[clang] [clang] Warn on signed char array constant conversion (PR #203792)

[via cfe-commits]

https://github.com/filaka771 updated https://github.com/llvm/llvm-project/pull/203792

>From f8eb7ae6127aa00c441f4f9e6242be9290449edb Mon Sep 17 00:00:00 2001
From: Alex Filak <filaka771 at gmail.com>
Date: Sun, 14 Jun 2026 00:40:00 +0300
Subject: [PATCH 1/4] [clang] Fix char-array constant-conversion suppression

---
 clang/lib/Sema/SemaChecking.cpp | 13 +++++++++----
 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index ef07e8c6133ed..24914c0057c7b 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -13251,7 +13251,8 @@ static void DiagnoseNullConversion(Sema &S, Expr *E, QualType T,
 }
 
 // Helper function to filter out cases for constant width constant conversion.
-// Don't warn on char array initialization or for non-decimal values.
+// Don't warn on char / unsigned char array initialization or for non-decimal
+// values.
 static bool isSameWidthConstantConversion(Sema &S, Expr *E, QualType T,
                                           SourceLocation CC) {
   // If initializing from a constant, and the constant starts with '0',
@@ -13264,12 +13265,16 @@ static bool isSameWidthConstantConversion(Sema &S, Expr *E, QualType T,
       return false;
   }
 
-  // If the CC location points to a '{', and the type is char, then assume
-  // assume it is an array initialization.
+  // If the CC location points to a '{', and the destination type is char or
+  // unsigned char, then assume this is an array initialization. Keep warning
+  // for signed char arrays, where values such as 255 change sign.
   if (CC.isValid() && T->isCharType()) {
+    const auto *BT =
+        dyn_cast<BuiltinType>(S.Context.getCanonicalType(T).getTypePtr());
     const char FirstContextCharacter =
         S.getSourceManager().getCharacterData(CC)[0];
-    if (FirstContextCharacter == '{')
+    if (BT && BT->getKind() != BuiltinType::SChar &&
+        FirstContextCharacter == '{')
       return false;
   }
 

>From ab55b35b3edabca58ae31cd2b2a93b4cf31da3fc Mon Sep 17 00:00:00 2001
From: Alex Filak <filaka771 at gmail.com>
Date: Sun, 14 Jun 2026 00:40:12 +0300
Subject: [PATCH 2/4] [clang] Add char-array constant-conversion tests

---
 clang/test/Sema/constant-conversion.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/clang/test/Sema/constant-conversion.c b/clang/test/Sema/constant-conversion.c
index ffc25b9cc4978..b40379e2180ce 100644
--- a/clang/test/Sema/constant-conversion.c
+++ b/clang/test/Sema/constant-conversion.c
@@ -125,6 +125,16 @@ void test9(void) {
   char macro_char_dec = CHAR_MACRO_DEC;  // expected-warning {{implicit conversion from 'int' to 'char' changes value from 255 to -1}}
 
   char array_init[] = { 255, 127, 128, 129, 0 };
+  unsigned char unsigned_array_init[] = { 255 };
+  unsigned char unsigned_array_init_multi[] = { 255, 127, 128, 129, 0 };
+  signed char signed_array_init[] = { 255 }; // expected-warning {{implicit conversion from 'int' to 'signed char' changes value from 255 to -1}}
+  signed char signed_array_init_multi[] = {
+    255, // expected-warning {{implicit conversion from 'int' to 'signed char' changes value from 255 to -1}}
+    127,
+    128, // expected-warning {{implicit conversion from 'int' to 'signed char' changes value from 128 to -128}}
+    129, // expected-warning {{implicit conversion from 'int' to 'signed char' changes value from 129 to -127}}
+    0
+  };
 }
 
 #define A 1

>From 1e6ec0df823dccd6a9eed9fe4697833bfb9d962f Mon Sep 17 00:00:00 2001
From: Alex Filak <filaka771 at gmail.com>
Date: Wed, 17 Jun 2026 21:53:22 +0300
Subject: [PATCH 3/4] [clang] Add release note for signed-char array warning

---
 clang/docs/ReleaseNotes.md | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md
index 3dd17d8c523bc..3b2dce7a2fa2e 100644
--- a/clang/docs/ReleaseNotes.md
+++ b/clang/docs/ReleaseNotes.md
@@ -325,6 +325,9 @@ features cannot lower the translation-unit ABI level;
 - Clang now attempts to print enumerator names rather than C-style cast expressions
   in more diagnostics.
 
+- Fixed a missing `-Wconstant-conversion` diagnostic for `signed char` array
+  initialization.
+
 
 ### Improvements to Clang's time-trace
 

>From e5c5b146eecdf8af9f7649b45e69e6d3628bf0b8 Mon Sep 17 00:00:00 2001
From: Alex Filak <filaka771 at gmail.com>
Date: Fri, 26 Jun 2026 07:02:08 +0300
Subject: [PATCH 4/4] [clang] Handle plain char signedness in array conversion
 warnings

---
 clang/docs/ReleaseNotes.md            |  3 +--
 clang/lib/Sema/SemaChecking.cpp       | 14 +++++---------
 clang/test/Sema/constant-conversion.c | 11 ++++++-----
 3 files changed, 12 insertions(+), 16 deletions(-)

diff --git a/clang/docs/ReleaseNotes.md b/clang/docs/ReleaseNotes.md
index 3b2dce7a2fa2e..dc7b9f7b17143 100644
--- a/clang/docs/ReleaseNotes.md
+++ b/clang/docs/ReleaseNotes.md
@@ -326,8 +326,7 @@ features cannot lower the translation-unit ABI level;
   in more diagnostics.
 
