[clang] Cap IntRange::Width to the expression's type size (PR #145356)

[Akira Hatanaka via cfe-commits]

https://github.com/ahatanak updated https://github.com/llvm/llvm-project/pull/145356

>From cecae359edce350139d845095e9ef9d1ec8933fd Mon Sep 17 00:00:00 2001
From: Akira Hatanaka <ahatanak at gmail.com>
Date: Mon, 23 Jun 2025 08:50:44 -0700
Subject: [PATCH 1/2] Cap IntRange::Width to the expression's type size

This commit addresses a fallout introduced by #126846.

Previously, TryGetExprRange would return an IntRange that has an active
range exceeding the maximum representable range for the expression's
underlying type. This led to clang erroneously issuing warnings about
implicit conversions losing integer precision.

This commit fixes the bug by capping IntRange::Width to the size of the
expression's type.

rdar://149444029
---
 clang/lib/Sema/SemaChecking.cpp                  | 6 ++++--
 clang/test/Sema/implicit-int-conversion-on-int.c | 8 ++++++++
 2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 69276ce418fa6..57cb810adceaa 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -10808,7 +10808,7 @@ static std::optional<IntRange> TryGetExprRange(ASTContext &C, const Expr *E,
       // sign bit. If the range was not non-negative, we need an extra bit
       // because the negation of the most-negative value is one bit wider than
       // that value.
-      return IntRange(SubRange->Width + 1, false);
+      return IntRange(std::min(SubRange->Width + 1, MaxWidth), false);
     }
 
     case UO_Not: {
@@ -10825,7 +10825,9 @@ static std::optional<IntRange> TryGetExprRange(ASTContext &C, const Expr *E,
 
       // The width increments by 1 if the sub-expression cannot be negative
       // since it now can be.
-      return IntRange(SubRange->Width + (int)SubRange->NonNegative, false);
+      return IntRange(
+          std::min(SubRange->Width + (int)SubRange->NonNegative, MaxWidth),
+          false);
     }
 
     default:
diff --git a/clang/test/Sema/implicit-int-conversion-on-int.c b/clang/test/Sema/implicit-int-conversion-on-int.c
index f555893d9e17a..3b1d81cd6a9ab 100644
--- a/clang/test/Sema/implicit-int-conversion-on-int.c
+++ b/clang/test/Sema/implicit-int-conversion-on-int.c
@@ -48,3 +48,11 @@ unsigned long long test_ull(unsigned long long x) {
 unsigned _BitInt(16) test_unsigned_bit_int(unsigned _BitInt(16) x) {
   return -x;
 }
+
+unsigned test_shift_minus(int i) {
+  return -(1 << i);
+}
+
+unsigned test_shift_not(int i) {
+  return ~(1 << i);
+}

>From acda73c7dc282c1e95eac36364205832cffd100e Mon Sep 17 00:00:00 2001
From: Akira Hatanaka <ahatanak at gmail.com>
Date: Tue, 24 Jun 2025 08:49:01 -0700
Subject: [PATCH 2/2] Improve comment

---
 clang/lib/Sema/SemaChecking.cpp | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 57cb810adceaa..60b23da3eecea 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -10540,6 +10540,8 @@ static std::optional<IntRange> TryGetExprRange(ASTContext &C, const Expr *E,
                                                bool Approximate) {
   E = E->IgnoreParens();
 
+  assert(MaxWidth == C.getIntWidth(GetExprType(E)));
+
   // Try a full evaluation first.
   Expr::EvalResult result;
   if (E->EvaluateAsRValue(result, C, InConstantContext))
@@ -10805,9 +10807,8 @@ static std::optional<IntRange> TryGetExprRange(ASTContext &C, const Expr *E,
         return std::nullopt;
 
       // If the range was previously non-negative, we need an extra bit for the
-      // sign bit. If the range was not non-negative, we need an extra bit
-      // because the negation of the most-negative value is one bit wider than
-      // that value.
+      // sign bit. Otherwise, we need an extra bit because the negation of the
+      // most-negative value is one bit wider than that value.
       return IntRange(std::min(SubRange->Width + 1, MaxWidth), false);
     }