[C2y] Add octal prefixes, deprecate unprefixed octals #131626

AaronBallman · 2025-03-17T15:16:02Z

WG14 N3353 added support for 0o and 0O as octal literal prefixes. It also deprecates use of octal literals without a prefix, except for the literal 0.

This feature is being exposed as an extension in older C language modes as well as in all C++ language modes.

WG14 N3353 added support for 0o and 0O as octal literal prefixes. It also deprecates use of octal literals without a prefix, except for the literal 0. This feature is being exposed as an extension in older C language modes as well as in all C++ language modes.

llvmbot · 2025-03-17T15:16:35Z

@llvm/pr-subscribers-clang

Author: Aaron Ballman (AaronBallman)

Changes

WG14 N3353 added support for 0o and 0O as octal literal prefixes. It also deprecates use of octal literals without a prefix, except for the literal 0.

This feature is being exposed as an extension in older C language modes as well as in all C++ language modes.

Full diff: https://github.com/llvm/llvm-project/pull/131626.diff

7 Files Affected:

(modified) clang/docs/LanguageExtensions.rst (+1)
(modified) clang/docs/ReleaseNotes.rst (+4)
(modified) clang/include/clang/Basic/DiagnosticGroups.td (+3-1)
(modified) clang/include/clang/Basic/DiagnosticLexKinds.td (+8)
(modified) clang/lib/Lex/LiteralSupport.cpp (+22)
(added) clang/test/C/C2y/n3353.c (+95)
(modified) clang/www/c_status.html (+1-1)

diff --git a/clang/docs/LanguageExtensions.rst b/clang/docs/LanguageExtensions.rst
index cc12ff5bad353..a96816916b5f2 100644
--- a/clang/docs/LanguageExtensions.rst
+++ b/clang/docs/LanguageExtensions.rst
@@ -1652,6 +1652,7 @@ Designated initializers (N494)                                                 C
 Array & element qualification (N2607)                                          C23           C89
 Attributes (N2335)                                                             C23           C89
 ``#embed`` (N3017)                                                             C23           C89, C++
+Octal literals prefixed with ``0o`` or ``0O``                                  C2y           C89, C++
 ============================================= ================================ ============= =============
 
 Builtin type aliases
diff --git a/clang/docs/ReleaseNotes.rst b/clang/docs/ReleaseNotes.rst
index 2a1c5ee2d788e..bfb6235247709 100644
--- a/clang/docs/ReleaseNotes.rst
+++ b/clang/docs/ReleaseNotes.rst
@@ -120,6 +120,10 @@ C2y Feature Support
 - Implemented `WG14 N3411 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3411.pdf>`_
   which allows a source file to not end with a newline character. This is still
   reported as a conforming extension in earlier language modes.
+- Implemented `WG14 N3353 <https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3353.htm>_`
+  which adds the new ``0o`` and ``0O`` ocal literal prefixes and deprecates
+  octal literals other than ``0`` which do not start with the new prefix. This
+  feature is exposed in earlier language modes and in C++ as an extension.
 
 C23 Feature Support
 ^^^^^^^^^^^^^^^^^^^
diff --git a/clang/include/clang/Basic/DiagnosticGroups.td b/clang/include/clang/Basic/DiagnosticGroups.td
index fac80fb4009aa..e386d8f2c3b27 100644
--- a/clang/include/clang/Basic/DiagnosticGroups.td
+++ b/clang/include/clang/Basic/DiagnosticGroups.td
@@ -92,6 +92,7 @@ def EnumCompare : DiagGroup<"enum-compare", [EnumCompareSwitch,
 def DeprecatedAnonEnumEnumConversion : DiagGroup<"deprecated-anon-enum-enum-conversion">;
 def DeprecatedEnumEnumConversion : DiagGroup<"deprecated-enum-enum-conversion">;
 def DeprecatedEnumFloatConversion : DiagGroup<"deprecated-enum-float-conversion">;
+def DeprecatedOctalLiterals : DiagGroup<"deprecated-octal-literals">;
 def AnonEnumEnumConversion : DiagGroup<"anon-enum-enum-conversion",
                                    [DeprecatedAnonEnumEnumConversion]>;
 def EnumEnumConversion : DiagGroup<"enum-enum-conversion",
@@ -235,7 +236,8 @@ def Deprecated : DiagGroup<"deprecated", [DeprecatedAnonEnumEnumConversion,
                                           DeprecatedVolatile,
                                           DeprecatedWritableStr,
                                           DeprecatedRedundantConstexprStaticDef,
-                                          DeprecatedMissingCommaVariadicParam
+                                          DeprecatedMissingCommaVariadicParam,
+                                          DeprecatedOctalLiterals
                                           ]>,
                  DiagCategory<"Deprecations">;
 
