Add tests for accessing an aggregate member constant through an ImplWitnessAccess (#5702)

toolchain/check/testdata/facet/aggregate_through_access.carbon

@@ -0,0 +1,72 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/facet/aggregate_through_access.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/facet/aggregate_through_access.carbon
+
+// --- fail_todo_tuple_access_through_witness.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {
+  let X:! type;
+}
+
+// CHECK:STDERR: fail_todo_tuple_access_through_witness.carbon:[[@LINE+4]]:34: error: type `type` does not support tuple indexing; only tuples can be indexed that way [TupleIndexOnANonTupleType]
+// CHECK:STDERR: fn F(T:! Z where .X = ({}, )) -> T.X.0 {
+// CHECK:STDERR:                                  ^~~~~
+// CHECK:STDERR:
+fn F(T:! Z where .X = ({}, )) -> T.X.0 {
+  return {};
+}
+
+// --- fail_todo_struct_access_through_witness.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {
+  let X:! type;
+}
+
+// CHECK:STDERR: fail_todo_struct_access_through_witness.carbon:[[@LINE+4]]:36: error: type `type` does not support qualified expressions [QualifiedExprUnsupported]
+// CHECK:STDERR: fn F(T:! Z where .X = {.t: ()}) -> T.X.t {
+// CHECK:STDERR:                                    ^~~~~
+// CHECK:STDERR:
+fn F(T:! Z where .X = {.t: ()}) -> T.X.t {
+  return ();
+}
+
+// --- fail_todo_array_access_through_witness.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {
+  let X:! type;
+}
+
+// CHECK:STDERR: fail_todo_array_access_through_witness.carbon:[[@LINE+4]]:40: error: type `type` does not support indexing [TypeNotIndexable]
+// CHECK:STDERR: fn F(T:! Z where .X = array({}, 1)) -> T.X[0] {
+// CHECK:STDERR:                                        ^~~~~~
+// CHECK:STDERR:
+fn F(T:! Z where .X = array({}, 1)) -> T.X[0] {
+  return {};
+}
+
+// --- impl_access_through_witness.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {
+  let X1:! type;
+}
+
+interface Y {
+  let X2:! type;
+}
+
+class C;
+impl C as Y where .X2 = {} {}
+
+fn F(T:! Z where .X1 = C) -> T.X1.(Y.X2) {
+  return {};
+}