[Clang] Force expressions with UO_Not to not be non-negative (#126846)

This PR addresses the bug of not throwing warnings for the following
code:
```c++
int test13(unsigned a, int *b) {
        return a > ~(95 != *b); // expected-warning {{comparison of integers of different signs}}
}
```

However, in the original issue, a comment mentioned that negation,
pre-increment, and pre-decrement operators are also incorrect in this
case.

Fixes #18878

1 files changed, 36 insertions, 0 deletions

diff --git a/clang/lib/Sema/SemaChecking.cpp b/clang/lib/Sema/SemaChecking.cpp
index 9cac9cf..a6cdbbaf 100644
--- a/clang/lib/Sema/SemaChecking.cpp
+++ b/clang/lib/Sema/SemaChecking.cpp
@@ -10620,6 +10620,42 @@ static std::optional<IntRange> TryGetExprRange(ASTContext &C, const Expr *E,
     case UO_AddrOf: // should be impossible
       return IntRange::forValueOfType(C, GetExprType(E));
 
+    case UO_Minus: {
+      if (E->getType()->isUnsignedIntegerType()) {
+        return TryGetExprRange(C, UO->getSubExpr(), MaxWidth, InConstantContext,
+                               Approximate);
+      }
+
+      std::optional<IntRange> SubRange = TryGetExprRange(
+          C, UO->getSubExpr(), MaxWidth, InConstantContext, Approximate);
+
+      if (!SubRange)
+        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.
+      return IntRange(SubRange->Width + 1, false);
+    }
+
+    case UO_Not: {
+      if (E->getType()->isUnsignedIntegerType()) {
+        return TryGetExprRange(C, UO->getSubExpr(), MaxWidth, InConstantContext,
+                               Approximate);
+      }
+
+      std::optional<IntRange> SubRange = TryGetExprRange(
+          C, UO->getSubExpr(), MaxWidth, InConstantContext, Approximate);
+
+      if (!SubRange)
+        return std::nullopt;
+
+      // 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);
+    }
+
     default:
       return TryGetExprRange(C, UO->getSubExpr(), MaxWidth, InConstantContext,
                              Approximate);