Add failing tests for exposing the value of .Self through an interface (#5957)

We test using .Self in a generic parameter of an interface and as the
value of an associated constant.

toolchain/check/testdata/facet/access.carbon

@@ -80,6 +80,207 @@ fn UseIndirect[T:! I](x: T) -> T {
 //@dump-sem-ir-end
 }
 
+// --- fail_todo_convert_from_period_self_to_full_facet_value.carbon
+library "[[@TEST_NAME]]";
+
+interface I {
+  let I1:! type;
+}
+
+fn F(U:! I where .I1 = .Self) {
+  // CHECK:STDERR: fail_todo_convert_from_period_self_to_full_facet_value.carbon:[[@LINE+4]]:3: error: cannot convert type `U` that implements `I where .(I.I1) = .Self` into type implementing `I where .(I.I1) = U` [ConversionFailureFacetToFacet]
+  // CHECK:STDERR:   U as (I where .I1 = U);
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  U as (I where .I1 = U);
+  // CHECK:STDERR: fail_todo_convert_from_period_self_to_full_facet_value.carbon:[[@LINE+4]]:3: error: cannot convert type `U` that implements `I where .(I.I1) = .Self` into type implementing `I where .(I.I1) = U` [ConversionFailureFacetToFacet]
+  // CHECK:STDERR:   (U as type) as (I where .I1 = U);
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  (U as type) as (I where .I1 = U);
+}
+
+// --- convert_to_period_self.carbon
+library "[[@TEST_NAME]]";
+
+interface I {
+  let I1:! type;
+  let I2:! type;
+}
+
+fn F(U:! I where .I1 = .Self and .I2 = ()) {
+  U as (I where .I1 = .Self);
+  (U as type) as (I where .I1 = .Self);
+}
+
+// --- fail_todo_access_through_call_once.carbon
+library "[[@TEST_NAME]]";
+
+// TODO: Merge this test with the one below once they both work.
+
+interface I {
+  let X:! type;
+  fn G() -> X*;
+}
+
+fn F2[U:! I](V: U*) {}
+
+fn F(U:! I where .X = .Self, V: U) {
+  // The returned value of `G` type `U` which has access to the methods of `I`.
+  U.G()->G();
+  // CHECK:STDERR: fail_todo_access_through_call_once.carbon:[[@LINE+4]]:3: error: type `type` does not support qualified expressions [QualifiedExprUnsupported]
+  // CHECK:STDERR:   (U as type).G()->G();
+  // CHECK:STDERR:   ^~~~~~~~~~~~~
+  // CHECK:STDERR:
+  (U as type).G()->G();
+
+  // The returned value of type `U` can be used as a value of type `U`.
+  F2(U.G());
+}
+
+// --- fail_todo_access_through_call.carbon
+library "[[@TEST_NAME]]";
+
+interface I {
+  let X:! type;
+  fn G() -> X*;
+}
+
+fn F2[U:! I](V: U*) {}
+fn F3[U:! I where .X = .Self](V: U*) {}
+
+fn F(U:! I where .X = .Self, V: U*) {
+  // The returned value of `G` type `U` which has access to the methods of `I`.
+  //
+  // TODO: These should work.
+  // - The first `.` is on a NameRef of type FacetType for `I where .X = .Self`.
+  //   - This finds `G` through the FacetType.
+  // - The second `.` is on a Call of type FacetAccessType into `BindSymbolicName` with type FacetType for `I`.
+  //   - This finds `G` through the FacetType (impl lookup strips off FacetAccessType).
+  // - The third `.` is on a Call of type FacetAccessType into `ImplWitnessAccess` of `I.X` into `LookupImplWitness`, which has type `type`
+  //   - Can't make calls on an `ImplWitnessAccess`.
+  //   - We could expect that the constant value of the `ImplWitnessAccess` would
+  //     be the same type that we got for the second lookup.
+  // CHECK:STDERR: fail_todo_access_through_call.carbon:[[@LINE+4]]:3: error: type `.(I.X)` does not support qualified expressions [QualifiedExprUnsupported]
+  // CHECK:STDERR:   U.G()->G()->G();
+  // CHECK:STDERR:   ^~~~~~~~~~~~~
+  // CHECK:STDERR:
+  U.G()->G()->G();
+  // CHECK:STDERR: fail_todo_access_through_call.carbon:[[@LINE+4]]:3: error: type `type` does not support qualified expressions [QualifiedExprUnsupported]
+  // CHECK:STDERR:   (U as type).G()->G()->G();
+  // CHECK:STDERR:   ^~~~~~~~~~~~~
+  // CHECK:STDERR:
+  (U as type).G()->G()->G();
+
+  // The returned value of type `U` can be used as a value of type `U`.
+  //
+  // TODO: This should work.
+  // CHECK:STDERR: fail_todo_access_through_call.carbon:[[@LINE+4]]:6: error: type `.(I.X)` does not support qualified expressions [QualifiedExprUnsupported]
+  // CHECK:STDERR:   F2(U.G()->G()->G());
+  // CHECK:STDERR:      ^~~~~~~~~~~~~
+  // CHECK:STDERR:
+  F2(U.G()->G()->G());
+
+  // The constraints in the type `U` are preserved.
+  //
+  // TODO: These should work.
