| Issue |
183597
|
| Summary |
Clang accepts invalid forward-declared scoped enum with dependent underlying type in class template
|
| Labels |
clang
|
| Assignees |
|
| Reporter |
vrukesh
|
**What code / commands /steps will reproduce the problem?**
Compile the below sample code:
`
template<class T>
struct color {
enum city : int;
};
template<class T>
enum color<T>::city : T {
london, ny, paris
};
int main() {
(void)color<int>::london;
}
`
Compiler Explorer link: https://godbolt.org/z/frcn8jqK6
**What is the expected result?**
This is ill-formed code and must be rejected by the compiler with compile-time error.
C++17 ISO/IEC 14882:2017 Section 10.2 para 6 [dcl.enum] mentions:
An enumeration whose underlying type is fixed is an incomplete type from its point of declaration to immediately after its enum-base (if any), at which point it becomes a complete type. An enumeration whose underlying type is not fixed is an incomplete type from its point of declaration to immediately after the closing } of its enum-specifier, at which point it becomes a complete type.
**What happens instead?**
g++ (GCC) compilation fails:
`
<source>:7:23: error: different underlying type in enum 'enum color<T>::city' [-Wtemplate-body]
7 | enum color<T>::city : T {
| ^
<source>:3:17: note: previous definition here
3 | enum city : int;
| ^~~
Compiler returned: 1
`
Clang compiles successfully without any error or warning.
A forward-declared scoped enum inside a class template fixes the underlying type to int, but Clang accepts a later definition that uses the template parameter as the underlying type when compiled under -std=c++17.
This must be fixed.
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