facetaccesstype-roundtrip

Author: danakj

toolchain/check/convert.cpp

@@ -1211,11 +1211,26 @@ static auto PerformBuiltinConversion(
 
     if (auto facet_access_type_inst =
             sem_ir.insts().TryGetAs<SemIR::FacetAccessType>(const_value_id)) {
-      // Conversion from a `FacetAccessType` to a `FacetValue` of the target
-      // `FacetType` if the instruction in the `FacetAccessType` is of a
-      // `FacetType` that satisfies the requirements of the target `FacetType`.
-      // If the `FacetType` exactly matches the target `FacetType` then we can
-      // shortcut and use that value, and avoid impl lookup.
+      // Lossless round trips through a FacetAccessType when converting back to
+      // the type of its original facet value.
+      //
+      // Given a symbolic facet value X, if a FacetAccessType(X) is converted to
+      // the type of X, we make the past introduction of the FacetAccessType
+      // preserve the original facet value by converting back into that original
+      // facet value. This is only needed when the (facet type) types match
+      // exactly, which allows the result to have equality with the original
+      // facet value.
+      //
+      // ```
+      // fn F(A:! Interface, B:! A) {
+      //    // typeof(B) is a FacetAccessType(A). We want the following to be
+      //    // true:
+      //    (typeof(B) as typeof(A)) == A;
+      // }
+      // ```
+      //
+      // See also test:
+      // facet_access_type_converts_back_to_original_facet_value.carbon
       auto facet_value_inst_id = facet_access_type_inst->facet_value_inst_id;
       if (sem_ir.insts().Get(facet_value_inst_id).type_id() == target.type_id) {
         return facet_value_inst_id;