From 85e41d06d363921590a383ad375fc434ab6fbd0a Mon Sep 17 00:00:00 2001
From: Niels Lohmann <mail@nlohmann.me>
Date: Wed, 1 Jul 2026 20:16:12 +0200
Subject: [PATCH] :green_heart: fix build

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
---
 include/nlohmann/json.hpp            | 10 +++-------
 single_include/nlohmann/json.hpp     | 10 +++-------
 tests/CMakeLists.txt                 | 16 +++++++++++++++-
 tests/fmt_formatter/project/main.cpp | 10 +++-------
 4 files changed, 24 insertions(+), 22 deletions(-)

diff --git a/include/nlohmann/json.hpp b/include/nlohmann/json.hpp
index 6a34898d7..6b9258efd 100644
--- a/include/nlohmann/json.hpp
+++ b/include/nlohmann/json.hpp
@@ -5408,11 +5408,11 @@ struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-c
             ++it;
         }
 
-        // ['#'] - "alternate form", used here to request pretty-printing
-        bool pretty = false;
+        // ['#'] - "alternate form", used here to request pretty-printing with a
+        // default indent of 4 (overridden by an explicit width below, if given)
         if (it != end && *it == '#')
         {
-            pretty = true;
+            indent = 4;
             ++it;
         }
 
@@ -5428,10 +5428,6 @@ struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-c
                 ++it;
             }
         }
-        else if (pretty)
-        {
-            indent = 4;
-        }
 
         // sign, the '0' flag, precision, locale-specific formatting ('L'), dynamic
         // width/precision ("{...}"), and type characters all have no meaning for
diff --git a/single_include/nlohmann/json.hpp b/single_include/nlohmann/json.hpp
index 544d8ddd5..3d55cb622 100644
--- a/single_include/nlohmann/json.hpp
+++ b/single_include/nlohmann/json.hpp
@@ -25934,11 +25934,11 @@ struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-c
             ++it;
         }
 
-        // ['#'] - "alternate form", used here to request pretty-printing
-        bool pretty = false;
+        // ['#'] - "alternate form", used here to request pretty-printing with a
+        // default indent of 4 (overridden by an explicit width below, if given)
         if (it != end && *it == '#')
         {
-            pretty = true;
+            indent = 4;
             ++it;
         }
 
@@ -25954,10 +25954,6 @@ struct formatter<nlohmann::NLOHMANN_BASIC_JSON_TPL, char> // NOLINT(cert-dcl58-c
                 ++it;
             }
         }
-        else if (pretty)
-        {
-            indent = 4;
-        }
 
         // sign, the '0' flag, precision, locale-specific formatting ('L'), dynamic
         // width/precision ("{...}"), and type characters all have no meaning for
diff --git a/tests/CMakeLists.txt b/tests/CMakeLists.txt
index 027881095..317ada91f 100644
--- a/tests/CMakeLists.txt
+++ b/tests/CMakeLists.txt
@@ -195,4 +195,18 @@ add_subdirectory(cmake_add_subdirectory)
 add_subdirectory(cmake_fetch_content)
 add_subdirectory(cmake_fetch_content2)
 add_subdirectory(cmake_target_include_directories)
-add_subdirectory(fmt_formatter)
+
+# fmt (fetched by tests/fmt_formatter) requires a genuinely modern, C++17-capable
+# toolchain; skip it on legacy/niche toolchains where fmt itself is known not to build
+set(JSON_FMT_FORMATTER_TEST_SUPPORTED ${compiler_supports_cpp_17})
+# fmt 12's 128-bit integer emulation does not build with 32-bit MinGW
+if (MINGW AND CMAKE_SIZEOF_VOID_P EQUAL 4)
+    set(JSON_FMT_FORMATTER_TEST_SUPPORTED FALSE)
+endif()
+# the MSVC STL rejects Clang versions older than 19 as a host compiler
+if (WIN32 AND CMAKE_CXX_COMPILER_ID STREQUAL "Clang" AND CMAKE_CXX_COMPILER_VERSION VERSION_LESS 19.0)
+    set(JSON_FMT_FORMATTER_TEST_SUPPORTED FALSE)
+endif()
+if (JSON_FMT_FORMATTER_TEST_SUPPORTED)
+    add_subdirectory(fmt_formatter)
+endif()
diff --git a/tests/fmt_formatter/project/main.cpp b/tests/fmt_formatter/project/main.cpp
index c0cecdc23..d50632249 100644
--- a/tests/fmt_formatter/project/main.cpp
+++ b/tests/fmt_formatter/project/main.cpp
@@ -35,11 +35,11 @@ struct fmt::formatter<nlohmann::json>
             ++it;
         }
 
-        // ['#'] - "alternate form", used here to request pretty-printing
-        bool pretty = false;
+        // ['#'] - "alternate form", used here to request pretty-printing with a
+        // default indent of 4 (overridden by an explicit width below, if given)
         if (it != end && *it == '#')
         {
-            pretty = true;
+            indent = 4;
             ++it;
         }
 
@@ -54,10 +54,6 @@ struct fmt::formatter<nlohmann::json>
                 ++it;
             }
         }
-        else if (pretty)
-        {
-            indent = 4;
-        }
 
         if (it != end && *it != '}')
         {