[clang-tidy] bugprone-string-integer-assignment: Reduce false positives.

[courbet]

Summary

Detect a few expressions as likely character expressions, see PR27723.

[alexfh]

clang-tidy/bugprone/StringIntegerAssignmentCheck.cpp

50

What about <mask> & <expr>?

57

Please remove the debug output.

59

What about (x % 26) + 'a'?

[courbet]

• address review comments

[courbet]

Thanks.

[courbet]

• more tests

[MyDeveloperDay]

clang-tidy/bugprone/StringIntegerAssignmentCheck.cpp

77

elide {} on small if statements

[courbet]

• cosmetics

[JonasToth]

clang-tidy/bugprone/StringIntegerAssignmentCheck.cpp

54

unnecessary braces.

[courbet]

• rebase
• -more cosmetics

[JonasToth]

LGTM.

[courbet]

Thanks.

clang-tidy/bugprone/StringIntegerAssignmentCheck.cpp

Finder->addMatcher(
                        unless(callExpr(callee(functionDecl(
                            hasAnyName("tolower", "std::tolower", "toupper",
                                       "std::toupper"))))))
                       .bind("expr"))),
           unless(isInTemplateInstantiation())),
       this);
 }
 
+static bool isLikelyCharExpression(const Expr *Argument,
+                                   const ASTContext &Ctx) {
+  const auto *BinOp = dyn_cast<BinaryOperator>(Argument);
+  if (!BinOp)
+    return false;
+  const auto *LHS = BinOp->getLHS()->IgnoreParenImpCasts();
+  const auto *RHS = BinOp->getRHS()->IgnoreParenImpCasts();
+  // <expr> & <mask>, mask is a compile time constant.
+  Expr::EvalResult RHSVal;
+  if (BinOp->getOpcode() == BO_And &&
+      (RHS->EvaluateAsInt(RHSVal, Ctx, Expr::SE_AllowSideEffects) ||
+       LHS->EvaluateAsInt(RHSVal, Ctx, Expr::SE_AllowSideEffects))) {
+    return true;
+  }
+  // <char literal> + (<expr> % <mod>), where <base> is a char literal.
+  const auto IsCharPlusModExpr = [](const Expr *L, const Expr *R) {
+    const auto *ROp = dyn_cast<BinaryOperator>(R);
+    return ROp && ROp->getOpcode() == BO_Rem && isa<CharacterLiteral>(L);
+  };
+  if (BinOp->getOpcode() == BO_Add) {
+    if (IsCharPlusModExpr(LHS, RHS) || IsCharPlusModExpr(RHS, LHS))
+      return true;
+  }
+  return false;
+}
+
 void StringIntegerAssignmentCheck::check(
     const MatchFinder::MatchResult &Result) {
   const auto *Argument = Result.Nodes.getNodeAs<Expr>("expr");
   SourceLocation Loc = Argument->getBeginLoc();
 
+  // Try to detect a few common expressions to reduce false positives.
+  if (isLikelyCharExpression(Argument, *Result.Context))
+    return;
+
   auto Diag =
       diag(Loc, "an integer is interpreted as a character code when assigning "
                 "it to a string; if this is intended, cast the integer to the "
                 "appropriate character type; if you want a string "
                 "representation, use the appropriate conversion facility");
 
   if (Loc.isMacroID())
     return;

test/clang-tidy/bugprone-string-integer-assignment.cpp

// CHECK-FIXES: {{^}}  ws += std::to_wstring(x);{{$}}
   std::basic_string<MyArcaneChar> as;
   as = 6;
 // CHECK-MESSAGES: :[[@LINE-1]]:8: warning: an integer is interpreted as a chara
 // CHECK-FIXES: {{^}}  as = 6;{{$}}
 
   s += toupper(x);
   s += tolower(x);
   s += std::tolower(x);
+
+  // Likely character expressions.
+  s += x & 0xff;
+  s += 0xff & x;
+
+  s += 'a' + (x % 26);
+  s += (x % 10) + 'b';
 }