'goto' skips over declaration triggered for unused variable

[dlangBugzillaToGithub]

Mathias LANG (@Geod24) reported this on 2019-08-29T03:11:02Z

Transferred from https://issues.dlang.org/show_bug.cgi?id=20179

CC List

• RazvanN

Description

The following code:
```
string foo () { return null; }

void main ()
{
    if (auto str = foo())
    {
	assert(str.length);
    L1:
    }
    else
        goto L1;
}
```

Produces "oa.d(10): Error: goto skips declaration of variable oa.main.str at oa.d(5)"
But the variable is not used after L1.

[dlangBugzillaToGithub]

razvan.nitu1305 commented on 2023-04-18T11:06:29Z

The compiler does not do any dataflow analysis so it cannot know whether `str` is used or not below L1. However, `str` could be used, therefore the compiler is conservative in this case and errors. In most cases, I think that the error could actually prevent wrong code.

Anyway, if last use analysis is implemented for move constructors/perfect fordwarding maybe this will have a chance of being fixed, however, chances are slim.