[clang] [clang-format] Allow `Language: Cpp` for C files (PR #133033)

[Owen Pan via cfe-commits]

https://github.com/owenca created https://github.com/llvm/llvm-project/pull/133033

Fix #132832

>From d03c3420218eed7c2db3a5c17abb6a0b3efbba87 Mon Sep 17 00:00:00 2001
From: Owen Pan <owenpiano at gmail.com>
Date: Tue, 25 Mar 2025 21:45:00 -0700
Subject: [PATCH] [clang-format] Allow `Language: Cpp` for C files

Fix #132832
---
 clang/lib/Format/Format.cpp                | 13 ++++++++++---
 clang/unittests/Format/ConfigParseTest.cpp |  6 ++++++
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/clang/lib/Format/Format.cpp b/clang/lib/Format/Format.cpp
index c67db4752e87b..87e36578d9958 100644
--- a/clang/lib/Format/Format.cpp
+++ b/clang/lib/Format/Format.cpp
@@ -2129,10 +2129,17 @@ std::error_code parseConfiguration(llvm::MemoryBufferRef Config,
       LanguageFound = true;
   }
   if (!LanguageFound) {
-    if (Styles.empty() || Styles[0].Language != FormatStyle::LK_None)
+    if (Styles.empty())
       return make_error_code(ParseError::Unsuitable);
-    FormatStyle DefaultStyle = Styles[0];
-    DefaultStyle.Language = Language;
+    auto DefaultStyle = Styles[0];
+    auto &DefaultLanguage = DefaultStyle.Language;
+    if (DefaultLanguage != FormatStyle::LK_None &&
+        // For backward compatibility.
+        !(DefaultLanguage == FormatStyle::LK_Cpp &&
+          Language == FormatStyle::LK_C)) {
+      return make_error_code(ParseError::Unsuitable);
+    }
+    DefaultLanguage = Language;
     StyleSet.Add(std::move(DefaultStyle));
   }
   *Style = *StyleSet.Get(Language);
diff --git a/clang/unittests/Format/ConfigParseTest.cpp b/clang/unittests/Format/ConfigParseTest.cpp
index cc42642f99252..4c17c1a50face 100644
--- a/clang/unittests/Format/ConfigParseTest.cpp
+++ b/clang/unittests/Format/ConfigParseTest.cpp
@@ -1230,6 +1230,12 @@ TEST(ConfigParseTest, ParsesConfigurationWithLanguages) {
               IndentWidth, 56u);
 }
 
+TEST(ConfigParseTest, AllowCppForC) {
+  FormatStyle Style = {};
+  Style.Language = FormatStyle::LK_C;
+  EXPECT_EQ(parseConfiguration("Language: Cpp", &Style), ParseError::Success);
+}
+
 TEST(ConfigParseTest, UsesLanguageForBasedOnStyle) {
   FormatStyle Style = {};
   Style.Language = FormatStyle::LK_JavaScript;