+  // CHECK:STDERR: fail_todo_access_through_call.carbon:[[@LINE+7]]:3: error: cannot convert type `.Self` that implements `I` into type implementing `I where .(I.X) = .Self` [ConversionFailureFacetToFacet]
+  // CHECK:STDERR:   F3(U.G());
+  // CHECK:STDERR:   ^~~~~~~~~
+  // CHECK:STDERR: fail_todo_access_through_call.carbon:[[@LINE-40]]:1: note: while deducing parameters of generic declared here [DeductionGenericHere]
+  // CHECK:STDERR: fn F3[U:! I where .X = .Self](V: U*) {}
+  // CHECK:STDERR: ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  F3(U.G());
+  // CHECK:STDERR: fail_todo_access_through_call.carbon:[[@LINE+4]]:6: error: type `.(I.X)` does not support qualified expressions [QualifiedExprUnsupported]
+  // CHECK:STDERR:   F3(U.G()->G()->G());
+  // CHECK:STDERR:      ^~~~~~~~~~~~~
+  // CHECK:STDERR:
+  F3(U.G()->G()->G());
+}
+
+// --- fail_todo_compound_access_through_call.carbon
+library "[[@TEST_NAME]]";
+
+interface I {
+  let X:! type;
+  fn G() -> X;
+}
+
+fn F(U:! I where .X = .Self) {
+  // Compound member lookup through a non-type value is possible for methods
+  // which take a `self` parameter. But it's not possible for methods without
+  // `self`. For those you need to go directly throug the type.
+  // See: https://github.com/carbon-language/carbon-lang/issues/6025
+
+  // TODO: This step should work.
+  //
+  // CHECK:STDERR: fail_todo_compound_access_through_call.carbon:[[@LINE+7]]:14: error: cannot implicitly convert expression of type `.Self` to `U` [ConversionFailure]
+  // CHECK:STDERR:   let u: U = U.(I.G)();
+  // CHECK:STDERR:              ^~~~~~~~~
+  // CHECK:STDERR: fail_todo_compound_access_through_call.carbon:[[@LINE+4]]:14: note: type `.Self` does not implement interface `Core.ImplicitAs(U)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   let u: U = U.(I.G)();
+  // CHECK:STDERR:              ^~~~~~~~~
+  // CHECK:STDERR:
+  let u: U = U.(I.G)();
+  // `u` is a non-type value. Can call methods with `self` through compound
+  // member lookup, but can't call methods without `self`. See the
+  // `compound_access_through_call_with_self_param.carbon` test for the former.
+  //
+  // CHECK:STDERR: fail_todo_compound_access_through_call.carbon:[[@LINE+7]]:3: error: cannot implicitly convert non-type value of type `U` into type implementing `I` [ConversionFailureNonTypeToFacet]
+  // CHECK:STDERR:   u.(I.G)();
+  // CHECK:STDERR:   ^~~~~~~
+  // CHECK:STDERR: fail_todo_compound_access_through_call.carbon:[[@LINE+4]]:3: note: type `U` does not implement interface `Core.ImplicitAs(I)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   u.(I.G)();
+  // CHECK:STDERR:   ^~~~~~~
+  // CHECK:STDERR:
+  u.(I.G)();
+
+  // This is the same as the above, since G() returns a non-type value of type
+  // `U`.
+  //
+  // CHECK:STDERR: fail_todo_compound_access_through_call.carbon:[[@LINE+7]]:3: error: cannot implicitly convert non-type value of type `.Self` into type implementing `I` [ConversionFailureNonTypeToFacet]
+  // CHECK:STDERR:   U.(I.G)().(I.G)();
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~
+  // CHECK:STDERR: fail_todo_compound_access_through_call.carbon:[[@LINE+4]]:3: note: type `.Self` does not implement interface `Core.ImplicitAs(I)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   U.(I.G)().(I.G)();
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  U.(I.G)().(I.G)();
+}
+
+// --- fail_todo_compound_access_through_call_with_self_param.carbon
+library "[[@TEST_NAME]]";
+
+interface I {
+  let X:! type;
+  fn G[self: Self]() -> X*;
+}
+
+fn F(U:! I where .X = .Self, v: U) {
+  // Compound member lookup through a non-type value is possible for methods
+  // which take a `self` parameter.
+
+  // TODO: This should all work.
+
+  // CHECK:STDERR: fail_todo_compound_access_through_call_with_self_param.carbon:[[@LINE+7]]:15: error: cannot implicitly convert expression of type `.Self*` to `U*` [ConversionFailure]
+  // CHECK:STDERR:   let u: U* = v.(I.G)();
+  // CHECK:STDERR:               ^~~~~~~~~
+  // CHECK:STDERR: fail_todo_compound_access_through_call_with_self_param.carbon:[[@LINE+4]]:15: note: type `.Self*` does not implement interface `Core.ImplicitAs(U*)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   let u: U* = v.(I.G)();
+  // CHECK:STDERR:               ^~~~~~~~~
+  // CHECK:STDERR:
+  let u: U* = v.(I.G)();
+  // `u` is a non-type value. Can call methods with `self` through compound
+  // member lookup, but can't call methods without `self`. See the
+  // `compound_access_through_call.carbon` test for the latter.
+  u->(I.G)();
+
+  // This is the same as the above, since G() returns a non-type value of type
+  // `U`. This works because G has a `self` parameter.
+  v.(I.G)()->(I.G)();
+}
+
 // --- fail_non_const_associated.carbon
 library "[[@TEST_NAME]]";
 

toolchain/check/testdata/facet/period_self.carbon

@@ -0,0 +1,525 @@
+// 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
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/facet/period_self.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/facet/period_self.carbon
+
+// --- period_self_param.carbon
+library "[[@TEST_NAME]]";
+
+//@dump-sem-ir-begin
+interface I(T:! type) {
+  let I1:! type;
+}
+
+fn F(T:! I(.Self) where .I1 = ()) -> T.I1 {
+  return ();
+}
+
+fn G(T:! I(.Self as type) where .I1 = ()) -> T.I1 {
+  return ();
+}
+//@dump-sem-ir-end
+
+// --- underscore_identifier_name.carbon
+library "[[@TEST_NAME]]";
+
+interface I(T:! type) {
+  let I1:! type;
+}
+
+// Underscore as the identifier name produces a different parse tree for the
+// binding pattern.
