Add some tests for .Self in the interface params, and comments about implied constraints (#5919)

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/testdata/facet/fail_todo_self_in_interface_param.carbon

@@ -0,0 +1,25 @@
+// 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/none.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/facet/fail_todo_self_in_interface_param.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/facet/fail_todo_self_in_interface_param.carbon
+
+interface I(T:! type) {
+  let I1:! type;
+}
+
+// @dump-sem-ir-begin
+// CHECK:STDERR: fail_todo_self_in_interface_param.carbon:[[@LINE+4]]:12: error: name `.Self` not found [NameNotFound]
+// CHECK:STDERR: fn F(T:! I(.Self) where .I1 = ()) -> T.I1 {
+// CHECK:STDERR:            ^~~~~
+// CHECK:STDERR:
+fn F(T:! I(.Self) where .I1 = ()) -> T.I1 {
+  return ();
+}
+// @dump-sem-ir-end

toolchain/check/testdata/facet/validate_impl_constraints.carbon

@@ -90,3 +90,45 @@ fn G(T:! L) {
   // CHECK:STDERR:
   F(T);
 }
+
+// --- fail_todo_self_in_interface_generic_param_unconstrained.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+interface I(T:! type) {}
+
+// CHECK:STDERR: fail_todo_self_in_interface_generic_param_unconstrained.carbon:[[@LINE+4]]:12: error: name `.Self` not found [NameNotFound]
+// CHECK:STDERR: fn F(T:! I(.Self) where .Self impls Z) {}
+// CHECK:STDERR:            ^~~~~
+// CHECK:STDERR:
+fn F(T:! I(.Self) where .Self impls Z) {}
+
+// CHECK:STDERR: fail_todo_self_in_interface_generic_param_unconstrained.carbon:[[@LINE+4]]:16: error: name `.Self` not found [NameNotFound]
+// CHECK:STDERR: fn G(T:! Z & I(.Self)) {
+// CHECK:STDERR:                ^~~~~
+// CHECK:STDERR:
+fn G(T:! Z & I(.Self)) {
+  F(T);
+}
+
+// --- fail_todo_self_in_interface_generic_param_constrained.carbon
+library "[[@TEST_NAME]]";
+
+interface Z {}
+interface I(T:! Z) {}
+
+// Implied constraint: .Self impls Z, which is satisfied and checked at the end
+// of the fn signature.
+// CHECK:STDERR: fail_todo_self_in_interface_generic_param_constrained.carbon:[[@LINE+4]]:12: error: name `.Self` not found [NameNotFound]
+// CHECK:STDERR: fn F(T:! I(.Self) where .Self impls Z) {}
+// CHECK:STDERR:            ^~~~~
+// CHECK:STDERR:
+fn F(T:! I(.Self) where .Self impls Z) {}
+
+// CHECK:STDERR: fail_todo_self_in_interface_generic_param_constrained.carbon:[[@LINE+4]]:16: error: name `.Self` not found [NameNotFound]
+// CHECK:STDERR: fn G(T:! Z & I(.Self)) {
+// CHECK:STDERR:                ^~~~~
+// CHECK:STDERR:
+fn G(T:! Z & I(.Self)) {
+  F(T);
+}

toolchain/check/testdata/impl/impl_assoc_const.carbon

@@ -354,6 +354,10 @@ interface Z {
 
 class C;
 impl C as Y {}
+// Implied constraint: .Self impls Y. Should be verified when finishing the impl
+// decl (where we know .Self is C). See:
+// https://discord.com/channels/655572317891461132/941071822756143115/1400953094015160370
+//
 // CHECK:STDERR: fail_todo_period_self_impl_lookup.carbon:[[@LINE+4]]:25: error: cannot convert type `.Self` that implements `Z` into type implementing `Y` [ConversionFailureFacetToFacet]
 // CHECK:STDERR: impl C as Z where .Z1 = .Self {}
 // CHECK:STDERR:                         ^~~~~
@@ -372,6 +376,9 @@ class C;
 impl C as Y {}
 impl C as Z where .Z1 = C {}
 
+// Implied constraint: .Self impls Y. Should be verified against the type of the
+// facet value replacing T when calling F.
+//
 // CHECK:STDERR: fail_todo_period_self_compared_with_concrete_self.carbon:[[@LINE+4]]:24: error: cannot convert type `.Self` that implements `Z` into type implementing `Y` [ConversionFailureFacetToFacet]
 // CHECK:STDERR: fn F(T:! Z where .Z1 = .Self) {}
 // CHECK:STDERR:                        ^~~~~
@@ -380,6 +387,9 @@ fn F(T:! Z where .Z1 = .Self) {}
 fn G() {
   F(C);
 
+  // Implied constraint: .Self impls Y. Should be verified against the type of
+  // the facet value of C when casting.
+  //
   // CHECK:STDERR: fail_todo_period_self_compared_with_concrete_self.carbon:[[@LINE+4]]:23: error: cannot convert type `.Self` that implements `Z` into type implementing `Y` [ConversionFailureFacetToFacet]
   // CHECK:STDERR:   C as (Z where .Z1 = .Self);
   // CHECK:STDERR:                       ^~~~~