Use FacetAccessType when converting to a value of type FacetType

toolchain/check/convert.cpp

@@ -991,7 +991,24 @@ static auto PerformBuiltinConversion(Context& context, SemIR::LocId loc_id,
   }
 
   if (sem_ir.types().Is<SemIR::FacetType>(target.type_id)) {
-    if (sem_ir.types().Is<SemIR::FacetType>(value_type_id)) {
+    auto lookup_inst = value_id;
+
+    // `FacetAccessType` wraps an instruction that evaluates to a facet value
+    // (of type `FacetType`). If the `FacetType` matches the target
+    // `FacetType` then we don't need to do impl lookup. Otherwise, we want to
+    // try convert the result of the instruction in the `FacetAccessType`.
+    if (auto facet_access_type_value =
+            context.insts().TryGetAs<SemIR::FacetAccessType>(lookup_inst)) {
+      auto facet_value_inst_id = facet_access_type_value->facet_value_inst_id;
+      if (context.insts().Get(facet_value_inst_id).type_id() ==
+          target.type_id) {
+        return facet_value_inst_id;
+      }
+      lookup_inst = facet_access_type_value->facet_value_inst_id;
+    }
+
+    if (sem_ir.types().Is<SemIR::FacetType>(
+            sem_ir.insts().Get(lookup_inst).type_id())) {
       // Conversion from a facet value (which has type `FacetType`) to a
       // different facet value (which has type `FacetType`), if the value's
       // `FacetType` satisfies the requirements of the target `FacetType`. The
@@ -1004,23 +1021,28 @@ static auto PerformBuiltinConversion(Context& context, SemIR::LocId loc_id,
       // so using that here would be like an implicit cast back to the concrete
       // type.
       context.TODO(loc_id, "Facet value converting to facet value");
-    } else if (sem_ir.types().Is<SemIR::TypeType>(value_type_id)) {
+      return value_id;
+    }
+
+    if (sem_ir.types().Is<SemIR::TypeType>(
+            sem_ir.insts().Get(lookup_inst).type_id())) {
       // Conversion from a type value (which has type `type`) to a facet value
       // (which has type `FacetType`), if the type satisfies the requirements of
       // the target `FacetType`, as determined by finding an impl witness. This
       // binds the value to the `FacetType` with a `FacetValue`.
+
       auto witness_inst_id = LookupImplWitness(
           context, loc_id,
           // The value instruction evaluates to a type value (which has type
           // `type`). This gets that type value if it's available at compile
           // time, as a constant value.
-          context.constant_values().Get(value_id),
+          context.constant_values().Get(lookup_inst),
           context.types().GetConstantId(target.type_id));
       if (witness_inst_id != SemIR::InstId::None) {
         return context.AddInst<SemIR::FacetValue>(
             loc_id, {
                         .type_id = target.type_id,
-                        .type_inst_id = value_id,
+                        .type_inst_id = lookup_inst,
                         .witness_inst_id = witness_inst_id,
                     });
       }