+fn G(_:! I(.Self) where .I1 = ()) {}
+
+// --- fail_period_self_as_type.carbon
+library "[[@TEST_NAME]]";
+
+// TODO: We should diagnose this use of `.Self` directly rather than later when
+// it is converted to `type`.
+interface I(T:! .Self) {
+  // CHECK:STDERR: fail_period_self_as_type.carbon:[[@LINE+7]]:13: error: cannot implicitly convert non-type value of type `.Self` to `type` [ConversionFailureNonTypeToFacet]
+  // CHECK:STDERR:   fn G() -> T;
+  // CHECK:STDERR:             ^
+  // CHECK:STDERR: fail_period_self_as_type.carbon:[[@LINE+4]]:13: note: type `.Self` does not implement interface `Core.ImplicitAs(type)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   fn G() -> T;
+  // CHECK:STDERR:             ^
+  // CHECK:STDERR:
+  fn G() -> T;
+}
+
+// --- fail_todo_convert_period_self_to_full_facet_value.carbon
+library "[[@TEST_NAME]]";
+
+interface I(T:! type) {}
+
+fn F(U:! I(.Self)) {
+  // CHECK:STDERR: fail_todo_convert_period_self_to_full_facet_value.carbon:[[@LINE+4]]:3: error: cannot convert type `U` that implements `I(.Self)` into type implementing `I(U)` [ConversionFailureFacetToFacet]
+  // CHECK:STDERR:   U as I(U);
+  // CHECK:STDERR:   ^~~~~~~~~
+  // CHECK:STDERR:
+  U as I(U);
+  // CHECK:STDERR: fail_todo_convert_period_self_to_full_facet_value.carbon:[[@LINE+4]]:3: error: cannot convert type `U` that implements `I(.Self)` into type implementing `I(U)` [ConversionFailureFacetToFacet]
+  // CHECK:STDERR:   (U as type) as I(U);
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  (U as type) as I(U);
+}
+
+// --- fail_todo_convert_period_self_to_full_facet_value_with_assoc_constant.carbon
+library "[[@TEST_NAME]]";
+
+interface I(T:! type) {
+  let X:! type;
+}
+
+fn F(U:! I(.Self) where .X = .Self) {
+  // CHECK:STDERR: fail_todo_convert_period_self_to_full_facet_value_with_assoc_constant.carbon:[[@LINE+4]]:3: error: cannot convert type `U` that implements `I(.Self) where .(I(.Self).X) = .Self` into type implementing `I(U) where .(I(U).X) = U` [ConversionFailureFacetToFacet]
+  // CHECK:STDERR:   U as (I(U) where .X = U);
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  U as (I(U) where .X = U);
+  // CHECK:STDERR: fail_todo_convert_period_self_to_full_facet_value_with_assoc_constant.carbon:[[@LINE+4]]:3: error: cannot convert type `U` that implements `I(.Self) where .(I(.Self).X) = .Self` into type implementing `I(U) where .(I(U).X) = U` [ConversionFailureFacetToFacet]
+  // CHECK:STDERR:   (U as type) as (I(U) where .X = U);
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  (U as type) as (I(U) where .X = U);
+}
+
+// --- fail_todo_return_of_type_period_self.carbon
+library "[[@TEST_NAME]]";
+
+interface I(T:! type) {
+  fn G() -> T*;
+}
+
+interface J(T:! type) {
+  fn J1() -> T*;
+}
+
+fn F2[U:! I(.Self)](T: U*) {}
+
+fn F(U:! I(.Self)) {
+  // Caller sees the returned `.Self` type from `F()` as the full `U`.
+  // CHECK:STDERR: fail_todo_return_of_type_period_self.carbon:[[@LINE+4]]:3: error: member name `G` not found [MemberNameNotFound]
+  // CHECK:STDERR:   U.G()->G()->G();
+  // CHECK:STDERR:   ^~~~~~~~
+  // CHECK:STDERR:
+  U.G()->G()->G();
+  // Conversion to `type` retains access to all of `U`.
+  // CHECK:STDERR: fail_todo_return_of_type_period_self.carbon:[[@LINE+4]]:3: error: type `type` does not support qualified expressions [QualifiedExprUnsupported]
+  // CHECK:STDERR:   (U as type).G()->G()->G();
+  // CHECK:STDERR:   ^~~~~~~~~~~~~
+  // CHECK:STDERR:
+  (U as type).G()->G()->G();
+
+  // Using the returned `.Self` as a facet value works, not just member lookup
+  // on its facet type.
+  // CHECK:STDERR: fail_todo_return_of_type_period_self.carbon:[[@LINE+4]]:6: error: member name `G` not found [MemberNameNotFound]
+  // CHECK:STDERR:   F2(U.G()->G()->G());
+  // CHECK:STDERR:      ^~~~~~~~
+  // CHECK:STDERR:
+  F2(U.G()->G()->G());
+}
+
+// --- fail_todo_return_of_type_period_self_extends_interface.carbon
+library "[[@TEST_NAME]]";
+
+interface I(T:! type) {
+  fn I1() -> T*;
+}
+
+interface J(T:! type) {
+  fn J1() -> T*;
+}
+
+fn F2[U:! I(.Self) & J(.Self)](T: U*) {}
+
+fn F(U:! I(.Self) & J(.Self)) {
+  // TODO: The returned value of `I1` and `J1` has type `U` which has access to
+  // the methods of `I` and `J`.
