[clang] [Clang] Defaults `-Wunicode-whitespace` to an error. (PR #210945)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin created https://github.com/llvm/llvm-project/pull/210945

Clang accepts some Unicode whitespaces in some context. There are a few issues with this:
 - The support is incomplete and inconsistent, as illustrated in #38934
 - We are not consistent with the Unicode specs (tr1, tr55) in that we treat U+0028/0+0029 as horizontal separators while Unicode consider them vertical.

Ultimately, Unicode whitespaces are more likely than not unintended.
Neither GCC nor MSVC support this extension.

Fixes #38934.

>From 9c66ee3a15d800101d63a02152ed5d567229822c Mon Sep 17 00:00:00 2001
From: Corentin Jabot <corentinjabot at gmail.com>
Date: Tue, 21 Jul 2026 12:39:29 +0200
Subject: [PATCH] [Clang] Defaults `-Wunicode-whitespace` to an error.

Clang accepts some Unicode whitespaces in some context.
There are a few issues with this:
 - The support is incomplete and inconsistant, as illustrated in #38934
 - We are not consistent with the Unicode specs (tr1, tr55)
   in that we treat U+0028/0+0029 as horizontal separators while
   Unicode consider them vertical.

Ultimately, Unicode whitespaces are more likely than not
unintended.
Neither GCC nor MSVC support this extension.

Fixes #38934.
---
 clang/docs/ReleaseNotes.md                      | 5 +++++
 clang/include/clang/Basic/DiagnosticLexKinds.td | 4 ++--
 clang/lib/Lex/Lexer.cpp                         | 1 +
 clang/test/Analysis/mig.mm                      | 2 +-
 clang/test/Lexer/unicode.c                      | 4 ++--
 5 files changed, 11 insertions(+), 5 deletions(-)

diff --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md
index 9301745b9628e..8e79ec77198a0 100644
--- a/clang/docs/ReleaseNotes.md
+++ b/clang/docs/ReleaseNotes.md
@@ -46,6 +46,11 @@ latest release, please see the [Clang Web Site](https://clang.llvm.org) or the
 
 ### C/C++ Language Potentially Breaking Changes
 
+- `-Wunicode-whitespace` now defaults to an error.
+The previous behavior can be restored with `-Wno-error=unicode-whitespace`.
+Clang will stop accepting non-ascii whitespaces as token seperator
+in a future version of Clang.
+
 ### C++ Specific Potentially Breaking Changes
 
 ### ABI Changes in This Version
diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
index 79e74a846e3ea..3995c757b99c4 100644
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -137,8 +137,8 @@ def err_character_not_allowed : Error<
 def err_character_not_allowed_identifier : Error<
   "character <U+%0> not allowed %select{in|at the start of}1 an identifier">;
 def ext_unicode_whitespace : ExtWarn<
-  "treating Unicode character as whitespace">,
-  InGroup<DiagGroup<"unicode-whitespace">>;
+  "treating character <U+%0> as whitespace">,
+  InGroup<DiagGroup<"unicode-whitespace">>, DefaultError;
 def warn_utf8_symbol_homoglyph : Warning<
   "treating Unicode character <U+%0> as an identifier character rather than "
   "as '%1' symbol">, InGroup<DiagGroup<"unicode-homoglyph">>;
diff --git a/clang/lib/Lex/Lexer.cpp b/clang/lib/Lex/Lexer.cpp
index d5085ca6d4c8a..07ddb18c52de7 100644
--- a/clang/lib/Lex/Lexer.cpp
+++ b/clang/lib/Lex/Lexer.cpp
@@ -3778,6 +3778,7 @@ bool Lexer::CheckUnicodeWhitespace(Token &Result, uint32_t C,
   if (!isLexingRawMode() && !PP->isPreprocessedOutput() &&
       isUnicodeWhitespace(C)) {
     Diag(BufferPtr, diag::ext_unicode_whitespace)
+      << EscapeSingleCodepointForDiagnostic(C)
       << makeCharRange(*this, BufferPtr, CurPtr);
 
     Result.setFlag(Token::LeadingSpace);
diff --git a/clang/test/Analysis/mig.mm b/clang/test/Analysis/mig.mm
index e8d08f355d3ea..954eccc1a01ca 100644
--- a/clang/test/Analysis/mig.mm
+++ b/clang/test/Analysis/mig.mm
@@ -166,7 +166,7 @@ void test_block() {
       ^MIG_SERVER_ROUTINE (mach_port_name_t port,
                            vm_address_t address, vm_size_t size) {
         vm_deallocate(port, address, size); // expected-note{{Value passed through parameter 'address' is deallocated}}
-        return KERN_ERROR; // expected-warning{{MIG callback fails with error after deallocating argument value. This is a use-after-free vulnerability because the caller will try to deallocate it again}}
+        return KERN_ERROR; // expected-warning{{MIG callback fails with error after deallocating argument value. This is a use-after-free vulnerability because the caller will try to deallocate it again}}
                            // expected-note at -1{{MIG callback fails with error after deallocating argument value. This is a use-after-free vulnerability because the caller will try to deallocate it again}}
       };
 }
diff --git a/clang/test/Lexer/unicode.c b/clang/test/Lexer/unicode.c
index 5add2e49e4dfb..4e71cdaed772a 100644
--- a/clang/test/Lexer/unicode.c
+++ b/clang/test/Lexer/unicode.c
@@ -7,8 +7,8 @@
 
 // This file contains Unicode characters; please do not "fix" them!
 
-extern int x; // expected-warning {{treating Unicode character as whitespace}}
-extern int　x; // expected-warning {{treating Unicode character as whitespace}}
+extern int x; // expected-error {{treating character <U+' ' U+00A0> as whitespace}}
+extern int　x; // expected-error {{treating character <U+'　' U+3000> as whitespace}}
 
 // CHECK: extern int {{x}}
 // CHECK: extern int　{{x}}