[Issue 2379] New: CTFE fails to evaluate if unless it is very straightforward

[d-bugmail at puremagic.com]

http://d.puremagic.com/issues/show_bug.cgi?id=2379

           Summary: CTFE fails to evaluate if unless it is very
                    straightforward
           Product: D
           Version: 2.019
          Platform: PC
        OS/Version: Windows
            Status: NEW
          Severity: critical
          Priority: P2
         Component: DMD
        AssignedTo: bugzilla at digitalmars.com
        ReportedBy: 2korden at gmail.com


First attempt:
-----------
template Test0(invariant(char)[] text)
{
    auto Test0 = text;
}

writefln(Test0!("hello")); // success
pragma(msg, Test0!("hello")); // failure:
// pragma msg string expected for message, not 'Test0'

Let's replace invariant(char)[] with alias :(
-----------
template Test0(alias text)
{
    auto Test0 = text;
}
pragma(msg, Test0!("hello")); // success

Let's make it slightly more complex:
-----------
template Test0(alias text)
{
    auto Test0 = text[0..1];
}
pragma(msg, Test0!("hello")); // failure:
// pragma msg string expected for message, not 'Test0'

Other example:
-----------
template Test0(invariant(char)[] text)
{
    auto Test0 = text;
}

template Test1(invariant(char)[] text)
{
    auto Test1 = Test0!(text);
}

writefln(Test0!("hello")); // success
writefln(Test1!("hello")); // failure:
// Error: non-constant expression Test0


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