+  // CHECK:STDERR: fail_todo_return_of_type_period_self_extends_interface.carbon:[[@LINE+4]]:3: error: member name `J1` not found [MemberNameNotFound]
+  // CHECK:STDERR:   U.I1()->J1()->I1()->J1()->I1()->J1();
+  // CHECK:STDERR:   ^~~~~~~~~~
+  // CHECK:STDERR:
+  U.I1()->J1()->I1()->J1()->I1()->J1();
+  // CHECK:STDERR: fail_todo_return_of_type_period_self_extends_interface.carbon:[[@LINE+4]]:3: error: type `type` does not support qualified expressions [QualifiedExprUnsupported]
+  // CHECK:STDERR:   (U as type).I1()->J1()->I1()->J1()->I1()->J1();
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  (U as type).I1()->J1()->I1()->J1()->I1()->J1();
+
+  // TODO: Using the returned value of type `U` as a facet value works.
+  // CHECK:STDERR: fail_todo_return_of_type_period_self_extends_interface.carbon:[[@LINE+4]]:6: error: member name `J1` not found [MemberNameNotFound]
+  // CHECK:STDERR:   F2(U.I1()->J1()->I1()->J1()->I1()->J1());
+  // CHECK:STDERR:      ^~~~~~~~~~
+  // CHECK:STDERR:
+  F2(U.I1()->J1()->I1()->J1()->I1()->J1());
+}
+
+// --- fail_todo_return_of_period_self_impls_interface.carbon
+library "[[@TEST_NAME]]";
+
+interface I(T:! type) {
+  fn I1() -> T;
+}
+
+interface J(T:! type) {
+  fn J1() -> T;
+}
+
+fn G(U:! I(.Self) where .Self impls J(.Self)) {
+  // Compound member lookup through a non-type value is possible for methods
+  // which take a `self` parameter. But it's not possible for methods without
+  // `self`. For those you need to go directly throug the type.
+  // See: https://github.com/carbon-language/carbon-lang/issues/6025
+
+  // TODO: This step should work.
+  //
+  // CHECK:STDERR: fail_todo_return_of_period_self_impls_interface.carbon:[[@LINE+7]]:14: error: cannot implicitly convert expression of type `.Self` to `U` [ConversionFailure]
+  // CHECK:STDERR:   let u: U = U.I1();
+  // CHECK:STDERR:              ^~~~~~
+  // CHECK:STDERR: fail_todo_return_of_period_self_impls_interface.carbon:[[@LINE+4]]:14: note: type `.Self` does not implement interface `Core.ImplicitAs(U)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   let u: U = U.I1();
+  // CHECK:STDERR:              ^~~~~~
+  // CHECK:STDERR:
+  let u: U = U.I1();
+  // `u` is a non-type value. Can call methods with `self` through compound
+  // member lookup, but can't call methods without `self`. See the
+  // `compound_access_through_call_with_self_param.carbon` test for the former.
+  //
+  // CHECK:STDERR: fail_todo_return_of_period_self_impls_interface.carbon:[[@LINE+4]]:6: error: type `<type of J>` does not support qualified expressions [QualifiedExprUnsupported]
+  // CHECK:STDERR:   u.(J.J1)();
+  // CHECK:STDERR:      ^~~~
+  // CHECK:STDERR:
+  u.(J.J1)();
+
+  // This is the same as the above, since U.I1() returns a non-type value of
+  // type `U`.
+  //
+  // CHECK:STDERR: fail_todo_return_of_period_self_impls_interface.carbon:[[@LINE+4]]:11: error: type `<type of J>` does not support qualified expressions [QualifiedExprUnsupported]
+  // CHECK:STDERR:   U.I1().(J.J1)();
+  // CHECK:STDERR:           ^~~~
+  // CHECK:STDERR:
+  U.I1().(J.J1)();
+}
+
+// --- fail_todo_return_of_type_period_self_has_type_u.carbon
+library "[[@TEST_NAME]]";
+
+interface I(T:! type) {
+  fn G() -> T;
+}
+
+fn F(U:! I(.Self)) {
+  // CHECK:STDERR: fail_todo_return_of_type_period_self_has_type_u.carbon:[[@LINE+7]]:14: error: cannot implicitly convert expression of type `.Self` to `U` [ConversionFailure]
+  // CHECK:STDERR:   let a: U = U.G();
+  // CHECK:STDERR:              ^~~~~
+  // CHECK:STDERR: fail_todo_return_of_type_period_self_has_type_u.carbon:[[@LINE+4]]:14: note: type `.Self` does not implement interface `Core.ImplicitAs(U)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   let a: U = U.G();
+  // CHECK:STDERR:              ^~~~~
+  // CHECK:STDERR:
+  let a: U = U.G();
+}
+
+// --- fail_todo_return_of_type_period_self_assoc_const_has_type_u.carbon
+library "[[@TEST_NAME]]";
+
+interface I {
+  let X:! type;
+  fn G() -> X;
+}
+
+fn F(U:! I where .X = .Self) {
+  // CHECK:STDERR: fail_todo_return_of_type_period_self_assoc_const_has_type_u.carbon:[[@LINE+7]]:14: error: cannot implicitly convert expression of type `.Self` to `U` [ConversionFailure]
+  // CHECK:STDERR:   let a: U = U.G();
+  // CHECK:STDERR:              ^~~~~
+  // CHECK:STDERR: fail_todo_return_of_type_period_self_assoc_const_has_type_u.carbon:[[@LINE+4]]:14: note: type `.Self` does not implement interface `Core.ImplicitAs(U)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   let a: U = U.G();
+  // CHECK:STDERR:              ^~~~~
+  // CHECK:STDERR:
+  let a: U = U.G();
+}
+
+// --- fail_todo_nested_period_self.carbon
+library "[[@TEST_NAME]]";
+
+interface I(T:! type) {
+  let A:! type;
+  let B:! type;
+  fn G() -> T;
+}
+
+// Both `.Self` refer to `T`. The first because it's the interface for the
+// binding. The second because it refers to the top level facet type which is
+// constraining the binding.
