Allow witnesses to come from a facet with partially identified type in the lookup target (#7260)

We allow impl lookup to use `Self` which is partially identified, but we
were only allowing this for the `Self` facet appearing in the impl
lookup query self. We should also allow it when `Self` appears in the
impl lookup query target facet type.

But demonstrate that `Self` appearing in the interface of an
ImplWitnessAccess (a compound member access) is not sufficient at this
time, as it does not put `Self` into the type structure.

toolchain/check/impl_lookup.cpp

@@ -432,7 +432,7 @@ static auto CollectFacetWitnessSources(
   {
     SemIR::TypeIterator iter(&context.sem_ir());
     iter.Add(context.insts().GetAs<SemIR::FacetType>(query_facet_type_inst_id));
-    collect_facets(iter, /*allow_partially_identified=*/false);
+    collect_facets(iter, /*allow_partially_identified=*/true);
   }
 
   if (!context.where_stack().empty()) {

toolchain/check/testdata/named_constraint/require.carbon

@@ -699,6 +699,46 @@ fn G(T:! Z) {
   T as N;
 }
 
+// --- partially_identified_witness_from_self_in_lookup_facet_type.carbon
+library "[[@TEST_NAME]]";
+
+interface Z(T:! type) {
+  let Z1:! type;
+}
+interface Y(T:! type) {}
+
+class C {}
+
+constraint N {
+  require C impls Z(Self);
+  // Test that we can find the `C impls Z(Self)` witness above through the
+  // `Self` in the lookup target facet type in `C as Z(Self)`.
+  require C.(Z(Self).Z1) impls Y(Self);
+}
+
+// --- fail_self_in_compound_member_access_is_not_in_type_structure.carbon
+library "[[@TEST_NAME]]";
+
+interface Z(T:! type) {
+  let Z1:! type;
+}
+interface Y {}
+
+class C {}
+
+constraint N {
+  require C impls Z(Self);
+  // The type structure is `? impls Y`, where the `?` is dependent on `Self` but
+  // it is not `Self`. Since `Self` is not directly part of the type structure,
+  // we don't accept this require decl.
+  //
+  // CHECK:STDERR: fail_self_in_compound_member_access_is_not_in_type_structure.carbon:[[@LINE+4]]:3: error: no `Self` reference found in `require` declaration; `Self` must appear in the self-type or as a generic argument for each required interface, but found interface `Y` without a `Self` argument [RequireImplsMissingSelf]
+  // CHECK:STDERR:   require C.(Z(Self).Z1) impls Y;
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  require C.(Z(Self).Z1) impls Y;
+}
+
 // CHECK:STDOUT: --- extend.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {