[Issue 9179] New: Invalid template instantiation attempt should result in a readable error message

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=9179

           Summary: Invalid template instantiation attempt should result
                    in a readable error message
           Product: D
           Version: D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody at puremagic.com
        ReportedBy: andrej.mitrovich at gmail.com


--- Comment #0 from Andrej Mitrovic <andrej.mitrovich at gmail.com> 2012-12-18 09:44:19 PST ---
template Pred(T)
{
    enum Pred = true;
}

void func(T)(T t)
    if (Pred(T))
{
}

void main()
{
    func(1);
}

Errors:
test.d(9): Error: template test.Pred does not match any function template
declaration. Candidates are:
test.d(3):        test.Pred(T)
test.d(9): Error: template test.Pred(T) cannot deduce template function from
argument types !()(int)
test.d(15): Error: template test.func does not match any function template
declaration. Candidates are:
test.d(8):        test.func(T)(T t) if (Pred(T))
test.d(15): Error: template test.func(T)(T t) if (Pred(T)) cannot deduce
template function from argument types !()(int)

The problem is Pred was used as a function instead of as a template, the fix
is:

void func(T)(T t)
    if (Pred!(T))  // note the !() syntax
{
}

There is no other way the Pred template can be called because it's not a
function template, therefore we should print out a better error message such
as:

test.d(9): Error: Need to use !() syntax to instantiate template test.Pred

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