+fn F(T:! I(.Self) where .A = ((I(.Self) where .B = {}) where .A = {}) and .B = {}, U:! T.A) {
+  // T.G() has type T.
+  // CHECK:STDERR: fail_todo_nested_period_self.carbon:[[@LINE+7]]:14: error: cannot implicitly convert expression of type `.Self` to `T` [ConversionFailure]
+  // CHECK:STDERR:   let t: T = T.G();
+  // CHECK:STDERR:              ^~~~~
+  // CHECK:STDERR: fail_todo_nested_period_self.carbon:[[@LINE+4]]:14: note: type `.Self` does not implement interface `Core.ImplicitAs(T)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   let t: T = T.G();
+  // CHECK:STDERR:              ^~~~~
+  // CHECK:STDERR:
+  let t: T = T.G();
+    // U.G() has type T.
+  // CHECK:STDERR: fail_todo_nested_period_self.carbon:[[@LINE+7]]:14: error: cannot implicitly convert expression of type `.Self` to `T` [ConversionFailure]
+  // CHECK:STDERR:   let u: T = U.G();
+  // CHECK:STDERR:              ^~~~~
+  // CHECK:STDERR: fail_todo_nested_period_self.carbon:[[@LINE+4]]:14: note: type `.Self` does not implement interface `Core.ImplicitAs(T)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   let u: T = U.G();
+  // CHECK:STDERR:              ^~~~~
+  // CHECK:STDERR:
+  let u: T = U.G();
+
+  // Shows both `I(.Self)` are `I(T)`.
+  // CHECK:STDERR: fail_todo_nested_period_self.carbon:[[@LINE+4]]:9: error: found cycle in facet type constraint for `.(I(T).A)` [FacetTypeConstraintCycle]
+  // CHECK:STDERR:   T as (I(T) where .A = (I(T) where .A = {} and .B = {}));
+  // CHECK:STDERR:         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  T as (I(T) where .A = (I(T) where .A = {} and .B = {}));
+  // CHECK:STDERR: fail_todo_nested_period_self.carbon:[[@LINE+4]]:3: error: cannot convert type `U` that implements `I(.Self) where .(I(.Self).B) = {} and .(I(.Self).A) = {}` into type implementing `I(T) where .(I(T).A) = {} and .(I(T).B) = {}` [ConversionFailureFacetToFacet]
+  // CHECK:STDERR:   U as (I(T) where .A = {} and .B = {});
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  U as (I(T) where .A = {} and .B = {});
+}
+
+// --- todo_fail_nested_period_self_ambiguous.carbon
+library "[[@TEST_NAME]]";
+
+interface I(T:! type) {
+  let A:! type;
+}
+
+// TODO: This should be an error: The third `.Self` becomes is not able to be
+// bound to anything unambiguous here, as it could refer to `T` or to something
+// later being constrained by `T.A`.
+fn F(T:! I(.Self) where .A = (I(.Self) where .A = I(.Self))) {}
+
+
+// --- period_self_parameter_sees_lhs_of_where_expr.carbon
+library "[[@TEST_NAME]]";
+
+interface I(T:! Core.Destroy) {}
+
+// The `.Self` can see the LHS of the `where` to know `U` impls Core.Destroy.
+fn F(U:! Core.Destroy where .Self impls I(.Self)) {}
+
+// CHECK:STDOUT: --- period_self_param.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %type: type = facet_type <type> [concrete]
+// CHECK:STDOUT:   %.Self.659: %type = bind_symbolic_name .Self [symbolic_self]
+// CHECK:STDOUT:   %T.8b3: type = bind_symbolic_name T, 0 [symbolic]
+// CHECK:STDOUT:   %pattern_type.98f: type = pattern_type type [concrete]
+// CHECK:STDOUT:   %I.type.dac: type = generic_interface_type @I [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %I.generic: %I.type.dac = struct_value () [concrete]
+// CHECK:STDOUT:   %I.type.b13: type = facet_type <@I, @I(%T.8b3)> [symbolic]
+// CHECK:STDOUT:   %Self.dae: %I.type.b13 = bind_symbolic_name Self, 1 [symbolic]
+// CHECK:STDOUT:   %I.assoc_type.1e5: type = assoc_entity_type @I, @I(%T.8b3) [symbolic]
+// CHECK:STDOUT:   %assoc0.ab0: %I.assoc_type.1e5 = assoc_entity element0, @I.%I1 [symbolic]
+// CHECK:STDOUT:   %.Self.binding.as_type.373: type = symbolic_binding_type .Self, %.Self.659 [symbolic_self]
+// CHECK:STDOUT:   %I.type.986: type = facet_type <@I, @I(%.Self.binding.as_type.373)> [symbolic_self]
+// CHECK:STDOUT:   %.Self.955: %I.type.986 = bind_symbolic_name .Self [symbolic_self]
+// CHECK:STDOUT:   %Self.f35: %I.type.986 = bind_symbolic_name Self, 1 [symbolic]
+// CHECK:STDOUT:   %I.assoc_type.4e2: type = assoc_entity_type @I, @I(%.Self.binding.as_type.373) [symbolic_self]
+// CHECK:STDOUT:   %assoc0.7d3: %I.assoc_type.4e2 = assoc_entity element0, @I.%I1 [symbolic_self]
+// CHECK:STDOUT:   %.Self.binding.as_type.320: type = symbolic_binding_type .Self, %.Self.955 [symbolic_self]
+// CHECK:STDOUT:   %I.lookup_impl_witness.462: <witness> = lookup_impl_witness %.Self.955, @I, @I(%.Self.binding.as_type.373) [symbolic_self]
+// CHECK:STDOUT:   %impl.elem0: type = impl_witness_access %I.lookup_impl_witness.462, element0 [symbolic_self]
+// CHECK:STDOUT:   %I_where.type: type = facet_type <@I, @I(%.Self.binding.as_type.373) where %impl.elem0 = %empty_tuple.type> [symbolic_self]