diff --git a/clang/include/clang/Basic/DiagnosticLexKinds.td b/clang/include/clang/Basic/DiagnosticLexKinds.td
index 0e5592d65669b..f662ad1211ba3 100644
--- a/clang/include/clang/Basic/DiagnosticLexKinds.td
+++ b/clang/include/clang/Basic/DiagnosticLexKinds.td
@@ -256,6 +256,14 @@ def warn_cxx17_hex_literal : Warning<
   "hexadecimal floating literals are incompatible with "
   "C++ standards before C++17">,
   InGroup<CXXPre17CompatPedantic>, DefaultIgnore;
+def ext_octal_literal : Extension<
+  "octal integer literals are a C2y extension">, InGroup<C2y>;
+def warn_c2y_compat_octal_literal : Warning<
+  "octal integer literals are incompatible with standards before C2y">,
+  InGroup<CPre2yCompat>, DefaultIgnore;
+def warn_unprefixed_octal_deprecated : Warning<
+  "octal literals without a '0o' prefix are deprecated">,
+  InGroup<DeprecatedOctalLiterals>;
 def ext_binary_literal : Extension<
   "binary integer literals are a C23 extension">, InGroup<C23>;
 def warn_c23_compat_binary_literal : Warning<
diff --git a/clang/lib/Lex/LiteralSupport.cpp b/clang/lib/Lex/LiteralSupport.cpp
index 69dc057d0df4b..0ab27b2d40373 100644
--- a/clang/lib/Lex/LiteralSupport.cpp
+++ b/clang/lib/Lex/LiteralSupport.cpp
@@ -21,6 +21,7 @@
 #include "clang/Lex/Preprocessor.h"
 #include "clang/Lex/Token.h"
 #include "llvm/ADT/APInt.h"
+#include "llvm/ADT/ScopeExit.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringSwitch.h"
@@ -1423,6 +1424,27 @@ void NumericLiteralParser::ParseNumberStartingWithZero(SourceLocation TokLoc) {
     return;
   }
 
+  // Parse a potential octal literal prefix.
+  bool SawOctalPrefix = false;
+  if ((c1 == 'O' || c1 == 'o') && (s[1] >= '0' && s[1] <= '7')) {
+    unsigned DiagId;
+    if (LangOpts.C2y)
+      DiagId = diag::warn_c2y_compat_octal_literal;
+    else
+      DiagId = diag::ext_octal_literal;
+    Diags.Report(TokLoc, DiagId);
+    ++s;
+    DigitsBegin = s;
+    SawOctalPrefix = true;
+  }
+
+  auto _ = llvm::make_scope_exit([&] {
+    // If we still have an octal value but we did not see an octal prefix,
+    // diagnose as being an obsolescent feature starting in C2y.
+    if (radix == 8 && LangOpts.C2y && !SawOctalPrefix && !hadError)
+      Diags.Report(TokLoc, diag::warn_unprefixed_octal_deprecated);
+  });
+
   // For now, the radix is set to 8. If we discover that we have a
   // floating point constant, the radix will change to 10. Octal floating
   // point constants are not permitted (only decimal and hexadecimal).
diff --git a/clang/test/C/C2y/n3353.c b/clang/test/C/C2y/n3353.c
new file mode 100644
index 0000000000000..4e8b7f0fc43ba
--- /dev/null
+++ b/clang/test/C/C2y/n3353.c
@@ -0,0 +1,95 @@
+// RUN: %clang_cc1 -verify=expected,c2y -pedantic -std=c2y %s
+// RUN: %clang_cc1 -verify=expected,c2y,compat -Wpre-c2y-compat -std=c2y %s
+// RUN: %clang_cc1 -verify=expected,ext -pedantic -std=c23 %s
+// RUN: %clang_cc1 -verify=expected,ext -pedantic -x c++ -Wno-c11-extensions %s
+
+
+/* WG14 N3353: Clang 21
+ * Obsolete implicitly octal literals and add delimited escape sequences
+ */
+
+constexpr int i = 0234;  // c2y-warning {{octal literals without a '0o' prefix are deprecated}}
+constexpr int j = 0o234; /* ext-warning {{octal integer literals are a C2y extension}}
+                            compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                          */
+
+static_assert(i == 156);
+static_assert(j == 156);
+
+// Show that 0O is the same as Oo (tested above)
+static_assert(0O1234 == 0o1234);  /* ext-warning 2 {{octal integer literals are a C2y extension}}
+                                     compat-warning 2 {{octal integer literals are incompatible with standards before C2y}}
+                                   */
+
+// Demonstrate that it works fine in the preprocessor.
+#if 0o123 != 0x53   /* ext-warning {{octal integer literals are a C2y extension}}
+                       compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                     */
+#error "oh no, math stopped working!"
+#endif
+
+// 0 by itself is not deprecated, of course.
+int k = 0;
+
+// Make sure there are no surprises with auto and type deduction. Promotion
+// turns this into an 'int', and 'constexpr' implies 'const'.
+constexpr auto l = 0o1234567; /* ext-warning {{octal integer literals are a C2y extension}}
+                                 compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                              */
+static_assert(l == 0x53977);
+static_assert(__extension__ _Generic(typeof(0o1), typeof(01) : 1, default : 0)); /* c2y-warning {{octal literals without a '0o' prefix are deprecated}}
+                                                                                    compat-warning {{passing a type argument as the first operand to '_Generic' is incompatible with C standards before C2y}}
+                                                                                    compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                                                                                  */
+static_assert(__extension__ _Generic(typeof(l), const int : 1, default : 0)); // compat-warning {{passing a type argument as the first operand to '_Generic' is incompatible with C standards before C2y}}
+
+// Note that 0o by itself is an invalid literal.
+int m = 0o; /* expected-error {{invalid suffix 'o' on integer constant}}
+               c2y-warning {{octal literals without a '0o' prefix are deprecated}}
+             */
+
+// Ensure negation works as expected.
+static_assert(-0o1234 == -668); /* ext-warning {{octal integer literals are a C2y extension}}
+                                   compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                                 */
+
+// FIXME: it would be better to not diagnose the compat and ext warnings when
+// the octal literal is invalid.
+// We expect diagnostics for non-octal digits.
+int n = 0o18; /* expected-error {{invalid digit '8' in octal constant}}
+                 compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                 ext-warning {{octal integer literals are a C2y extension}}
+               */
+int o1 = 0o8; /* expected-error {{invalid suffix 'o8' on integer constant}}
+                 c2y-warning {{octal literals without a '0o' prefix are deprecated}}
+               */
+// FIXME: however, it matches the behavior for hex literals in terms of the
+// error reported. Unfortunately, we then go on to think 0 is an octal literal
+// without a prefix, which is again a bit confusing.
+int o2 = 0xG; /* expected-error {{invalid suffix 'xG' on integer constant}}
+                 c2y-warning {{octal literals without a '0o' prefix are deprecated}}
+               */
+
+// Ensure digit separators work as expected.
+constexpr int p = 0o0'1'2'3'4'5'6'7; /* compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                                        ext-warning {{octal integer literals are a C2y extension}}
+                                      */
+static_assert(p == 01234567); // c2y-warning {{octal literals without a '0o' prefix are deprecated}}
+int q = 0o'0'1; /* expected-error {{invalid suffix 'o'0'1' on integer constant}}
+                   c2y-warning {{octal literals without a '0o' prefix are deprecated}}
+                 */
+
+#ifdef __cplusplus
+template <unsigned N>
+struct S {
+  static_assert(N == 0o567); /* ext-warning {{octal integer literals are a C2y extension}}
+                                compat-warning {{octal integer literals are incompatible with standards before C2y}}
+                              */
+};
+
+void foo() {
+  S<0o567> s; /* ext-warning {{octal integer literals are a C2y extension}}
+                 compat-warning {{octal integer literals are incompatible with standards before C2y}}
+               */
+}
+#endif
diff --git a/clang/www/c_status.html b/clang/www/c_status.html
index d68e8d6441ed2..7cf50bfdb6639 100644
--- a/clang/www/c_status.html
+++ b/clang/www/c_status.html
@@ -176,7 +176,7 @@ <h2 id="c2y">C2y implementation status</h2>
     <tr>
       <td>Obsolete implicitly octal literals and add delimited escape sequences</td>
       <td><a href="https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3353.htm">N3353</a></td>
-      <td class="none" align="center">No</td>
+      <td class="unreleased" align="center">Clang 21</td>
     </tr>
     <tr>
       <td>'if' declarations, v2</td>

