[Issue 8714] New: Instantiate template within CTFE with same CTFE returned value

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=8714

           Summary: Instantiate template within CTFE with same CTFE
                    returned value
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: jens.k.mueller at gmx.de


--- Comment #0 from jens.k.mueller at gmx.de 2012-09-23 14:08:29 PDT ---
To better integrate CTFE and templates it may be worthwhile to allow code as
follows

string foo(string f)
{
   if (f == "somestring")
   {
       return "got somestring";
   }
   return bar!(foo("somestring"));
}

template bar(string s)
{
   enum bar = s;
}

Note, that the return value of foo("somestring") is "got somestring" which
should be possible to pass to some template.

Code like

string foo(string f)                                                            
{                                                                               
    if (f == "somestring")                                                      
    {                                                                           
        return "got somestring";                                                
    }                                                                           
    return bar!(foo("somestring"))();                                           
}                                                                               

string bar(string s)()                                                          
{                                                                               
    return s;                                                                   
}

should compile as well.

Both fail with dmd 2.060.
The first code fails with:
test.d(7):        called from here: foo("somestring")
test.d(7):        called from here: foo("somestring")
test.d(7):        called from here: foo("somestring")
test.d(7): Error: expression foo("somestring") is not a valid template value
argument
test.d(12):        called from here: foo("somestring")
test.d(12):        called from here: foo("somestring")
test.d(7): Error: template instance test.bar!(foo("somestring")) error
instantiating

and the second with:
test.d(7):        called from here: foo("somestring")
test.d(7):        called from here: foo("somestring")
test.d(7):        called from here: foo("somestring")
test.d(7): Error: expression foo("somestring") is not a valid template value
argument
test.d(7): Error: template instance test.bar!(foo("somestring")) error
instantiating

If code like above should stay invalid the compiler diagnostics should be
improved. I.e. recursive dependency on foo when analyzing foo in CTFE.

See also the discussion on the list
http://forum.dlang.org/thread/mailman.148.1348176167.5162.digitalmars-d@puremagic.com
http://forum.dlang.org/thread/20120920212236.GD3056@minet.uni-jena.de

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