[Issue 21456] New: std.format does not accept enum member with string base type as template parameter

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21456

          Issue ID: 21456
           Summary: std.format does not accept enum member with string
                    base type as template parameter
           Product: D
           Version: D2
          Hardware: x86_64
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: rmaicle at gmail.com

D 2.094.2
Linux 5.9.11-3-MANJARO x86_64
-----------------------------

The std.format overload accepts a compile-time string as format string.

This code compiles:

    enum FMTS = "%s is %s";
    assert (format!FMTS("Pi", 3.14) == "Pi is 3.14");

This code also compiles:

    enum {
        FMTS = "%s is %s"
    }
    assert (format!FMTS("Pi", 3.14) == "Pi is 3.14");

This code does not compile:

    enum FMTS : string {
        ONE = "%s is %s"
    }
    assert (format!FMTS("Pi", 3.14) == "Pi is 3.14");

Formatted error message:

    Error: template std.format.format cannot deduce function from argument 
           types !("%s is %s")(string, double), candidates are:
    ../src/phobos/std/format.d(6514,13): format(alias fmt, Args...)(Args args)
      with fmt = ONE,
           Args = (string, double)
      must satisfy the following constraint:
            isSomeString!(typeof(fmt))

The std.format overload accepting compile-time string as format string 
should accept named enums with a string base type since the enum member 
value can be determined during compile-time.

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