cor3ntin

Thanks for working on that Aaron

clang/test/C/C2y/n3353.c

clang/include/clang/Basic/DiagnosticLexKinds.td

clang/www/c_status.html

clang/test/C/C2y/n3353.c

github-actions · 2025-03-17T17:00:47Z

✅ With the latest revision this PR passed the C/C++ code formatter.

cor3ntin · 2025-03-17T17:00:53Z

clang/include/clang/Lex/Lexer.h

+  /// Diagnose use of a delimited or named escape sequence.
+  static void DiagnoseDelimitedOrNamedEscapeSequence(SourceLocation Loc,
+                                                     bool Named,
+                                                     const LangOptions &Opts,
+                                                     DiagnosticsEngine &Diags);
+


This could just be a non-member static function Lexer.cpp

It's called from LiteralSupport.cpp, so it could be a non-member function, but I think it's a bit cleaner to scope it to the Lexer to make it clear this is emitting lexer-based diagnostics. WDYT?

clang/docs/ReleaseNotes.rst

clang/test/C/C2y/n3353.c

cor3ntin

Thanks Aaron!

AaronBallman added clang:frontend Language frontend issues, e.g. anything involving "Sema" extension:clang c2y labels Mar 17, 2025

AaronBallman requested review from jrtc27, jyknight, cor3ntin and tahonermann March 17, 2025 15:16

llvmbot added the clang Clang issues not falling into any other category label Mar 17, 2025

cor3ntin reviewed Mar 17, 2025

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AaronBallman added 2 commits March 17, 2025 12:01

Add C++-specific extension diagnostic

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Update the support for escape sequences

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Fix formatting, add test case for #line directives

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AaronBallman added 2 commits March 17, 2025 13:14

Update release note for escape sequences

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AaronBallman merged commit 9cf46fb into llvm:main Mar 18, 2025
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AaronBallman deleted the aballman-wg14-n3353 branch March 18, 2025 11:29

naveen-seth mentioned this pull request May 28, 2025

[C2y] Handle FP-suffixes on prefixed octals (#141230) #141695

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