[clang-format] Correctly parse C99 digraphs: "<:", ":>", "<%", "%>", "%:", "%:%:".

[curdeius]

Summary

Fixes https://github.com/llvm/llvm-project/issues/31592.

This commits enables lexing of digraphs in C++11 and onwards.
Enabling them in C++03 is error-prone, as it would unconditionally treat sequences like "<:" as digraphs, even if they are followed by a single colon, e.g. "<::" would be treated as "[:" instead of "<" followed by "::". Lexing in C++11 doesn't have this problem as it looks ahead the following token.
The relevant excerpt from Lexer::LexTokenInternal:

// C++0x [lex.pptoken]p3:
//  Otherwise, if the next three characters are <:: and the subsequent
//  character is neither : nor >, the < is treated as a preprocessor
//  token by itself and not as the first character of the alternative
//  token <:.

Also, note that both clang and gcc turn on digraphs by default (-fdigraphs), so clang-format should match this behaviour.

[owenpan]

clang/lib/Format/Format.cpp

3246

“unconditionally”?

[HazardyKnusperkeks]

What the fuck. I know of trigraphs, and that we got rid of them, but these...

clang/lib/Format/Format.cpp

3246

yeah

[curdeius]

clang/lib/Format/Format.cpp

3246

Good catch. Thanks!

clang/lib/Format/Format.cpp

LangOptions getFormattingLangOpts(const FormatStyle &Style) {
   if (LexingStd == FormatStyle::LS_Latest)
     LexingStd = FormatStyle::LS_Cpp20;
   LangOpts.CPlusPlus = 1;
   LangOpts.CPlusPlus11 = LexingStd >= FormatStyle::LS_Cpp11;
   LangOpts.CPlusPlus14 = LexingStd >= FormatStyle::LS_Cpp14;
   LangOpts.CPlusPlus17 = LexingStd >= FormatStyle::LS_Cpp17;
   LangOpts.CPlusPlus20 = LexingStd >= FormatStyle::LS_Cpp20;
   LangOpts.Char8 = LexingStd >= FormatStyle::LS_Cpp20;
+  // Turning on digraphs in standards before C++0x is error-prone, because e.g.
+  // the sequence "<::" will be inconditionally treated as "[:".
+  // Cf. Lexer::LexTokenInternal.
+  LangOpts.Digraphs = 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.

clang/unittests/Format/FormatTest.cpp

TEST_F(FormatTest, AlignAfterOpenBracketBlockIndentForStatement) {
   verifyFormat("for (int myReallyLongCountVariable = 0; "
                "myReallyLongCountVariable < count;\n"
                "     myReallyLongCountVariable++) {\n"
                "  doSomething();\n"
                "}",
                Style);
 }
 
+TEST_F(FormatTest, UnderstandsDigraphs) {
+  verifyFormat("int arr<:5:> = {};");
+  verifyFormat("int arr[5] = <%%>;");
+  verifyFormat("int arr<:::qualified_variable:> = {};");
+  verifyFormat("int arr[::qualified_variable] = <%%>;");
+  verifyFormat("%:include <header>");
+  verifyFormat("%:define A x##y");
+  verifyFormat("#define A x%:%:y");
+}
+
 } // namespace
 } // namespace format
 } // namespace clang