+// CHECK:STDOUT:   %T.797: %I_where.type = bind_symbolic_name T, 0 [symbolic]
+// CHECK:STDOUT:   %pattern_type.85a: type = pattern_type %I_where.type [symbolic_self]
+// CHECK:STDOUT:   %T.binding.as_type: type = symbolic_binding_type T, 0, %T.797 [symbolic]
+// CHECK:STDOUT:   %I.lookup_impl_witness.920: <witness> = lookup_impl_witness %T.797, @I, @I(%.Self.binding.as_type.373) [symbolic]
+// CHECK:STDOUT:   %I.facet: %I.type.986 = facet_value %T.binding.as_type, (%I.lookup_impl_witness.920) [symbolic]
+// CHECK:STDOUT:   %pattern_type.cb1: type = pattern_type %empty_tuple.type [concrete]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_tuple: %empty_tuple.type = tuple_value () [concrete]
+// CHECK:STDOUT:   %G.type: type = fn_type @G [concrete]
+// CHECK:STDOUT:   %G: %G.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   %I.decl: %I.type.dac = interface_decl @I [concrete = constants.%I.generic] {
+// CHECK:STDOUT:     %T.patt: %pattern_type.98f = symbolic_binding_pattern T, 0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:     %T.loc4_13.2: type = bind_symbolic_name T, 0 [symbolic = %T.loc4_13.1 (constants.%T.8b3)]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
+// CHECK:STDOUT:     %T.patt: %pattern_type.85a = symbolic_binding_pattern T, 0 [concrete]
+// CHECK:STDOUT:     %return.patt: %pattern_type.cb1 = return_slot_pattern [concrete]
+// CHECK:STDOUT:     %return.param_patt: %pattern_type.cb1 = out_param_pattern %return.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %T.ref: %I_where.type = name_ref T, %T.loc8_6.2 [symbolic = %T.loc8_6.1 (constants.%T.797)]
+// CHECK:STDOUT:     %.loc8_39.1: %I.assoc_type.4e2 = specific_constant @I1.%assoc0, @I(constants.%.Self.binding.as_type.373) [symbolic_self = constants.%assoc0.7d3]
+// CHECK:STDOUT:     %I1.ref.loc8_39: %I.assoc_type.4e2 = name_ref I1, %.loc8_39.1 [symbolic_self = constants.%assoc0.7d3]
+// CHECK:STDOUT:     %T.as_type: type = facet_access_type %T.ref [symbolic = %T.binding.as_type (constants.%T.binding.as_type)]
+// CHECK:STDOUT:     %.loc8_39.2: type = converted %T.ref, %T.as_type [symbolic = %T.binding.as_type (constants.%T.binding.as_type)]
+// CHECK:STDOUT:     %impl.elem0.loc8_39: type = impl_witness_access constants.%I.lookup_impl_witness.920, element0 [concrete = constants.%empty_tuple.type]
+// CHECK:STDOUT:     %.loc8_19.1: type = splice_block %.loc8_19.2 [symbolic_self = constants.%I_where.type] {
+// CHECK:STDOUT:       <elided>
+// CHECK:STDOUT:       %I.ref: %I.type.dac = name_ref I, file.%I.decl [concrete = constants.%I.generic]
+// CHECK:STDOUT:       %.Self.ref.loc8_12: %type = name_ref .Self, %.Self.1 [symbolic_self = constants.%.Self.659]
+// CHECK:STDOUT:       %.Self.as_type.loc8_17: type = facet_access_type %.Self.ref.loc8_12 [symbolic_self = constants.%.Self.binding.as_type.373]
+// CHECK:STDOUT:       %.loc8_17: type = converted %.Self.ref.loc8_12, %.Self.as_type.loc8_17 [symbolic_self = constants.%.Self.binding.as_type.373]
+// CHECK:STDOUT:       %I.type: type = facet_type <@I, @I(constants.%.Self.binding.as_type.373)> [symbolic_self = constants.%I.type.986]
+// CHECK:STDOUT:       <elided>
+// CHECK:STDOUT:       %.Self.ref.loc8_25: %I.type.986 = name_ref .Self, %.Self.2 [symbolic_self = constants.%.Self.955]
+// CHECK:STDOUT:       %.loc8_25.1: %I.assoc_type.4e2 = specific_constant @I1.%assoc0, @I(constants.%.Self.binding.as_type.373) [symbolic_self = constants.%assoc0.7d3]
+// CHECK:STDOUT:       %I1.ref.loc8_25: %I.assoc_type.4e2 = name_ref I1, %.loc8_25.1 [symbolic_self = constants.%assoc0.7d3]
+// CHECK:STDOUT:       %.Self.as_type.loc8_25: type = facet_access_type %.Self.ref.loc8_25 [symbolic_self = constants.%.Self.binding.as_type.320]
+// CHECK:STDOUT:       %.loc8_25.2: type = converted %.Self.ref.loc8_25, %.Self.as_type.loc8_25 [symbolic_self = constants.%.Self.binding.as_type.320]
+// CHECK:STDOUT:       %impl.elem0.loc8_25: type = impl_witness_access constants.%I.lookup_impl_witness.462, element0 [symbolic_self = constants.%impl.elem0]
+// CHECK:STDOUT:       %.loc8_32.1: %empty_tuple.type = tuple_literal ()
+// CHECK:STDOUT:       %.loc8_32.2: type = converted %.loc8_32.1, constants.%empty_tuple.type [concrete = constants.%empty_tuple.type]
+// CHECK:STDOUT:       %.loc8_19.2: type = where_expr %.Self.2 [symbolic_self = constants.%I_where.type] {
+// CHECK:STDOUT:         requirement_base_facet_type constants.%I.type.986
+// CHECK:STDOUT:         requirement_rewrite %impl.elem0.loc8_25, %.loc8_32.2
+// CHECK:STDOUT:       }
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %T.loc8_6.2: %I_where.type = bind_symbolic_name T, 0 [symbolic = %T.loc8_6.1 (constants.%T.797)]
+// CHECK:STDOUT:     %return.param: ref %empty_tuple.type = out_param call_param0
