http://d.puremagic.com/issues/show_bug.cgi?id=9715

           Summary: Implement basic template error checking
           Product: D
           Version: D1 & D2
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: enhancement
          Priority: P2
         Component: DMD
        AssignedTo: nobody@puremagic.com
        ReportedBy: clugdbug@yahoo.com.au


--- Comment #0 from Don <clugdbug@yahoo.com.au> 2013-03-14 02:18:34 PDT ---
int foo(T)(T x, int yesterday)
{
    return yesteray;
}
----
There is a typo in this template, it will fail for any type T. Would be nice if this reported an error, without having to instantiate the template.

We could do this by creating a new Unspecified type and UnspecifiedExp in the compiler. Whenever a template declaration is encountered, instantiate it with Unspecified types. These guys would propagate exactly like ErrorExp.

Then, we'd get error messages for the simple cases like those shown above.

You could actually do 'static if' the same way (types declared in is() expressions inside static if will be Unspecified type if they depend on template parameters), but the challenge is when you have multiple independent static ifs in a template:

static if (T.sizeof == 2)  int x = 3;
static if (T.sizeof > 2)  int x = 4;  // this isn't a redefinition error

There are ways to deal with this, eg treating the inside of the static if as a
scope, and when the static if has been analyzed, moving everything declared in
that scope into the parent scope, marking it as PossiblyDeclared.
(so that redefining it doesn't cause an error, but it's OK to use it from
inside another static if).

The simpler option would be to stop analysis after the first 'static if' is encountered. That would still catch errors in a lot of useful cases.

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Walter Bright <bugzilla@digitalmars.com> changed:

           What    |Removed                     |Added
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                 CC|                            |bugzilla@digitalmars.com


--- Comment #1 from Walter Bright <bugzilla@digitalmars.com> 2013-03-14 13:04:34 PDT ---
It's a good idea, but the pervasiveness of ErrorExp is still pretty incomplete.

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--- Comment #2 from Walter Bright <bugzilla@digitalmars.com> 2013-03-14 14:34:05 PDT ---
Another issue is what happens with value and alias parameters. A fair amount of reengineering of the internals would be necessary to make this work.

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