[clang] [clang-format] Handle multi-section config with InheritParentConfig (PR #224816)

[via cfe-commits]

https://github.com/owenca created https://github.com/llvm/llvm-project/pull/224816

Fixes #217085

>From 8ce4d7c7f71e12bfe24a304c6f7b5ff6c21179ea Mon Sep 17 00:00:00 2001
From: Owen Pan <owenpiano at gmail.com>
Date: Sat, 19 Sep 2026 00:04:04 -0700
Subject: [PATCH] [clang-format] Handle multi-section config with
 InheritParentConfig

Fixes #217085
---
 clang/lib/Format/Format.cpp                 | 30 ++++++++++++---------
 clang/test/Format/inherit-parent-config.cpp | 21 +++++++++++++++
 2 files changed, 38 insertions(+), 13 deletions(-)
 create mode 100644 clang/test/Format/inherit-parent-config.cpp

diff --git a/clang/lib/Format/Format.cpp b/clang/lib/Format/Format.cpp
index 4c78c1dbe9f80f..a60301d78520b7 100644
--- a/clang/lib/Format/Format.cpp
+++ b/clang/lib/Format/Format.cpp
@@ -2568,30 +2568,34 @@ std::error_code parseConfiguration(llvm::MemoryBufferRef Config,
       CPos = I;
   }
 
+  auto &Style0 = Styles[0];
+  const bool HasDefaultStyle = Style0.Language == FormatStyle::LK_None;
+
   // If Language is not found, use the default style if there is one. Otherwise,
   // use the C style for C++ .h files and for backward compatibility, the C++
   // style for .c files.
   if (LanguagePos < 0) {
-    if (Styles[0].Language == FormatStyle::LK_None) // Default style.
+    if (HasDefaultStyle) {
       LanguagePos = 0;
-    else if (IsDotHFile && Language == FormatStyle::LK_Cpp)
-      LanguagePos = CPos;
-    else if (!IsDotHFile && Language == FormatStyle::LK_C)
-      LanguagePos = CppPos;
-    if (LanguagePos < 0)
-      return make_error_code(ParseError::Unsuitable);
+      Style0.Language = Language;
+    } else {
+      if (IsDotHFile && Language == FormatStyle::LK_Cpp)
+        LanguagePos = CPos;
+      else if (!IsDotHFile && Language == FormatStyle::LK_C)
+        LanguagePos = CppPos;
+      if (LanguagePos < 0)
+        return make_error_code(ParseError::Unsuitable);
+      Language = Styles[LanguagePos].Language;
+    }
   }
 
   for (const auto &S : llvm::reverse(llvm::drop_begin(Styles)))
     Style->StyleSet.Add(S);
 
-  *Style = Styles[LanguagePos];
+  if (!HasDefaultStyle || LanguagePos == 0)
+    Style->StyleSet.Add(Style0);
 
-  if (LanguagePos == 0) {
-    if (Style->Language == FormatStyle::LK_None) // Default style.
-      Style->Language = Language;
-    Style->StyleSet.Add(*Style);
-  }
+  *Style = *Style->StyleSet.Get(Language);
 
   if (Style->InsertTrailingCommas != FormatStyle::TCS_None &&
       (Style->PackArguments.BinPack == FormatStyle::BPAS_BinPack ||
diff --git a/clang/test/Format/inherit-parent-config.cpp b/clang/test/Format/inherit-parent-config.cpp
new file mode 100644
index 00000000000000..9b70b8cabc6d8e
--- /dev/null
+++ b/clang/test/Format/inherit-parent-config.cpp
@@ -0,0 +1,21 @@
+// RUN: rm -rf %t.dir
+// RUN: mkdir -p %t.dir/code
+
+// RUN: cd %t.dir
+// RUN: echo "BasedOnStyle: Google" > .clang-format
+// RUN: echo "IndentWidth: 4" >> .clang-format
+
+// RUN: cd code
+// RUN: echo "BasedOnStyle: InheritParentConfig" > .clang-format
+// RUN: echo "---" >> .clang-format
+// RUN: echo "Language: Cpp" >> .clang-format
+
+// RUN: clang-format -style=file:.clang-format %s \
+// RUN:   | FileCheck %s --strict-whitespace
+// CHECK: {{^ {8}//}}
+
+s = R"CPP(
+    void foo() {
+  // "IndentWidth: 4" applies here, resulting in 8 leading spaces.
+    }
+)CPP";