[Format] Make getFormattingLangOpts clear LS_Auto uses LS_Cpp11 lexing rule

[MaskRay]

Summary

Suggested by sammccall in post-commit review of D65183

[MaskRay]

getFormattingLangOpts is called with unadjusted FormatStyle (deriveLocalStyle doesn't modify Style). For example, GoogleStyle uses FormatStyle::LS_Auto. We're relying on FormatStyle::LS_Auto being larger than FormatStyle::LS_Cpp11, which is brittle.

Below is a case getFormattingLangOpts is called with LS_Auto:

TEST_F(CleanUpReplacementsTest, InsertMultipleIncludesGoogleStyle) {
...
  Style = format::getGoogleStyle(format::FormatStyle::LanguageKind::LK_Cpp);
  EXPECT_EQ(Expected, apply(Code, Replaces));

[sammccall]

Thanks! (Sorry for the comment arriving late, didn't expect you to be awake!)

lib/Format/Format.cpp

tooling::Replacements sortUsingDeclarations(const FormatStyle &Style,
                                             StringRef FileName) {
   return UsingDeclarationsSorter(Environment(Code, FileName, Ranges), Style)
       .process()
       .first;
 }
 
 LangOptions getFormattingLangOpts(const FormatStyle &Style) {
   LangOptions LangOpts;
+  FormatStyle::LanguageStandard LexingStd =
+      Style.Standard == FormatStyle::LS_Auto ? FormatStyle::LS_Cpp11
+                                             : Style.Standard;
   LangOpts.CPlusPlus = 1;
-  LangOpts.CPlusPlus11 = Style.Standard >= FormatStyle::LS_Cpp11;
-  LangOpts.CPlusPlus14 = Style.Standard >= FormatStyle::LS_Cpp11;
-  LangOpts.CPlusPlus17 = Style.Standard >= FormatStyle::LS_Cpp11;
-  LangOpts.CPlusPlus2a = Style.Standard >= FormatStyle::LS_Cpp11;
+  LangOpts.CPlusPlus11 = LexingStd >= FormatStyle::LS_Cpp11;
+  LangOpts.CPlusPlus14 = LexingStd >= FormatStyle::LS_Cpp11;
+  LangOpts.CPlusPlus17 = LexingStd >= FormatStyle::LS_Cpp11;
+  LangOpts.CPlusPlus2a = LexingStd >= FormatStyle::LS_Cpp11;
   LangOpts.LineComment = 1;
   bool AlternativeOperators = Style.isCpp();
   LangOpts.CXXOperatorNames = AlternativeOperators ? 1 : 0;
   LangOpts.Bool = 1;
   LangOpts.ObjC = 1;
   LangOpts.MicrosoftExt = 1;    // To get kw___try, kw___finally.
   LangOpts.DeclSpecKeyword = 1; // To get __declspec.
   return LangOpts;