[clang] [clang-format] Propagate `LeadingEmptyLinesAffected` when joining lines (PR #146761)

[Eric Li via cfe-commits]

https://github.com/tJener updated https://github.com/llvm/llvm-project/pull/146761

>From 61e2f8e1d79563ad9bd9478e97ee1d597518f305 Mon Sep 17 00:00:00 2001
From: Eric Li <li.zhe.hua at gmail.com>
Date: Wed, 2 Jul 2025 14:46:45 -0400
Subject: [PATCH 1/2] [clang-format] Propagate `LeadingEmptyLinesAffected` when
 joining lines

Before this commit, when `LineJoiner` joins a line with affected
leading whitespace, it would drop the knowledge of this
entirely. However, when the `AffectedRangeManager` is computing the
affected lines, the leading empty whitespace lines are potentially
considered for non-first tokens in the `AnnotatedLine`. This causes a
discrepancy in behavior when an `AnnotatedLine` is put together from
joining multiple lines versus when it is not.

We change `LineJoiner::join` to follow `AffectedRangeManager`'s logic,
considering the leading whitespace when determining `Affected` for a
token if the previous token did not have children.

https://github.com/llvm/llvm-project/blob/a63f57262898588b576d66e5fd79c0aa64b35f2d/clang/lib/Format/AffectedRangeManager.cpp#L111-L130

Fixes #138942.
---
 clang/lib/Format/UnwrappedLineFormatter.cpp    | 2 ++
 clang/unittests/Format/FormatTestSelective.cpp | 2 ++
 2 files changed, 4 insertions(+)

diff --git a/clang/lib/Format/UnwrappedLineFormatter.cpp b/clang/lib/Format/UnwrappedLineFormatter.cpp
index f2ed027b2c047..d22bfc61ed736 100644
--- a/clang/lib/Format/UnwrappedLineFormatter.cpp
+++ b/clang/lib/Format/UnwrappedLineFormatter.cpp
@@ -988,6 +988,8 @@ class LineJoiner {
     assert(!B.First->Previous);
     if (B.Affected)
       A.Affected = true;
+    else if (B.LeadingEmptyLinesAffected && A.Last->Children.empty())
+      A.Affected = true;
     A.Last->Next = B.First;
     B.First->Previous = A.Last;
     B.First->CanBreakBefore = true;
diff --git a/clang/unittests/Format/FormatTestSelective.cpp b/clang/unittests/Format/FormatTestSelective.cpp
index 624684c7a079b..3500b7d47e99d 100644
--- a/clang/unittests/Format/FormatTestSelective.cpp
+++ b/clang/unittests/Format/FormatTestSelective.cpp
@@ -41,6 +41,8 @@ TEST_F(FormatTestSelective, RemovesTrailingWhitespaceOfFormattedLine) {
   EXPECT_EQ("int a;", format("int a;         ", 0, 0));
   EXPECT_EQ("int a;\n", format("int a;  \n   \n   \n ", 0, 0));
   EXPECT_EQ("int a;\nint b;    ", format("int a;  \nint b;    ", 0, 0));
+
+  EXPECT_EQ("void f() {}\n", format("void f() {\n  \n}\n", 11, 0));
 }
 
 TEST_F(FormatTestSelective, FormatsCorrectRegionForLeadingWhitespace) {

>From 3f76d11c572d3f58c2314c90047e6e6fb120694b Mon Sep 17 00:00:00 2001
From: Eric Li <li.zhe.hua at gmail.com>
Date: Wed, 2 Jul 2025 15:49:41 -0400
Subject: [PATCH 2/2] fixup! [clang-format] Propagate
 `LeadingEmptyLinesAffected` when joining lines

---
 clang/lib/Format/UnwrappedLineFormatter.cpp | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/clang/lib/Format/UnwrappedLineFormatter.cpp b/clang/lib/Format/UnwrappedLineFormatter.cpp
index d22bfc61ed736..6432465ce57f3 100644
--- a/clang/lib/Format/UnwrappedLineFormatter.cpp
+++ b/clang/lib/Format/UnwrappedLineFormatter.cpp
@@ -986,9 +986,7 @@ class LineJoiner {
   void join(AnnotatedLine &A, const AnnotatedLine &B) {
     assert(!A.Last->Next);
     assert(!B.First->Previous);
-    if (B.Affected)
-      A.Affected = true;
-    else if (B.LeadingEmptyLinesAffected && A.Last->Children.empty())
+    if (B.Affected || (B.LeadingEmptyLinesAffected && A.Last->Children.empty()))
       A.Affected = true;
     A.Last->Next = B.First;
     B.First->Previous = A.Last;