[libcxx-commits] [clang] [libcxx] [Clang] Normalize constraints before checking for satisfaction (PR #141776)

[Younan Zhang via libcxx-commits]

================
@@ -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
----------------
zyn0217 wrote:

```suggestion
Checking satisfaction is done by ``ConstraintSatisfactionChecker``, recursively
walking ``NormalizedConstraint``. At each level, we substitute the outermost
```

https://github.com/llvm/llvm-project/pull/141776