https://issues.dlang.org/show_bug.cgi?id=20101

          Issue ID: 20101
           Summary: BetterC: Template instantiation in CTFE only context
                    should skip codegen / nogc / ... Phases
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody@puremagic.com
          Reporter: johannespfau@gmail.com

In D, we often uses helpers to generate code for string mixins and similar purposes:

--------------------------------------
public string ctfeHelper()(string a)
{
    return "int " ~ a ~ " = 42;";
}

extern(C) int main() @nogc
{
    mixin(ctfeHelper("a"));
    return a;
}
--------------------------------------

With -betterC, this code currently fails with this message:
test.d(3): Error: array concatenation of expression "int " ~ a ~ " = 42;"
requires the GC which is not available with -betterC

However, the code is exclusively used in a CTFE-only context where the GC is conceptionally always available.

Note that if ctfeHelper was a regular function, we would have to emit runtime code, as it is public and could be called from external code. So in that case, the error would be correct and there's no way to make this work (except for making ctfeHelper private, proofing that it's never used at runtime and that it's unreachable from other modules (which is never the case, if you consider explicitly linking to the mangled name to be valid)).

But for this example, where `ctfeHelper` is a template, we could argue that CTFE-only instatiations should be separated from runtime instantiations: If we do not generate code for the template in CTFE contexts, that's still perfectly fine: Anyone calling ctfeHelper at runtime will instantiate the template again, so the codegen can be executed there. And the user should get the errors there, when instantiating the template in a runtime context.

Fixing this would probably involve marking an TemplateInstance as ctfeOnly. However, it seems the implementation would be 99% identical to isSpeculative templates. Consider this code, which already works with -betterC:


--------------------------------------
string ctfeHelper()(string a)
{
    return "int " ~ a ~ " = 42;";
}

extern(C) int main() @nogc
{
    return  __traits(compiles, ctfeHelper("a"));
}

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