CTAD: implement DR2628 implicit deduction guides should propagate constraints

[hokein]

https://cplusplus.github.io/CWG/issues/2628.html

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: Haojian Wu (hokein)

https://cplusplus.github.io/CWG/issues/2628.html

[shafik]

Can you provide and example that would demonstrate the difference? I am looking at the example from the DR: https://godbolt.org/z/WEaqzd9he

and it is not obvious what behavior I should see and how I would determine it is correct.

[hokein]

If we look at it from a different angle (the AST point of view), it becomes more obvious https://godbolt.org/z/zed6846re:

template<class T> concept True = true;
template<class T> concept True2 = true;

template<class T>
requires True2<T>
 struct X {
  template<class U> requires True<U> X(T, U(&)[3]); 
};

double arr3[3];
X z(3, arr3);  

The implicit deduction guide is:

|-FunctionTemplateDecl <line:8:3, col:50> col:38 implicit <deduction guide for X>
| |-TemplateTypeParmDecl <line:5:10, col:16> col:16 referenced class depth 0 index 0 T
| |-TemplateTypeParmDecl <line:8:12, col:18> col:18 class depth 0 index 1 U
| |-ConceptSpecializationExpr <col:30, col:36> 'bool' Concept 0xcfa0880 'True'
| | |-ImplicitConceptSpecializationDecl <line:2:27> col:27
| | | `-TemplateArgument type 'type-parameter-0-1'
| | |   `-TemplateTypeParmType 'type-parameter-0-1' dependent depth 0 index 1
| | `-TemplateArgument <line:8:35> type 'type-parameter-0-1'
| |   `-TemplateTypeParmType 'type-parameter-0-1' dependent depth 0 index 1
| |-CXXDeductionGuideDecl <col:38, col:50> col:38 implicit <deduction guide for X> 'auto (T, type-parameter-0-1 (&)[3]) -> X<T>'
| | |-ParmVarDecl <col:40> col:41 'T'
| | `-ParmVarDecl <col:43, col:49> col:46 'type-parameter-0-1 (&)[3]'

The associated constraints is True<U>, which is wrong. It should be True<U> && True2<T>.