Unexpected issue with std.format

[Steven Schveighoffer]

On 2/1/20 8:39 AM, Saurabh Das wrote:
> I faced this issue while working with custom formatting for a struct. I 
> have reduced the error down to this test program:
> 
> import std.format, std.stdio, std.array;
> 
> struct Test1
> {
>      void toString(W, C)(ref W w, scope const ref FormatSpec!C fmt)
>      {
>          pragma(msg, "Test1 function compiled with W=" ~ W.stringof);
>          // formatValue(w, this, fmt);
>      }
> }
> 
> struct Test2
> {
>      void toString(W, C)(ref W w, scope const ref FormatSpec!C fmt)
>      {
>          pragma(msg, "Test2 function compiled with W=" ~ W.stringof);
>          formatValue(w, this, fmt);
>      }
> }
> 
> void main()
> {
>      Test1 t1;
>      Test2 t2;
> 
>      Appender!string writer;
>      auto ff = singleSpec("%s");
> 
>      formatValue(writer, t1, ff);
>      formatValue(writer, t2, ff);
> }
> 
> When compiled, the output is:
> 
> Test1 function compiled with W=S
> Test1 function compiled with W=Appender!string
> Test2 function compiled with W=S
> 
> 1. Why was Test2 never compiled with W=Appender!string?
> 2. What is "S"?
> 
> Essentially, my custom struct was not being formatted using the toString 
> method that I had written. Reducing the issue, it seems like a call to 
> formatValue with the same type caused the issue. If someone can explain 
> what I am doing wrong here, it would really help a lot.
> 
> Thanks,
> Saurabh
> 

Something very weird is happening.

I switched to fullyQualifiedName!W, and I get *no output*.

The "S" comes from hasToString 
here:https://github.com/dlang/phobos/blob/9fe5cd354f0166b11d32a5c1214932757d8e7eba/std/format.d#L3876-L3899

I tried copying the implementation to a local file, and as expected, I 
get customPutWriterFormatSpec for both types.

But it's only calling one of them in the implementation.

I think the only true way to diagnose this is to instrument std.format 
and see what it's doing with more pragma(msg) calls. Don't have the time 
right now.

-Steve