[PATCH] Add new warning to -Wparentheses for mixing overloaded shift operators and comparison operators

[David Blaikie]

On Apr 10, 2013 10:04 AM, "Richard Trieu" <rtrieu at google.com> wrote:
>
> Streams commonly overload the << and >> operators for input and output.
 However, these operators have higher precedence than comparison operators.
 Code written like this:
>
> cout << 5 == 4;
>
> Is interpreted as:
>
> (cout << 5) == 4;
>
> Instead of:
>
> cout << (5 == 4);

What sort of statistics on success rate do you have with this warning? I
wonder if it would find many intentional uses when dealing with numeric
user defined tyows,.

>
> http://llvm-reviews.chandlerc.com/D650
>
> Files:
>   lib/Sema/SemaExpr.cpp
>   test/Sema/parentheses.cpp
>   include/clang/Basic/DiagnosticSemaKinds.td
>   include/clang/Basic/DiagnosticGroups.td
>
> Index: lib/Sema/SemaExpr.cpp
> ===================================================================
> --- lib/Sema/SemaExpr.cpp
> +++ lib/Sema/SemaExpr.cpp
> @@ -8819,6 +8819,27 @@
>    }
>  }
>
> +static void DiagnoseShiftCompare(Sema &S, SourceLocation OpLoc,
> +                                 Expr *LHSExpr, Expr *RHSExpr) {
> +  CXXOperatorCallExpr *OCE = dyn_cast<CXXOperatorCallExpr>(LHSExpr);
> +  if (!OCE) return;

These early returns seem a little hard to spot in a fairly dense/flat piece
if code. Might benefit from putting the return on their own line and/or
some comments (or perhaps this is easy enough to read with syntax
highlighting)

> +  FunctionDecl *FD = OCE->getDirectCallee();
> +  if (!FD || !FD->isOverloadedOperator()) return;
> +  OverloadedOperatorKind Kind = FD->getOverloadedOperator();
> +  if (Kind != OO_LessLess && Kind != OO_GreaterGreater) return;
> +  S.Diag(OpLoc, diag::warn_overloaded_shift_in_comparison)
> +      << LHSExpr->getSourceRange() << RHSExpr->getSourceRange()
> +      << (Kind == OO_LessLess);
> +  SuggestParentheses(S, OpLoc,
> +                     S.PDiag(diag::note_evaluate_comparison_first),
> +                     SourceRange(OCE->getArg(1)->getLocStart(),
> +                                 RHSExpr->getLocEnd()));
> +  SuggestParentheses(S, OCE->getOperatorLoc(),
> +                     S.PDiag(diag::note_precedence_silence)
> +                         << (Kind == OO_LessLess ? "<<" : ">>"),
> +                     OCE->getSourceRange());
> +}
> +
>  /// DiagnoseBinOpPrecedence - Emit warnings for expressions with tricky
>  /// precedence.
>  static void DiagnoseBinOpPrecedence(Sema &Self, BinaryOperatorKind Opc,
> @@ -8847,6 +8868,11 @@
>      DiagnoseAdditionInShift(Self, OpLoc, LHSExpr, Shift);
>      DiagnoseAdditionInShift(Self, OpLoc, RHSExpr, Shift);
>    }
> +
> +  // Warn on overloaded shift operators and comparisons, such as:
> +  // cout << 5 == 4;
> +  if (BinaryOperator::isComparisonOp(Opc))
> +    DiagnoseShiftCompare(Self, OpLoc, LHSExpr, RHSExpr);
>  }
>
>  // Binary Operators.  'Tok' is the token for the operator.
> Index: test/Sema/parentheses.cpp
> ===================================================================
> --- test/Sema/parentheses.cpp
> +++ test/Sema/parentheses.cpp
> @@ -29,6 +29,12 @@
>    (void)(s << b ? "foo" : "bar"); // expected-warning {{operator '?:'
has lower precedence than '<<'}} \
>                                    // expected-note {{place parentheses
around the '?:' expression to evaluate it first}} \
>                                    // expected-note {{place parentheses
around the '<<' expression to silence this warning}}
> +  (void)(s << 5 == 1); // expected-warning {{overloaded operator << has
lower precedence than comparison operator}} \
> +                       // expected-note {{place parentheses around the
'<<' expression to silence this warning}} \
> +                       // expected-note {{place parentheses around
comparison expression to evaluate it first}}
> +  (void)(s >> 5 == 1); // expected-warning {{overloaded operator >> has
lower precedence than comparison operator}} \
> +                       // expected-note {{place parentheses around the
'>>' expression to silence this warning}} \
> +                       // expected-note {{place parentheses around
comparison expression to evaluate it first}}
>  }
>
>  struct S {
> Index: include/clang/Basic/DiagnosticSemaKinds.td
> ===================================================================
> --- include/clang/Basic/DiagnosticSemaKinds.td
> +++ include/clang/Basic/DiagnosticSemaKinds.td
> @@ -4025,6 +4025,13 @@
>  def warn_logical_and_in_logical_or : Warning<
>    "'&&' within '||'">, InGroup<LogicalOpParentheses>;
>
> +def warn_overloaded_shift_in_comparison :Warning<
> +  "overloaded operator %select{>>|<<}0 has lower precedence than "
> +  "comparison operator">,
> +  InGroup<OverloadedShiftOpParentheses>;
> +def note_evaluate_comparison_first :Note<
> +  "place parentheses around comparison expression to evaluate it first">;
> +
>  def warn_addition_in_bitshift : Warning<
>    "operator '%0' has lower precedence than '%1'; "
>    "'%1' will be evaluated first">, InGroup<ShiftOpParentheses>;
> Index: include/clang/Basic/DiagnosticGroups.td
> ===================================================================
> --- include/clang/Basic/DiagnosticGroups.td
> +++ include/clang/Basic/DiagnosticGroups.td
> @@ -122,6 +122,7 @@
>  def BitwiseOpParentheses: DiagGroup<"bitwise-op-parentheses">;
>  def LogicalOpParentheses: DiagGroup<"logical-op-parentheses">;
>  def ShiftOpParentheses: DiagGroup<"shift-op-parentheses">;
> +def OverloadedShiftOpParentheses:
DiagGroup<"overloaded-shift-op-parentheses">;
>  def DanglingElse: DiagGroup<"dangling-else">;
>  def DanglingField : DiagGroup<"dangling-field">;
>  def DistributedObjectModifiers :
DiagGroup<"distributed-object-modifiers">;
> @@ -352,6 +353,7 @@
>                              [LogicalOpParentheses,
>                               BitwiseOpParentheses,
>                               ShiftOpParentheses,
> +                             OverloadedShiftOpParentheses,
>                               ParenthesesOnEquality,
>                               DanglingElse]>;
>
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