When something becomes a runtime value?

Help
,
1 
fn StructFieldDefaultValue(comptime sf: std.builtin.Type.StructField) *const sf.type {
    const fi_type_dvalue = sf.default_value.?;
    const fi_type_aligned: *align(sf.alignment) const anyopaque = @alignCast(fi_type_dvalue);
    return @as(*const sf.type, @ptrCast(fi_type_aligned));
}

fn CreateDefaultStruct(comptime T: type) T {
    const type_info = @typeInfo(T);
    if (type_info != .@"struct") {
        @compileError("expected tuple or struct argument, found " ++ @typeName(T));
    }
    const fields_info = type_info.@"struct".fields;
    var instance: T = undefined;

    inline for (fields_info) |fi| {
        if (fi.default_value) |_| {
            @field(instance, fi.name) = StructFieldDefaultValue(fi).*;
        } else {
            @field(instance, fi.name) = undefined;
        }
    }

    return instance;
}

for normal struct types, the above example works as expected,
but for this comptime generated type(if you really want to test create a test project and paste it in the build.zig file) I get this error:

error: cannot store runtime value in compile time variable
            @field(instance, fi.name) = StructFieldDefaultValue(fi).*;

But the thing is if I replace

@field(instance, fi.name) = StructFieldDefaultValue(fi).*;

with

const dvalue_aligned: *align(fi.alignment) const anyopaque = @alignCast(dvalue);
const value = @as(*const fi.type, @ptrCast(dvalue_aligned)).*;
@field(instance, fi.name) = value;

It(value) somehow becomes comptime value? Why?

2

It looks like comptimeness is being lost across the function call. You could try returning a value instead of a pointer from StructFieldDefaultValue. If this isn’t enough, you can inline it.
With that said, the whole thing looks quite convoluted. If you want some of your struct fields to be default initialized to undefined, simply define undefined as the default value in the struct definition.