DLang - semantic analysis on imports

x.d

module x;

import y;

/*
Semantic analysis 1 = only TOP-LEVEL declarations are analyzed
By TOP-LEVEL declarations I understand: - functions (the header of the function, the parameters and the return type; 
                                                        *the body IS NOT included*)
                                        - global variables (templates instantiations included)
                                        - struct/classes/templates declarations
                                        
Semantic analysis 2 = solves the things that couldn't be solved in Semantic 1 (more complicated things)
                                        (ex. templates definitions and instantiations, classes, structs etc.)
                                        
Semantic analysis 3 = the final analysis and the most complete and invasive one. 
                        - e.g. the bodies of the functions and some properties of the templates that weren't solved in semantic 1 and semantic 2
                        - THE MOST IMPORTANT THING: the code on which semantic 3 is performed GETS A CHANCE TO BE GENERATED.  
                      

Now is important to understand how semantic analysis works *on imports*:
  - Firstly, only semantic analysis 1 is done on imports. 
    - observation 1: as I mentioned before, in semantic1 the bodies of functions are not included, so that's why
                      there won't be an error for 'WrongCodeThatPassesLexicalAnalysis' (see y.d below) (both "dmd -c x.d" and 
                       "dmd -c -unittest x.d" will work)
    - observation 2: most of version(unittest) {} blocks are at the TOP-LEVEL; that means "dmd -c -unittest x.d" will trigger
                     semantic 1 on every statement that's present inside a version block (Phobos, druntime included) -> overhead
                     - observation 2.1 : there are still many version (unittest) in the Phobos library. 
                                         I thought that Steven’s PR removed everything, but then I verified myself and 
                                         I realized it’s not true. I tried to replace them automatically using 
                                         sed -i version(unittest)/ version(somethingElse)/g but this didn’t work 
                                         and resulted in different errors while recompiling Phobos. 
                                         The conclusion is that we must remove one version block at a time 
                                         because the code inside each version() {} has to be understood and moved correctly. 
                                         There are version (unittest) {} blocks even in the druntime, and they are 
                                         semantic analyzed (when we use -unittest) even if there are 0 import statements. 

  - After the semantic 1 finishes, semantic analysis 2 will begin its job on the statements that couldn't be handled by semantic1
  - Now, very important, BY DEFAULT THERE IS NO semantic3 done on non-root modules => no semantic3 on imports that are not
                            also passed as command-line arguments. 
                            e.g. "dmd -c x.d" won't trigger semantic3 on y.d because y is not a root module;
                            
                            
 Now, again, very important: the special behaviour done when -unittest switch is used can be summed up as follows:
                    "-unittest is activating semantic analysis 3 on non-root modules (i.e. imports not given in the command line) 
                      by considering them root modules"
 Why? Because some templates from non-root modules might need code generation. Why again?
 
 All the answers are here: https://wiki.dlang.org/Template_Instantiation_Strategy
*/

y.d

module y;

struct S(T)
{ 
  T something;
  void method() {};
}

unittest 
{
  WrongCodeThatPassesLexicalAnalysis;
}


version (unittest)
{
  import std.uni;
  import std.regex;
  // other imports
  // template instantiations, other things...
}