diff --git a/proposals/p6395.md b/proposals/p6395.md
new file mode 100644
index 000000000000..38ddef8e08c1
--- /dev/null
+++ b/proposals/p6395.md
@@ -0,0 +1,190 @@
+# Type completeness in extend
+
+<!--
+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
+-->
+
+[Pull request](https://github.com/carbon-language/carbon-lang/pull/6395)
+
+<!-- toc -->
+
+## Table of contents
+
+-   [Abstract](#abstract)
+-   [Problem](#problem)
+-   [Background](#background)
+-   [Proposal](#proposal)
+-   [Details](#details)
+-   [Rationale](#rationale)
+-   [Alternatives considered](#alternatives-considered)
+
+<!-- tocstop -->
+
+## Abstract
+
+Require any target scopes in an `extend` declaration to be complete, since
+`extend` changes the scopes which are looked in for name lookup to include those
+named in the declaration.
+
+## Problem
+
+Proposal
+[#5168](https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p5168.md)
+laid out rules for when a facet type needs to be identified or complete. When
+`require impls X` is written, then `X` must be identified. However it does not
+specify any rule for `extend require impls X`. And we lack completeness rules
+for other `extend` declarations, including `extend impl as X`, `extend base: X`,
+and `extend adapt X`.
+
+An `extend` declaration always names one or more target entities which will be
+included in the search for names when looking in the enclosing scope of the
+`extend` declaration. In order to do name lookup into an entity, it must be
+complete to avoid poisoning names in the entity.
+
+## Background
+
+-   Proposal
+    [#5168](https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p5168.md):
+    Forward `impl` declaration of an incomplete interface
+-   Proposal
+    [#2760](https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p2760.md):
+    Consistent `class` and `interface` syntax
+-   Proposal
+    [#0777](https://github.com/carbon-language/carbon-lang/blob/trunk/proposals/p0777.md):
+    Inheritance
+
+## Proposal
+
+An `extend` declaration declares that the enclosing scope extends the scope
+associated with any target entity named in the declaration. That is to say name
+lookups into the enclosing scope will also look into the scopes which are
+nominated by the `extend` declaration. The `extend` declaration requires that
+the target scopes named in an `extend` declaration are complete, or if the
+target is a generic parameter, requires the type of the parameter to be
+complete.
+
+The scope of a facet type formed by a `where` declaration extends the scope of
+its first operand. And the scope of a facet type formed by a `&` operator
+extends the scopes of both of its operands. In either case, the scope of the
+facet type is complete if every scope it extends is complete. The facet type
+`type` is associated with an empty scope and is complete.
+
+| Facet type                   | Requirement                           |
+| ---------------------------- | ------------------------------------- |
+| `I`                          | Requires `I` is complete.             |
+| `I & J`                      | Requires `I` and `J` are complete.    |
+| `type where U impls J`       | Requires `type` is complete.          |
+| `I & (type where U impls J)` | Requires `I` and `type` are complete. |
+
+## Details
+
+To `extend` an entity `Y` with another `Z` means that name lookups into `Y` will
+also look into `Z`. Immediately after the `extend` operation, members of `Z`
+should also be found when doing name lookup into `Y`, both from outside and from
+inside the definition of `Y`. In order to be able to perform lookups into `Z`,
+we require that `extend` operations only target scopes that are complete.
+
+This requirement functions recursively. Given an interface `B` that extends
+another interface `A`: By naming `A` in an extend declaration, we require `A` is
+complete. This provides that its entire definition is known, and thus its
+`extend` relationship to `B`. The `extend` relationship there also provides that
+`B` is complete.
+
+If the target scope of an `extend` declaration is a generic parameter, its type
+must be complete where the `extend` declaration is written, as name lookups into
+the extended scope will look into the type of the generic parameter.
+
+```carbon
+interface I {
+  fn F();
+}
+
+class C(T:! I) {
+  extend base: T;
+  // `F` names `T.F` here, found in `I`.
+  fn G() { F(); }
+}
+```
+
+As any generic parameter in the enclosing scope is replaced by a more specific
+value, extended scopes that depend on a generic parameter must remain complete.
+This includes forming a specific for the extended scope involving the parameter
+in order to surface any monomorphization errors in the resulting specific.
