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clang-concepts-roadmap

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Roadmap for implementation of Concepts in the Clang compiler.

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# Concepts ([P0734R0][1]) Implementation in Clang
Roadmap for implementation of Concepts in the Clang compiler.

## Roadmap

1. There are no longer variable and function concepts, and using VarTemplateDecl to represent concepts is unnecessarily complex and would require many workarounds to prevent said variables from actually being instantiated.
   Therefore - remove the support for them (currently there is a flag in TemplateDecl and a 'concept' storage specifier, and a bunch of diagnostics which are no longer relevant).
> Addressed in [D40380][2]
   
2. Add a new "ConceptDecl" AST node which represents a concept declaration.
    - inherits from TemplateDecl and not RedeclarableTemplateDecl, because concepts are not redeclarable.
    - stores an Expr \* representing the constraint expression in the function's body.
> Addressed in [D40381][3]
   
3. Add code to parse "ConceptDecl"s.
> Addressed in [D40381][3]

4. Add a new "ConceptSpecializationExpr" expression which is a concept name with template arguments, which stores a pointer to ConceptDecl and a TemplateArgumentListInfo.
    - when a ConceptSpecializationExpr is created without dependent template arguments, it is evaluated for satisfaction, the value stored within the object.
    - translate during constant evaluation and code generation to a bool.
> Addressed in [D41217][4]
    
5. Add code to parse "ConceptSpecializationExpr"s.
> Addressed in [D41217][4]

-  There was already a "RequiresClause" field (by means of TrailingObjects) added to TemplateParameterList.
   
-  There was already a "ConstrainedTemplateDeclInfo" struct which contains a TemplateParameterList and a ConstraintExpression added to TemplateDecl. 
   Keep that for later caching a combined constraint expression with both the requires clause, constraints arising from constrained template parameters (a.k.a template\<Callable C\>), and from trailing requires clauses on functions (a.k.a void foo() requires sizeof(int) == 4). 
   
6. Remove ConstrainedTemplateDeclInfo from TemplateDecl constructors and instead create one in the constructor if there are any associated constraints in the TemplateParameterList passed in.
> Addressed in [D41284][5]

7. Add ConstrainedTemplateDeclInfo to VarPartialSpecializationTemplateDecl and ClassPartialSpecializationTemplateDecl as well.
> Addressed in [D41284][5]

8. Add a function to ClassPartialSpecializationTemplateDecl, VarPartialSpecializationTemplateDecl and TemplateDecl called 
   "getAssociatedConstraints", which will return the total associated constraints, collected from all sources mentioned above (this method will not do any calculation/creation of "and" expressions - the expression should be created in the ctor of said classes if it requires calculation).
> Addressed in [D41284][5]

9. Enforce RequiresClause on template specialization in the following functions: 
    - CheckTemplateArgumentList which is ran before a primary template is instantiated with arguments.
    - ConvertDeducedTemplateArguments which is ran before partial specializations of all kinds are instantiated with arguments.
    - Both above functions will use a single template function which checks whether a bunch of arguments satisfy the constraints imposed by a 'templatedecl-like' object - it would basically instantiate the constraint expression returned by the object's getAssociatedConstraints() function.
> Addressed in [D41569][6]
  
10. Create a function which given an Expr \* representing a constraint expression known to have not been satisfied, emits diagnostics as to why it wasn't (it would have special cases for BinaryOperators && and ||, as well as ConceptSpecializationExprs).
    Call this function in TemplateSpecCandidateSet::NoteCandidates when appropriate.
> Addressed in [D41569][6]
  
11. Add code to isAtLeastAsSpecializedAs to regard constraint expressions (with partial ordering, subsumption and normalization).
> Addressed in [D41910][7]

12. Add code to parse a trailing requires-clause in a function declaration.
> Addressed in [D43357][8]

13. Add a ConstraintExpression field to FunctionDecl, add code to TemplateDecl to take the constraint into account (and store it in the constraint stored in ConstrainedTemplateDeclInfo).
    - Go over all places where FunctionDecls are created and make sure constraints are passed in if needed.
> Addressed in [D43357][8]
  
14. Add name mangling for the added ConstraintExpression (which is now part of the signature).
> On hold until itanium decides what the mangling's going to be.

15. Add ConstraintExpression checking for redeclarations of functions (consider whether or not the ConstraintExpression matches when determining if a function declaration is a redeclaration).
> Addressed in [D43357][8]
 
16. Prohibit virtual functions from being declared with constraints.
> Addressed in [D43357][8]

17. Change Sema::IsOverload to regard functions with matching signatures but different constraints as overloads.
> Addressed in [D43357][8]

18. Make sure functions whose constraints are not satisfied cannot be referenced.
> Addressed in [D43357][8]

19. Change addOverload and addTemplateOverload to disallow adding functions whose constraints are not met.
> Addressed in [D43357][8]

20. Change isBetter to regard a candidate whose constraints subsume the other to be better.
> Addressed in [D43357][8]

21. Add a ConstraintExpression field to TypeTemplateParmDecl, TemplateTemplateParmDecl and NonTypeTemplateParmDecl, to represent constraints imposed by 'constrained template parameters' (e.g. things such as template<Callable C>).
> Addressed in [D44352][9]

22. Add a "calculateAssociatedConstraints" function to TemplateParameterList  which returns the requires clause constraint-expression ANDed with all constrained template parameter's cosntraint-expressions, make TemplateDecl, VarPartialSpecializationTemplateDecl and     ClassPartialSpecializationTemplateDecl use this function when calculating the associated constraints in TemplateDecl.
> Addressed in [D44352][9]

23. Add code to parse constrained template parameters and generate the imposed constraint-expression, storing them in the created 
    TemplateParameters.
> Addressed in [D44352][9]

24. Add a new CXXRequiresExpr expression class (e.g. requires(T t) { t.foo(); }).
> Addressed in [D50360][10]

25. Add code to parse CXXRequiresExpr expressions.
> Addressed in [D50360][10]

26. Add code to the diagnostics functions we'd previously implemented to introspect into requires expressions which weren't satisfied to further explain why they weren't.
> Addressed in [D50360][10]

[1]: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/p0734r0.pdf
[2]: https://reviews.llvm.org/D40380
[3]: https://reviews.llvm.org/D40381
[4]: https://reviews.llvm.org/D41217
[5]: https://reviews.llvm.org/D41284
[6]: https://reviews.llvm.org/D41569
[7]: https://reviews.llvm.org/D41910
[8]: https://reviews.llvm.org/D43357
[9]: https://reviews.llvm.org/D44352
[10]: https://reviews.llvm.org/D50360

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