[llvm] r289853 - [StringRef] Add enable-if to StringLiteral.

[Zachary Turner via llvm-commits]

Grr.  Is there an easy way to disable this warning?

On Thu, Dec 15, 2016 at 11:21 AM David Blaikie <dblaikie at gmail.com> wrote:

> Build's a bit noisy:
>
> llvm/src/include/llvm/ADT/StringRef.h:851:22: warning: 'enable_if' is a
> clang extension [-Wgcc-compat]
>         __attribute((enable_if(__builtin_strlen(Str) == N - 1,
>                      ^
> 1 warning generated.
>
> (I get a lot of those... )
>
> On Thu, Dec 15, 2016 at 11:13 AM Zachary Turner via llvm-commits <
> llvm-commits at lists.llvm.org> wrote:
>
> Author: zturner
> Date: Thu Dec 15 13:02:43 2016
> New Revision: 289853
>
> URL: http://llvm.org/viewvc/llvm-project?rev=289853&view=rev
> Log:
> [StringRef] Add enable-if to StringLiteral.
>
> to prevent StringLiteral from being created with a non-literal
> char array, clang has a macro enable_if() that can be used
> in such a way as to guarantee that the constructor is disabled
> unless the length fo the string can be computed at compile time.
>
> This only works on clang, but at least it should allow bots
> to catch abuse of StringLiteral.
>
> Differential Revision: https://reviews.llvm.org/D27780
>
> Modified:
>     llvm/trunk/include/llvm/ADT/StringRef.h
>
> Modified: llvm/trunk/include/llvm/ADT/StringRef.h
> URL:
> http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/ADT/StringRef.h?rev=289853&r1=289852&r2=289853&view=diff
>
> ==============================================================================
> --- llvm/trunk/include/llvm/ADT/StringRef.h (original)
> +++ llvm/trunk/include/llvm/ADT/StringRef.h Thu Dec 15 13:02:43 2016
> @@ -838,22 +838,21 @@ namespace llvm {
>
>    /// A wrapper around a string literal that serves as a proxy for
> constructing
>    /// global tables of StringRefs with the length computed at compile
> time.
> -  /// Using this class with a non-literal char array is considered
> undefined
> -  /// behavior.  To prevent this, it is recommended that StringLiteral
> *only*
> -  /// be used in a constexpr context, as such:
> +  /// In order to avoid the invocation of a global constructor,
> StringLiteral
> +  /// should *only* be used in a constexpr context, as such:
>    ///
>    /// constexpr StringLiteral S("test");
>    ///
> -  /// Note: There is a subtle behavioral difference in the constructor of
> -  /// StringRef and StringLiteral, as illustrated below:
> -  ///
> -  /// constexpr StringLiteral S("a\0b");  // S.size() == 3
> -  /// StringRef S("a\0b");  // S.size() == 1
> -  ///
>    class StringLiteral : public StringRef {
>    public:
>      template <size_t N>
> -    constexpr StringLiteral(const char (&Str)[N]) : StringRef(Str, N - 1)
> {}
> +    constexpr StringLiteral(const char (&Str)[N])
> +#if __has_attribute(enable_if)
> +        __attribute((enable_if(__builtin_strlen(Str) == N - 1,
> +                               "invalid string literal")))
> +#endif
> +        : StringRef(Str, N - 1) {
> +    }
>    };
>
>    /// @name StringRef Comparison Operators
> @@ -865,9 +864,7 @@ namespace llvm {
>    }
>
>    LLVM_ATTRIBUTE_ALWAYS_INLINE
> -  inline bool operator!=(StringRef LHS, StringRef RHS) {
> -    return !(LHS == RHS);
> -  }
> +  inline bool operator!=(StringRef LHS, StringRef RHS) { return !(LHS ==
> RHS); }
>
>    inline bool operator<(StringRef LHS, StringRef RHS) {
>      return LHS.compare(RHS) == -1;
>
>
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