| Issue |
120822
|
| Summary |
Off-by-one error in _expression_ mangling of parameter reference in lambda
|
| Labels |
clang,
c++14,
miscompilation
|
| Assignees |
|
| Reporter |
hubert-reinterpretcast
|
According to the Itanium C++ ABI, the _expression_ mangling for a function parameter involves a number `L` where `L` is `1` when referencing a parameter of the current function declarator within its parameter declaration clause.
In the case where `L` is one, the parameter reference is represented with a prefix of `fL0p` (that is with the value of `L` - 1 after the "L").
Consider the source below.
The value of `L` should be `1`; however, the mangling used for `x` is `_ZZZ1fvENKUlT_DtfL1p_EE_clIiEEDaS_S0_E1x` (with `fL1p` instead of the expected `fL0p`). Oddly enough, the typeinfo string for the closure type (`Z1fvEUlT_DtfL0p_EE_`) has `fL0p` as expected.
GCC has an off-by-one error in the opposite direction: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118144
Online compiler link: https://godbolt.org/z/51E9Yjvxf
### SOURCE (`<stdin>`)
```cpp
inline auto f() {
return [](auto p, decltype(p)) {
static int x = 0;
return &x;
};
}
void g() { throw f(); }
auto h() { return f()(0, 0); }
```
### COMPILER INVOCATION
```
clang++ -std=c++17 -xc++ - -O -S -emit-llvm -o -
```
### ACTUAL COMPILER OUTPUT (partial)
```
ret ptr @_ZZZ1fvENKUlT_DtfL1p_EE_clIiEEDaS_S0_E1x
```
### EXPECTED COMPILER OUTPUT (partial)
```
ret ptr @_ZZZ1fvENKUlT_DtfL0p_EE_clIiEEDaS_S0_E1x
```
### COMPILER VERSION INFO (`clang++ -v`)
```
clang version 20.0.0git (https://github.com/llvm/llvm-project.git 44514316bd5ef656076b6baaf6bccb298d98f0ea)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /opt/wandbox/clang-head/bin
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/13
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/14
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/14
Candidate multilib: .;@m64
Selected multilib: .;@m64
```
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