+// CHECK:STDOUT:     %return: ref %empty_tuple.type = return_slot %return.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {
+// CHECK:STDOUT:     %T.patt: %pattern_type.85a = symbolic_binding_pattern T, 0 [concrete]
+// CHECK:STDOUT:     %return.patt: %pattern_type.cb1 = return_slot_pattern [concrete]
+// CHECK:STDOUT:     %return.param_patt: %pattern_type.cb1 = out_param_pattern %return.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %T.ref: %I_where.type = name_ref T, %T.loc12_6.2 [symbolic = %T.loc12_6.1 (constants.%T.797)]
+// CHECK:STDOUT:     %.loc12_47.1: %I.assoc_type.4e2 = specific_constant @I1.%assoc0, @I(constants.%.Self.binding.as_type.373) [symbolic_self = constants.%assoc0.7d3]
+// CHECK:STDOUT:     %I1.ref.loc12_47: %I.assoc_type.4e2 = name_ref I1, %.loc12_47.1 [symbolic_self = constants.%assoc0.7d3]
+// CHECK:STDOUT:     %T.as_type: type = facet_access_type %T.ref [symbolic = %T.binding.as_type (constants.%T.binding.as_type)]
+// CHECK:STDOUT:     %.loc12_47.2: type = converted %T.ref, %T.as_type [symbolic = %T.binding.as_type (constants.%T.binding.as_type)]
+// CHECK:STDOUT:     %impl.elem0.loc12_47: type = impl_witness_access constants.%I.lookup_impl_witness.920, element0 [concrete = constants.%empty_tuple.type]
+// CHECK:STDOUT:     %.loc12_27.1: type = splice_block %.loc12_27.2 [symbolic_self = constants.%I_where.type] {
+// CHECK:STDOUT:       <elided>
+// CHECK:STDOUT:       %I.ref: %I.type.dac = name_ref I, file.%I.decl [concrete = constants.%I.generic]
+// CHECK:STDOUT:       %.Self.ref.loc12_12: %type = name_ref .Self, %.Self.1 [symbolic_self = constants.%.Self.659]
+// CHECK:STDOUT:       %.Self.as_type.loc12_18: type = facet_access_type %.Self.ref.loc12_12 [symbolic_self = constants.%.Self.binding.as_type.373]
+// CHECK:STDOUT:       %.loc12_18: type = converted %.Self.ref.loc12_12, %.Self.as_type.loc12_18 [symbolic_self = constants.%.Self.binding.as_type.373]
+// CHECK:STDOUT:       %I.type: type = facet_type <@I, @I(constants.%.Self.binding.as_type.373)> [symbolic_self = constants.%I.type.986]
+// CHECK:STDOUT:       <elided>
+// CHECK:STDOUT:       %.Self.ref.loc12_33: %I.type.986 = name_ref .Self, %.Self.2 [symbolic_self = constants.%.Self.955]
+// CHECK:STDOUT:       %.loc12_33.1: %I.assoc_type.4e2 = specific_constant @I1.%assoc0, @I(constants.%.Self.binding.as_type.373) [symbolic_self = constants.%assoc0.7d3]
+// CHECK:STDOUT:       %I1.ref.loc12_33: %I.assoc_type.4e2 = name_ref I1, %.loc12_33.1 [symbolic_self = constants.%assoc0.7d3]
+// CHECK:STDOUT:       %.Self.as_type.loc12_33: type = facet_access_type %.Self.ref.loc12_33 [symbolic_self = constants.%.Self.binding.as_type.320]
+// CHECK:STDOUT:       %.loc12_33.2: type = converted %.Self.ref.loc12_33, %.Self.as_type.loc12_33 [symbolic_self = constants.%.Self.binding.as_type.320]
+// CHECK:STDOUT:       %impl.elem0.loc12_33: type = impl_witness_access constants.%I.lookup_impl_witness.462, element0 [symbolic_self = constants.%impl.elem0]
+// CHECK:STDOUT:       %.loc12_40.1: %empty_tuple.type = tuple_literal ()
+// CHECK:STDOUT:       %.loc12_40.2: type = converted %.loc12_40.1, constants.%empty_tuple.type [concrete = constants.%empty_tuple.type]
+// CHECK:STDOUT:       %.loc12_27.2: type = where_expr %.Self.2 [symbolic_self = constants.%I_where.type] {
+// CHECK:STDOUT:         requirement_base_facet_type constants.%I.type.986
+// CHECK:STDOUT:         requirement_rewrite %impl.elem0.loc12_33, %.loc12_40.2
+// CHECK:STDOUT:       }
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %T.loc12_6.2: %I_where.type = bind_symbolic_name T, 0 [symbolic = %T.loc12_6.1 (constants.%T.797)]
+// CHECK:STDOUT:     %return.param: ref %empty_tuple.type = out_param call_param0
+// CHECK:STDOUT:     %return: ref %empty_tuple.type = return_slot %return.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic interface @I(%T.loc4_13.2: type) {
+// CHECK:STDOUT:   %T.loc4_13.1: type = bind_symbolic_name T, 0 [symbolic = %T.loc4_13.1 (constants.%T.8b3)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %I.assoc_type: type = assoc_entity_type @I, @I(%T.loc4_13.1) [symbolic = %I.assoc_type (constants.%I.assoc_type.1e5)]
+// CHECK:STDOUT:   %assoc0: @I.%I.assoc_type (%I.assoc_type.1e5) = assoc_entity element0, %I1 [symbolic = %assoc0 (constants.%assoc0.ab0)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   interface {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:     %I1: type = assoc_const_decl @I1 [concrete] {
+// CHECK:STDOUT:       %assoc0: @I.%I.assoc_type (%I.assoc_type.1e5) = assoc_entity element0, @I.%I1 [symbolic = @I.%assoc0 (constants.%assoc0.ab0)]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .Self = %Self.1
+// CHECK:STDOUT:     .I1 = @I1.%assoc0
