[Issue 23601] New: [std.format] acceptedSpecs do match baseOfSpec

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23601

          Issue ID: 23601
           Summary: [std.format] acceptedSpecs do match baseOfSpec
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: jlourenco5691 at gmail.com

```d
import std;

void main()
{
    int i;
    "12.34".formattedRead!"%f"(i);
}
```

This code sample compiles incorrectly to then fail at run time. The validation
fails because it validates 'i' as a valid argument for the used spec. Any of
[FfGgAaEe] counts as a valid base for any integral argument (int, bool, char,
...) and when checking acceptedSpecs at run time they do not match.

It seems to me that baseOfSpec is the one in the wrong here, but the real issue
is using the writing format to validate the reading format since 'int' can
format into a 'float' but not the other way around.

Moreover, 'a' is missing from acceptedSpecs, and all the upper case versions of
spec are allowed by the 'write' version but not the 'read' alternative, which
also leads to the run-time error `not supported with formatted read` instead of
a compile-time assertion.

Formatted read should validate the format specs with its own implementation if
it differs this much from the 'write' version.

--