[Issue 23088] New: spurious case of "`expression`" has no effect

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23088

          Issue ID: 23088
           Summary: spurious case of "`expression`" has no effect
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Keywords: diagnostic, rejects-valid
          Severity: normal
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: b2.temp at gmx.com

For the following code

```
static int v;

extern(C) int rand();

ref int a()
{
    return v = rand();
}

int main()
{
    a == 0;
    return v ? 1 : 0;
}             
```

we get, with -de, the output

> /tmp/temp_7F3C12CCD670.d:12:5: Error: `a() == 0` has no effect

but the effect of the call directly influences the return code of the program.

The compiler, by digging in the nested expressions could determine that it
contains one call to an impure function and consequently allow (or not warn)
about the comparison.

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