warning C5240: 'nodiscard': attribute is ignored in this syntactic position

Question

Recently version 16.9.5 of Visual Studio 2019 has been released. It apparently introduced new warning:

[[nodiscard]] __declspec(dllexport) bool foo(); //ok
__declspec(dllexport) [[nodiscard]] bool bar(); // warning C5240: 'nodiscard': attribute is ignored in this syntactic position

Actually I thought that both nodiscard and dllexport are attributes that can appear in any order, or it is not?

One is standard, the other is implementation-specific. There's not really a "correct" behavior here in terms of any specification. As a QoI issue, I'd agree this is surprising/vexing. — StoryTeller - Unslander Monica, May 12 '21 at 11:25

score 11 · Accepted Answer · answered May 12 '21 at 20:20

I got this warning today too, so decided to look into it. This requires looking a bit at the standard, and putting different sections together.

According to [dcl.fct.def.general], a function is defined as:

function-definition:
   attribute-specifier-seq_opt decl-specifier-seq_opt declarator virt-specifier-seq_opt function-body

The reason this is important, is that it specifies that the optional decl-specifier-seq comes after the attribute-specifier-seq (which includes [[nodiscard]]).

Now, according to [dcl.spec.general], a decl-specifier-seq is defined as:

decl-specifier-seq:
   decl-specifier attribute-specifier-seq_opt
   decl-specifier decl-specifier-seq

According to Microsoft's documentation of __declspec, it is defined as a decl-specifier; thus, the __declspec(dllexport) must come after the [[nodiscard]] attribute.

Note:

The optional attribute-specifier-seq in a decl-specifier-seq appertains to the type determined by the preceding decl-specifiers

Thus, the [[nodiscard]] after the __declspec(dllexport) applies only to the __declspec(dllexport)

warning C5240: 'nodiscard': attribute is ignored in this syntactic position

1 Answers1