| Issue |
175954
|
| Summary |
[[maybe_unused]] rejected on typedef (valid in C++17)
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
vrukesh
|
**What code / commands /steps will reproduce the problem?**
Compile the below code for clang C++17 and g++ C++17.
typedef int [[maybe_unused]] a;
int main() { return 0; }
**Compiler Explorer link:** https://godbolt.org/z/YxbfP5Yb5
**What is the expected result?**
Compilation should pass.
**What happens instead?**
Compilation fails on clang.
Compilation is pass on gcc.
$ g++ helloworld.cpp -o helloworld -std=c++17
helloworld.cpp:1:13: warning: attribute ignored [-Wattributes]
1 | typedef int [[maybe_unused]] a;
| ^
helloworld.cpp:1:13: note: an attribute that appertains to a type-specifier is ignored
$ /usr/bin/clang++ helloworld.cpp -o helloworld -std=c++17
helloworld.cpp:1:15: error: 'maybe_unused' attribute cannot be applied to types
1 | typedef int [[maybe_unused]] a;
| ^
1 error generated.
As per C++17 standard, `[[maybe_unused]]` may appertain to a class, typedef, variable (including structured bindings), non-static data member, function, enumeration, or enumerator, and no diagnostic is required if it ends up unused. Attributes placed in the declaration-specifiers of a typedef apply to the typedef itself, so the usage is valid.
GCC accepts this code (only warns that the attribute is ignored). Per C++17 [dcl.attr.unused]/1, maybe_unused can appertain to a typedef, so this should be accepted.
So, Clang needs to accept or at most warn, matching the standard.
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