[SUGGESTION] Independent order and type data members #406

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1. realgdman

   added

   suggestion

   on May 1, 2023
2. JohelEGP commented on Jul 31, 2023

   

   JohelEGP

   on Jul 31, 2023

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   There's also no order independence in a type's Cpp1 declaration (template parameters and requires clause).
   This fails because origin has not been declared (https://cpp2.godbolt.org/z/9WeKsef3j):

   unit: @struct type       = { }
   quantity_point: @struct <O: * origin, U: * unit, N> type = { }
   origin: @struct type = { }
   main: () = { }
   

   main.cpp2:2:10: error: unknown type name 'origin'
       2 | template<origin* O, unit* U, typename N> class quantity_point;
         |          ^
   
3. JohelEGP commented on Jul 31, 2023

   

   JohelEGP

   on Jul 31, 2023

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   Here is the order of emitted Cpp1
   (template implied where possible):

   • Phase 1 "Cpp2 type declarations"
     first emits #include "cpp2util.h".
   • Phase 1 "Cpp2 type declarations"
     then emits, in source order:
     • Of a Cpp2 type, its Cpp1 type declaration.
   • Phase 2 "Cpp2 type definitions and function declarations"
     then emits, in source order:
     • Of a Cpp2 type, its Cpp1 type definition and member function declarations.
     • Of a Cpp2 function, its Cpp1 declaration.
     • Of a Cpp2 non-expression alias, its Cpp1 declaration.
     • Of a Cpp2 expression alias, its Cpp1 definition.
     • An import declaration is emitted as-is.
     • Cpp1 code is emitted as-is.
       That includes #include directives, even if for a Cpp2 source file (see [BUG] Can't use #included declaration in requires clause #470).
   • Phase 3 "Cpp2 function definitions"
     then emits, in source order:
     • Of a Cpp2 type, its member function definitions.
     • Of a Cpp2 function, its Cpp1 definition.

   That results in these order dependencies:

   • The body of a function is order independent.
     I think Cpp2 guarantees this.
   • Anything else can be source code order dependent according to how things are emitted above.
     Notably, a function with a deduced return type can't be used before it's defined.
     E.g., if A and B are Cpp2 types,
     you can use the incomplete type A in type B's template parameter list or requires clause
     if A comes before B in source code.
4. JohelEGP commented on Aug 10, 2023

   

   JohelEGP

   on Aug 10, 2023

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   Probably hard to solve, requiring some sort of dependency graph

   See the last paragraph of #75 (comment).
5. JohelEGP commented on Aug 11, 2023

   

   JohelEGP

   on Aug 11, 2023

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   @hsutter What do you think about concepts and order independence?

   Right now, a concept is emitted during Phase 2 "Cpp2 type definitions and function declarations".
   But that's no good if you want to use it to constrain template types in the same TU.
6. Repository owner locked and limited conversation to collaborators on Aug 30, 2023
7. hsutter

   converted this issue into a discussion #641 on Aug 30, 2023