+// CHECK:STDOUT:     witness = (%I1)
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic assoc_const @I1(@I.%T.loc4_13.2: type, @I.%Self.1: @I.%I.type (%I.type.b13)) {
+// CHECK:STDOUT:   assoc_const I1:! type;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @F(%T.loc8_6.2: %I_where.type) {
+// CHECK:STDOUT:   %T.loc8_6.1: %I_where.type = bind_symbolic_name T, 0 [symbolic = %T.loc8_6.1 (constants.%T.797)]
+// CHECK:STDOUT:   %T.binding.as_type: type = symbolic_binding_type T, 0, %T.loc8_6.1 [symbolic = %T.binding.as_type (constants.%T.binding.as_type)]
+// CHECK:STDOUT:   %I.lookup_impl_witness: <witness> = lookup_impl_witness %T.loc8_6.1, @I, @I(constants.%.Self.binding.as_type.373) [symbolic = %I.lookup_impl_witness (constants.%I.lookup_impl_witness.920)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn() -> %empty_tuple.type {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     %.loc9_11.1: %empty_tuple.type = tuple_literal ()
+// CHECK:STDOUT:     %.loc9_11.2: init %empty_tuple.type = tuple_init () to %return [concrete = constants.%empty_tuple]
+// CHECK:STDOUT:     %.loc9_12: init %empty_tuple.type = converted %.loc9_11.1, %.loc9_11.2 [concrete = constants.%empty_tuple]
+// CHECK:STDOUT:     return %.loc9_12 to %return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @G(%T.loc12_6.2: %I_where.type) {
+// CHECK:STDOUT:   %T.loc12_6.1: %I_where.type = bind_symbolic_name T, 0 [symbolic = %T.loc12_6.1 (constants.%T.797)]
+// CHECK:STDOUT:   %T.binding.as_type: type = symbolic_binding_type T, 0, %T.loc12_6.1 [symbolic = %T.binding.as_type (constants.%T.binding.as_type)]
+// CHECK:STDOUT:   %I.lookup_impl_witness: <witness> = lookup_impl_witness %T.loc12_6.1, @I, @I(constants.%.Self.binding.as_type.373) [symbolic = %I.lookup_impl_witness (constants.%I.lookup_impl_witness.920)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn() -> %empty_tuple.type {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     %.loc13_11.1: %empty_tuple.type = tuple_literal ()
+// CHECK:STDOUT:     %.loc13_11.2: init %empty_tuple.type = tuple_init () to %return [concrete = constants.%empty_tuple]
+// CHECK:STDOUT:     %.loc13_12: init %empty_tuple.type = converted %.loc13_11.1, %.loc13_11.2 [concrete = constants.%empty_tuple]
+// CHECK:STDOUT:     return %.loc13_12 to %return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @I(constants.%T.8b3) {
+// CHECK:STDOUT:   %T.loc4_13.1 => constants.%T.8b3
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @I1(constants.%T.8b3, constants.%Self.dae) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @I(constants.%.Self.binding.as_type.373) {
+// CHECK:STDOUT:   %T.loc4_13.1 => constants.%.Self.binding.as_type.373
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %I.type => constants.%I.type.986
+// CHECK:STDOUT:   %Self.2 => constants.%Self.f35
+// CHECK:STDOUT:   %I.assoc_type => constants.%I.assoc_type.4e2
+// CHECK:STDOUT:   %assoc0 => constants.%assoc0.7d3
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @I1(constants.%.Self.binding.as_type.373, constants.%.Self.955) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @I1(constants.%.Self.binding.as_type.373, constants.%I.facet) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @F(constants.%T.797) {
+// CHECK:STDOUT:   %T.loc8_6.1 => constants.%T.797
+// CHECK:STDOUT:   %T.binding.as_type => constants.%T.binding.as_type
+// CHECK:STDOUT:   %I.lookup_impl_witness => constants.%I.lookup_impl_witness.920
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @G(constants.%T.797) {
+// CHECK:STDOUT:   %T.loc12_6.1 => constants.%T.797
+// CHECK:STDOUT:   %T.binding.as_type => constants.%T.binding.as_type
+// CHECK:STDOUT:   %I.lookup_impl_witness => constants.%I.lookup_impl_witness.920
+// CHECK:STDOUT: }
+// CHECK:STDOUT:

toolchain/check/testdata/generic/template/fail_todo_template_access_assoc_const.carbon

@@ -0,0 +1,24 @@
+// 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
+//
+// INCLUDE-FILE: toolchain/testing/testdata/min_prelude/convert.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/generic/template/fail_todo_template_access_assoc_const.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/generic/template/fail_todo_template_access_assoc_const.carbon
+
+library "[[@TEST_NAME]]";
+
+// TODO: This should check. The `T.I1` access should be deferred until we know
+// the value of the `template T`.
+//
+// CHECK:STDERR: fail_todo_template_access_assoc_const.carbon:[[@LINE+4]]:36: error: cannot evaluate type expression [TypeExprEvaluationFailure]
+// CHECK:STDERR: interface I(template T:! type, N:! T.I1) {
+// CHECK:STDERR:                                    ^~~~
+// CHECK:STDERR:
+interface I(template T:! type, N:! T.I1) {
+    let I1:! type;
+}