https://gcc.gnu.org/bugzilla/show_bug.cgi?id=119182
Bug ID: 119182
Summary: Compiler Fails to Diagnose Redefinition of Type Alias
in Lambda Scope
Product: gcc
Version: 15.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: qurong at ios dot ac.cn
Target Milestone: ---
The compiler silently accepts code that redefines a type alias (`using`) within
the same lambda scope, while Clang correctly issues a warning. This violates
C++17 §10.1.3, which prohibits redefining type aliases in the same scope and
requires a diagnostic.
**Steps to Reproduce**:
1. Save the following code as `test.cpp`:
```cpp
template <typename T>
struct Test {
int f() {
int x = 0;
[x = x, &r = x] {
using ty1 = int; // First definition
using ty1 = decltype(x); // Redefinition (invalid)
};
return 0;
}
};
int main() {
Test<int> obj;
obj.f();
return 0;
}
```
2. Compile with GCC:
g++ -std=c++17 test.cpp
Expected Result:
A diagnostic (error/warning) about the redefinition of ty1.
Actual Result:
GCC compile the code without any errors or warnings.
Environment:
Compiler: GCC 13.2.0
Flags: -std=c++17
Proposal:
Reject the code or emit a diagnostic for redefined type aliases in the same
scope, as required by the standard.
Additional Notes:
Standard Reference: C++17 §10.1.3:
"A type-name introduced by a using declaration shall be unique in the
declarative region in which it appears."
Clang Behavior: Correctly warns with warning: redefinition of type alias 'ty1'.
Compiler Explorer link: https://godbolt.org/z/xEMrnbrW4