 - Fixed a missing `-Wconstant-conversion` diagnostic for `signed char` array
-  initialization.
-
+  initialization. (#GH181730)
 
 ### Improvements to Clang's time-trace
 
diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 24914c0057c7b..debf1a8198b63 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -13251,7 +13251,7 @@ static void DiagnoseNullConversion(Sema &S, Expr *E, QualType T,
 }
 
 // Helper function to filter out cases for constant width constant conversion.
-// Don't warn on char / unsigned char array initialization or for non-decimal
+// Don't warn on unsigned character array initialization or for non-decimal
 // values.
 static bool isSameWidthConstantConversion(Sema &S, Expr *E, QualType T,
                                           SourceLocation CC) {
@@ -13265,16 +13265,12 @@ static bool isSameWidthConstantConversion(Sema &S, Expr *E, QualType T,
       return false;
   }
 
-  // If the CC location points to a '{', and the destination type is char or
-  // unsigned char, then assume this is an array initialization. Keep warning
-  // for signed char arrays, where values such as 255 change sign.
-  if (CC.isValid() && T->isCharType()) {
-    const auto *BT =
-        dyn_cast<BuiltinType>(S.Context.getCanonicalType(T).getTypePtr());
+  // If the CC location points to a '{' and the type is an unsigned character
+  // type, assume it is an array initialization.
+  if (T->isCharType() && !T->isSignedIntegerType() && CC.isValid()) {
     const char FirstContextCharacter =
         S.getSourceManager().getCharacterData(CC)[0];
-    if (BT && BT->getKind() != BuiltinType::SChar &&
-        FirstContextCharacter == '{')
+    if (FirstContextCharacter == '{')
       return false;
   }
 
diff --git a/clang/test/Sema/constant-conversion.c b/clang/test/Sema/constant-conversion.c
index b40379e2180ce..adba3315cc9b4 100644
--- a/clang/test/Sema/constant-conversion.c
+++ b/clang/test/Sema/constant-conversion.c
@@ -1,5 +1,6 @@
-// RUN: %clang_cc1 -fsyntax-only -ffreestanding -verify=expected,one-bit -triple x86_64-apple-darwin %s
-// RUN: %clang_cc1 -fsyntax-only -ffreestanding -Wno-single-bit-bitfield-constant-conversion -verify -triple x86_64-apple-darwin %s
+// RUN: %clang_cc1 -fsyntax-only -ffreestanding -verify=expected,signed-plain-char,one-bit -triple x86_64-apple-darwin %s
+// RUN: %clang_cc1 -fsyntax-only -ffreestanding -Wno-single-bit-bitfield-constant-conversion -verify=expected,signed-plain-char -triple x86_64-apple-darwin %s
+// RUN: %clang_cc1 -fsyntax-only -ffreestanding -Wno-single-bit-bitfield-constant-conversion -verify -triple x86_64-apple-darwin -fno-signed-char %s
 
 #include <stdbool.h>
 
@@ -103,7 +104,7 @@ void test9(void) {
   const int max_short_plus_one = (int)max_short + 1;
   const long max_int_plus_one = (long)max_int + 1;
 
-  char new_char = max_char_plus_one;  // expected-warning {{implicit conversion from 'const short' to 'char' changes value from 128 to -128}}
+  char new_char = max_char_plus_one;  // signed-plain-char-warning {{implicit conversion from 'const short' to 'char' changes value from 128 to -128}}
   short new_short = max_short_plus_one;  // expected-warning {{implicit conversion from 'const int' to 'short' changes value from 32768 to -32768}}
   int new_int = max_int_plus_one;  // expected-warning {{implicit conversion from 'const long' to 'int' changes value from 2147483648 to -2147483648}}
 
@@ -122,9 +123,9 @@ void test9(void) {
 #define CHAR_MACRO_HEX 0xff
   char macro_char_hex = CHAR_MACRO_HEX;
 #define CHAR_MACRO_DEC 255
-  char macro_char_dec = CHAR_MACRO_DEC;  // expected-warning {{implicit conversion from 'int' to 'char' changes value from 255 to -1}}
+  char macro_char_dec = CHAR_MACRO_DEC;  // signed-plain-char-warning {{implicit conversion from 'int' to 'char' changes value from 255 to -1}}
 
-  char array_init[] = { 255, 127, 128, 129, 0 };
+  char array_init[] = { 255 }; // signed-plain-char-warning {{implicit conversion from 'int' to 'char' changes value from 255 to -1}}
   unsigned char unsigned_array_init[] = { 255 };
   unsigned char unsigned_array_init_multi[] = { 255, 127, 128, 129, 0 };
   signed char signed_array_init[] = { 255 }; // expected-warning {{implicit conversion from 'int' to 'signed char' changes value from 255 to -1}}