574b72d317f908a3 Mon Sep 17 00:00:00 2001
From: Barry Revzin <barry.revzin@gmail.com>
Date: Sun, 27 Apr 2025 11:32:10 -0500
Subject: [PATCH] Avoiding __builtin_strlen (#4429)

---
 include/fmt/base.h |  2 +-
 test/base-test.cc  | 23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/include/fmt/base.h b/include/fmt/base.h
index 39ef747c..3395e4ab 100644
--- a/include/fmt/base.h
+++ b/include/fmt/base.h
@@ -539,7 +539,7 @@ template <typename Char> class basic_string_view {
 #endif
   FMT_CONSTEXPR20 basic_string_view(const Char* s) : data_(s) {
 #if FMT_HAS_BUILTIN(__builtin_strlen) || FMT_GCC_VERSION || FMT_CLANG_VERSION
-    if (std::is_same<Char, char>::value) {
+    if (std::is_same<Char, char>::value && !detail::is_constant_evaluated()) {
       size_ = __builtin_strlen(detail::narrow(s));
       return;
     }
diff --git a/test/base-test.cc b/test/base-test.cc
index f8af275d..0d8ae91b 100644
--- a/test/base-test.cc
+++ b/test/base-test.cc
@@ -92,6 +92,29 @@ TEST(string_view_test, compare) {
   check_op<std::greater_equal>();
 }
 
+#if FMT_USE_CONSTEVAL
+namespace {
+
+template <size_t N>
+struct fixed_string {
+    char data[N] = {};
+
+    constexpr fixed_string(char const (&m)[N]) {
+        for (size_t i = 0; i != N; ++i) {
+            data[i] = m[i];
+        }
+    }
+};
+
+}
+
+TEST(string_view_test, from_constexpr_fixed_string) {
+  static constexpr auto fs = fixed_string<5>("x={}");
+  static constexpr auto fmt = fmt::string_view(fs.data);
+  EXPECT_EQ(fmt, "x={}");
+}
+#endif
+
 TEST(base_test, is_locking) {
   EXPECT_FALSE(fmt::detail::is_locking<const char(&)[3]>());
 }