[Issue 24395] New: Allow braced statements in loop conditions

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=24395

          Issue ID: 24395
           Summary: Allow braced statements in loop conditions
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: qs.il.paperinik at gmail.com

For loops allow a braced sequence of statements in the first section:
```d
// Valid D code:
for ({int i = 1; int n = 10;} i < n; ++i) { } 
```
Variables declared there are visible in the following two sections and the loop
body.

I propose to allow this for the condition as well:
Like the initial section, the condition could be a braced sequence of
statements, where the last statement would be special-cased in the sense that
it must evaluate to something that can be interpreted as a Boolean condition.

E.g.:
```d
// Proposed D code:
for (int i = 0; {int n = 10 - i; i < n;} ++i) { }
```

The main difference between this and an immediately invoked lambda is probably
scope: The braces as part of the `for` loop construct do not create a scope;
the variable `n` in the example would be visible in the condition, the
increment (what `++i` is) and the body of the for loop.

It would make sense to allow the same for the condition of the `while` loop.
That allows e.g. to rewrite the standard C `fgetc` pattern in a cleaner
fashion:
```d
while ({int c = std::fgetc(fp); c != EOF;}) { ... }
```
To be consistent, it should also work on a `do…while` loop, albeit not being
that useful as the scope of variables declared in its condition should not be
visible in the loop body.

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