[clang] [clang] Don't warn on zero literals with -std=c2y (PR #149688)

[Timothy Herchen via cfe-commits]

https://github.com/anematode created https://github.com/llvm/llvm-project/pull/149688

Fixes #149669; the old check compared with the end of the literal, but we can just check that after parsing digits, we're pointing to one character past the token start.

>From 5def3c282d446e6ac687cdef37710e9545814cbf Mon Sep 17 00:00:00 2001
From: Timothy Herchen <timothy.herchen at gmail.com>
Date: Sat, 19 Jul 2025 18:55:17 -0700
Subject: [PATCH] [clang] Don't warn on zero literals with -std=c2y

Fixes #149669
---
 clang/lib/Lex/LiteralSupport.cpp | 2 +-
 clang/test/C/C2y/n3353.c         | 8 +++-----
 2 files changed, 4 insertions(+), 6 deletions(-)

diff --git a/clang/lib/Lex/LiteralSupport.cpp b/clang/lib/Lex/LiteralSupport.cpp
index a62508e3e27bf..5b08d7f0efe5a 100644
--- a/clang/lib/Lex/LiteralSupport.cpp
+++ b/clang/lib/Lex/LiteralSupport.cpp
@@ -1467,7 +1467,7 @@ void NumericLiteralParser::ParseNumberStartingWithZero(SourceLocation TokLoc) {
   if (s != PossibleNewDigitStart)
     DigitsBegin = PossibleNewDigitStart;
   else
-    IsSingleZero = (s == ThisTokEnd); // Is the only thing we've seen a 0?
+    IsSingleZero = (s == ThisTokBegin + 1);
 
   if (s == ThisTokEnd)
     return; // Done, simple octal number like 01234
diff --git a/clang/test/C/C2y/n3353.c b/clang/test/C/C2y/n3353.c
index cd61cbf039067..44dc1cabae2a1 100644
--- a/clang/test/C/C2y/n3353.c
+++ b/clang/test/C/C2y/n3353.c
@@ -44,7 +44,9 @@ static const void *ptr = 0o0;  /* ext-warning {{octal integer literals are a C2y
 #endif
 
 // 0 by itself is not deprecated, of course.
-int k = 0;
+int k1 = 0;
+long k2 = 0l;
+unsigned long long k3 = 0ull;
 
 // Test a preprocessor use of 0 by itself, which is also not deprecated.
 #if 0
@@ -65,7 +67,6 @@ static_assert(__extension__ _Generic(typeof(l), const int : 1, default : 0)); //
 
 // 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.
@@ -83,13 +84,11 @@ int n = 0o18; /* expected-error {{invalid digit '8' in octal constant}}
                  cpp-warning {{octal integer literals are a Clang 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}}
                */
 
 // Show that floating-point suffixes on octal literals are rejected.
@@ -130,7 +129,6 @@ constexpr int p = 0o0'1'2'3'4'5'6'7; /* compat-warning {{octal integer literals
                                       */
 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}}
                  */
 
 #define M 0o123