[clang-format] Handle Trailing Whitespace After Line Continuation (P2223R2) (#145243)

Fixes #145226.

Implement
[P2223R2](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2021/p2223r2.pdf)
in clang-format to correctly handle cases where a backslash '\\' is
followed by trailing whitespace before the newline.
Previously, `clang-format` failed to properly detect and handle such
cases, leading to misformatted code.

With this, `clang-format` matches the behavior already implemented in
Clang's lexer and `DependencyDirectivesScanner.cpp`, which allow
trailing whitespace after a line continuation in any C++ standard.

1 files changed, 15 insertions, 0 deletions

diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index c0633ba..a05bf83 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -25768,6 +25768,21 @@ TEST_F(FormatTest, OperatorPassedAsAFunctionPtr) {
   verifyFormat("foo(operator, , -42);", Style);
 }
 
+TEST_F(FormatTest, LineSpliceWithTrailingWhitespace) {
+  auto Style = getLLVMStyle();
+  Style.AlignEscapedNewlines = FormatStyle::ENAS_DontAlign;
+  Style.UseTab = FormatStyle::UT_Never;
+
+  verifyFormat("int i;", "  \\  \n"
+                         "  int i;");
+  verifyFormat("#define FOO(args) \\\n"
+               "  struct a {};",
+               "#define FOO( args )   \\   \n"
+               "struct a{\\\t\t\t\n"
+               "  };",
+               Style);
+}
+
 TEST_F(FormatTest, WhitespaceSensitiveMacros) {
   FormatStyle Style = getLLVMStyle();
   Style.WhitespaceSensitiveMacros.push_back("FOO");