[Issue 5042] New: format("%s") of struct without toString

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=5042

           Summary: format("%s") of struct without toString
           Product: D
           Version: D2
          Platform: x86
        OS/Version: Windows
            Status: NEW
          Keywords: rejects-valid
          Severity: normal
          Priority: P2
         Component: Phobos
        AssignedTo: nobody at puremagic.com
        ReportedBy: bearophile_hugs at eml.cc


--- Comment #0 from bearophile_hugs at eml.cc 2010-10-11 15:18:36 PDT ---
Given a plain struct like Foo, the to!string creates a good enough textual
representation of it:


import std.conv: to;
struct Foo { int x; }
void main() {
    assert(to!string(Foo(1)) == "Foo(1)");
}



A nomal writefln("%s") just prints "Foo":


import std.stdio: writeln, writefln;
struct Foo { int x; }
void main() {
    writeln(Foo(1)); // ==> Foo
    writefln("%s", Foo(1)); // ==> Foo
}



But the format("%s") doesn't work:


import std.string: format;
struct Foo { int x; }
void main() {
    assert(format("%s", Foo(1)) == "Foo(1)");
}


DMD 2.049 gives the error:
std.format.FormatError: std.format Can't convert test.Point to string: "string
toString()" not defined


I expect this format("%s") to return "Foo(1)" or "Foo" (I prefer "Foo(1)").

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