[clang-format] Correctly handle backward compatibility of C headers (llvm#159908)

This in effect reverts 05fb840 and
7e1a88b, the latter of which
erroneously changed the behavior of formatting `ObjC` header files when
both the default and `ObjC` styles were absent. Now the previous
behavior of treating that as an error is restored.

Fixes llvm#158704

(cherry picked from commit d7921de)

Author: owenca

clang/lib/Format/Format.cpp

@@ -2133,47 +2133,68 @@ std::error_code parseConfiguration(llvm::MemoryBufferRef Config,
   if (Input.error())
     return Input.error();
 
-  for (unsigned i = 0; i < Styles.size(); ++i) {
-    // Ensures that only the first configuration can skip the Language option.
-    if (Styles[i].Language == FormatStyle::LK_None && i != 0)
+  assert(!Styles.empty());
+  const auto StyleCount = Styles.size();
+
+  // Start from the second style as (only) the first one may be the default.
+  for (unsigned I = 1; I < StyleCount; ++I) {
+    const auto Lang = Styles[I].Language;
+    if (Lang == FormatStyle::LK_None)
       return make_error_code(ParseError::Error);
     // Ensure that each language is configured at most once.
-    for (unsigned j = 0; j < i; ++j) {
-      if (Styles[i].Language == Styles[j].Language) {
+    for (unsigned J = 0; J < I; ++J) {
+      if (Lang == Styles[J].Language) {
         LLVM_DEBUG(llvm::dbgs()
                    << "Duplicate languages in the config file on positions "
-                   << j << " and " << i << "\n");
+                   << J << " and " << I << '\n');
         return make_error_code(ParseError::Error);
       }
     }
   }
-  // Look for a suitable configuration starting from the end, so we can
-  // find the configuration for the specific language first, and the default
-  // configuration (which can only be at slot 0) after it.
-  FormatStyle::FormatStyleSet StyleSet;
-  bool LanguageFound = false;
-  for (const FormatStyle &Style : llvm::reverse(Styles)) {
-    const auto Lang = Style.Language;
-    if (Lang != FormatStyle::LK_None)
-      StyleSet.Add(Style);
-    if (Lang == Language ||
-        // For backward compatibility.
-        (Lang == FormatStyle::LK_Cpp && Language == FormatStyle::LK_C)) {
-      LanguageFound = true;
-    } else if (IsDotHFile && Language == FormatStyle::LK_Cpp &&
-               (Lang == FormatStyle::LK_C || Lang == FormatStyle::LK_ObjC)) {
-      Language = Lang;
-      LanguageFound = true;
+
+  int LanguagePos = -1; // Position of the style for Language.
+  int CppPos = -1;      // Position of the style for C++.
+  int CPos = -1;        // Position of the style for C.
+
+  // Search Styles for Language and store the positions of C++ and C styles in
+  // case Language is not found.
+  for (unsigned I = 0; I < StyleCount; ++I) {
+    const auto Lang = Styles[I].Language;
+    if (Lang == Language) {
+      LanguagePos = I;
+      break;
     }
-  }
-  if (!LanguageFound) {
-    if (Styles.empty() || Styles[0].Language != FormatStyle::LK_None)
+    if (Lang == FormatStyle::LK_Cpp)
+      CppPos = I;
+    else if (Lang == FormatStyle::LK_C)
+      CPos = I;
+  }
+
+  // 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.
+      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);
-    FormatStyle DefaultStyle = Styles[0];
-    DefaultStyle.Language = Language;
-    StyleSet.Add(std::move(DefaultStyle));
   }
-  *Style = *StyleSet.Get(Language);
+
+  for (const auto &S : llvm::reverse(llvm::drop_begin(Styles)))
+    Style->StyleSet.Add(S);
+
+  *Style = Styles[LanguagePos];
+
+  if (LanguagePos == 0) {
+    if (Style->Language == FormatStyle::LK_None) // Default style.
+      Style->Language = Language;
+    Style->StyleSet.Add(*Style);
+  }
+
   if (Style->InsertTrailingCommas != FormatStyle::TCS_None &&
       Style->BinPackArguments) {
     // See comment on FormatStyle::TSC_Wrapped.
@@ -2204,14 +2225,8 @@ FormatStyle::FormatStyleSet::Get(FormatStyle::LanguageKind Language) const {
   if (!Styles)
     return std::nullopt;
   auto It = Styles->find(Language);
-  if (It == Styles->end()) {
-    if (Language != FormatStyle::LK_C)
-      return std::nullopt;
-    // For backward compatibility.
-    It = Styles->find(FormatStyle::LK_Cpp);
-    if (It == Styles->end())
-      return std::nullopt;
-  }
+  if (It == Styles->end())
+    return std::nullopt;
   FormatStyle Style = It->second;
   Style.StyleSet = *this;
   return Style;

clang/unittests/Format/ConfigParseTest.cpp

@@ -1269,7 +1269,7 @@ TEST(ConfigParseTest, AllowCppForC) {
             ParseError::Success);
 }
 
-TEST(ConfigParseTest, HandleNonCppDotHFile) {
+TEST(ConfigParseTest, HandleDotHFile) {
   FormatStyle Style = {};
   Style.Language = FormatStyle::LK_Cpp;
   EXPECT_EQ(parseConfiguration("Language: C", &Style,
@@ -1280,11 +1280,14 @@ TEST(ConfigParseTest, HandleNonCppDotHFile) {
 
   Style = {};
   Style.Language = FormatStyle::LK_Cpp;
-  EXPECT_EQ(parseConfiguration("Language: ObjC", &Style,
+  EXPECT_EQ(parseConfiguration("Language: Cpp\n"
+                               "...\n"
+                               "Language: C",
+                               &Style,
                                /*AllowUnknownOptions=*/false,
                                /*IsDotHFile=*/true),
             ParseError::Success);
-  EXPECT_EQ(Style.Language, FormatStyle::LK_ObjC);
+  EXPECT_EQ(Style.Language, FormatStyle::LK_Cpp);
 }
 
 TEST(ConfigParseTest, UsesLanguageForBasedOnStyle) {