================ @@ -2859,6 +2859,65 @@ This library is called by the :ref:`Parser library <Parser>` during parsing to do semantic analysis of the input. For valid programs, Sema builds an AST for parsed constructs. + +Concept Satisfaction Checking and Subsumption +--------------------------------------------- + +As per the C++ standard, constraints are + `normalized <https://eel.is/c++draft/temp.constr.normal>`_ +and the normal form is used both for subsumption, and constraint checking. +Both depend on a parameter mapping that substitutes lazily. In particular, +we should not substitute in unused arguments. + +Clang follows the order of operations prescribed by the standard. + +Normalization happens prior to satisfaction and subsumption +and is handled by ``NormalizedConstraint``. + +Clang preserves in the normalized form intermediate concept-ids +(``ConceptIdConstraint``) This is used for diagnostics only and no substitution +happens in a ConceptIdConstraint if its expression is satisfied. + +The normal form of the associated constraints of a declaration is cached in +Sema::NormalizationCache such that it is only computed once. + +a ``NormalizedConstraint`` is a recursive data structure, where each node +contains a parameter mapping, represented by the indexes of all parameter +being used. + +Checking satisfaction is done by ConstraintSatisfactionChecker, recursively +walking NormalizedConstraint. At each level, we substitute the outermost +level of the template arguments referenced in the parameter mapping of a +normalized expression (``MultiLevelTemplateArgumentList``). + +.. code-block:: c++ + + template <typename T> + concept A = __is_same(T, int); + + template <typename U> + concept B = A<U> && __is_same(U, int); + +The normal form of B is + +.. code-block:: c++ + __is_same(T, int) /*T->U, innermost level*/ + && __is_same(U, int) {U->U} /*T->U, outermost most level*/ + +After substitution in the mapping, we substitute in the constraint expression +using that copy of the MultiLevelTemplateArgumentList, and then evaluate it. ---------------- zyn0217 wrote:
```suggestion using that copy of the ``MultiLevelTemplateArgumentList``, and then evaluate it. ``` https://github.com/llvm/llvm-project/pull/141776 _______________________________________________ cfe-commits mailing list cfe-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
