[clang] [clang-format] Fix a bug in recognizing trailing comments (PR #206393)

[via cfe-commits]

https://github.com/owenca created https://github.com/llvm/llvm-project/pull/206393

Test cases are borrowed/adapted from #196760.

Fixes #196663

>From 3f01a59c87268272f8533ab9ba0f78c6fc16de5b Mon Sep 17 00:00:00 2001
From: Owen Pan <owenpiano at gmail.com>
Date: Sun, 28 Jun 2026 20:02:27 -0700
Subject: [PATCH] [clang-format] Fix a bug in recognizing trailing comments

Test cases are borrowed/adapted from #196760.

Fixes #196663
---
 clang/lib/Format/WhitespaceManager.cpp        |  2 ++
 clang/unittests/Format/FormatTestComments.cpp | 36 +++++++++++++++++++
 2 files changed, 38 insertions(+)

diff --git a/clang/lib/Format/WhitespaceManager.cpp b/clang/lib/Format/WhitespaceManager.cpp
index 4a72abbfa9ec3..5596532556538 100644
--- a/clang/lib/Format/WhitespaceManager.cpp
+++ b/clang/lib/Format/WhitespaceManager.cpp
@@ -154,6 +154,7 @@ void WhitespaceManager::calculateLineBreakInformation() {
     auto &C = Changes[I];
     auto &P = Changes[I - 1];
     auto &PrevTokLength = P.TokenLength;
+    const auto *PP = I > 1 ? &Changes[I - 2] : nullptr;
     SourceLocation OriginalWhitespaceStart =
         C.OriginalWhitespaceRange.getBegin();
     SourceLocation PreviousOriginalWhitespaceEnd =
@@ -214,6 +215,7 @@ void WhitespaceManager::calculateLineBreakInformation() {
         (C.NewlinesBefore > 0 || C.Tok->is(tok::eof) ||
          (C.IsInsideToken && C.Tok->is(tok::comment))) &&
         P.Tok->is(tok::comment) &&
+        (P.NewlinesBefore == 0 || !PP || PP->IsTrailingComment) &&
         // FIXME: This is a dirty hack. The problem is that
         // BreakableLineCommentSection does comment reflow changes and here is
         // the aligning of trailing comments. Consider the case where we reflow
diff --git a/clang/unittests/Format/FormatTestComments.cpp b/clang/unittests/Format/FormatTestComments.cpp
index 707016096f7d2..d9a9f4c18efec 100644
--- a/clang/unittests/Format/FormatTestComments.cpp
+++ b/clang/unittests/Format/FormatTestComments.cpp
@@ -3060,6 +3060,42 @@ TEST_F(FormatTestComments, AlignTrailingCommentsLeave) {
                "}",
                Style);
 
+  // Move comments along, when it appears, that the indentation changed when a
+  // scope has been added or removed.
+  verifyFormat("void func() {\n"
+               "  int i;\n"
+               "  // comment\n"
+               "  // comment 2\n"
+               "}",
+               "void func() {\n"
+               "    int i;\n"
+               "    // comment\n"
+               "    // comment 2\n"
+               "}",
+               Style);
+
+  verifyFormat("void func() {\n"
+               "  // comment\n"
+               "  // comment 2\n"
+               "  int i;\n"
+               "}",
+               "void func() {\n"
+               "    // comment\n"
+               "    // comment 2\n"
+               "    int i;\n"
+               "}",
+               Style);
+
+  verifyFormat("void func() {\n"
+               "  // non-trailing comment\n"
+               "  int i;\n"
+               "}",
+               "void func() {\n"
+               "     // non-trailing comment\n"
+               "    int i;\n"
+               "}",
+               Style);
+
   Style.AlignEscapedNewlines = FormatStyle::ENAS_Left;
   verifyNoChange("#define FOO    \\\n"
                  "  /* foo(); */ \\\n"