[PATCH] D118706: [clang-format] Correctly parse C99 digraphs: "<:", ":>", "<%", "%>", "%:", "%:%:".

[Marek Kurdej via Phabricator via cfe-commits]

curdeius created this revision.
curdeius added reviewers: MyDeveloperDay, HazardyKnusperkeks, owenpan.
curdeius requested review of this revision.
Herald added a project: clang.
Herald added a subscriber: cfe-commits.

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 inconditionnally 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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D118706

Files:
  clang/lib/Format/Format.cpp
  clang/unittests/Format/FormatTest.cpp


Index: clang/unittests/Format/FormatTest.cpp
===================================================================
--- clang/unittests/Format/FormatTest.cpp
+++ clang/unittests/Format/FormatTest.cpp
@@ -24219,6 +24219,16 @@
                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
Index: clang/lib/Format/Format.cpp
===================================================================
--- clang/lib/Format/Format.cpp
+++ clang/lib/Format/Format.cpp
@@ -3242,6 +3242,10 @@
   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();


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