+
+In the next example, the `extend` declaration in interface `A(N)` names a
+symbolic facet type which can produce monomorphization errors when a negative
+value is provided for `N`. When a more specific value for the target `B(N)` is
+provided, we require the specific value to be complete as well by forming the
+specific. A diagnostic error would be produced while checking `C(-1)` for
+completeness, as it requires `A(-1)` to be complete, which requires
+`B(array(i32, -1))` to be complete, and that contains an invalid type.
+
+```carbon
+interface B(T:! type) {}
+
+interface A(N:! i32) {
+  // Requires `B(N)` to be complete.
+  extend require impls B(array(i32, N)) {}
+}
+
+class C(N! i32) {
+  // Requires `A(N)` to be complete, which requires `B(N)` to be complete.
+  extend impl as A(N);
+}
+
+fn F() {
+  // Requires `C(-1)` to be complete, which requires `A(-1)` to be complete, which requires `B(array(i32, -1))` to be complete.
+  var c: C(-1);
+}
+```
+
+These rules prohibit an `extend` declaration from naming its enclosing scope,
+since by being part of the definition of that scope, it is implied that the
+enclosing scope is not complete. This seems reasonable as all names available
+inside the enclosing interface or named constraint are already available or
+would conflict with the ones that are.
+
+## Rationale
+
+This is based on the principle of
+[Code that is easy to read, understand, and write](/docs/project/goals.md#code-that-is-easy-to-read-understand-and-write).
+For code to be easy to write, the rules need to be consistent. Once an `extend`
+declaration has been written, the names inside should become available through
+the enclosing scope immediately. If we allow an interface named in an `extend`
+declaration to be incomplete, then name lookup will fail ambiguously. Those same
+names may become valid later once the interface is defined.
+
+## Alternatives considered
+
+We considered not requiring that the scope named in an `extend` declaration be
+complete where it is written, but only when the enclosing scope is required to
+be complete. This is more flexible, allowing entities to be defined later in the
+program. However this does not allow the use of names from the target scope in
+the `extend` to be used from within the enclosing definition scope. They would
+not become available until the enclosing definition scope was closed and
+complete.
+
+In particular, we want this to work:
+
+```carbon
+interface Z {
+  let X:! type;
+  fn F() -> X;
+}
+class C {
+  extend impl as Z where .X = () {
+    // Names `X` directly.
+    fn F() -> X;
+  }
+
+  // Names `X` and `F` directly.
+  fn G() -> X { return F(); }
+}
+
+// Now `C` is complete, `C.X` and `C.F` are also available.
+fn H() -> C.X {
+  return C.F();
+}
+```
diff --git a/toolchain/check/testdata/impl/use_assoc_entity.carbon b/toolchain/check/testdata/impl/use_assoc_entity.carbon
index 3f115580b315..02f4de6c4e46 100644
--- a/toolchain/check/testdata/impl/use_assoc_entity.carbon
+++ b/toolchain/check/testdata/impl/use_assoc_entity.carbon
@@ -141,6 +141,9 @@ interface J {
 
 class E {
   extend impl as J where .U = i32 {
+    // TODO: U should be in scope and usable as soon as we finish the
+    // declaration / enter the definition.
+    //
     // CHECK:STDERR: fail_todo_use_associated_constant_in_impl.carbon:[[@LINE+14]]:13: error: cannot implicitly convert non-type value of type `<associated entity in J>` to `type` [ConversionFailureNonTypeToFacet]
     // CHECK:STDERR:     fn F(u: U) -> U {
     // CHECK:STDERR:             ^
@@ -161,6 +164,61 @@ class E {
   }
 }
 
+// --- fail_todo_call_extend_fn.carbon
+library "[[@TEST_NAME]]";
+
+interface J {
+  fn F() {}
+}
+
+class E {
+  extend impl as J {
+    fn F() {}
+  }
+
+  // CHECK:STDERR: fail_todo_call_extend_fn.carbon:[[@LINE+4]]:12: error: value of type `<associated entity in J>` is not callable [CallToNonCallable]
+  // CHECK:STDERR:   fn G() { F(); }
+  // CHECK:STDERR:            ^~~
+  // CHECK:STDERR:
+  fn G() { F(); }
+}
+
+// --- call_extend_method.carbon
+library "[[@TEST_NAME]]";
+
+interface J {
+  fn F[self: Self]() {}
+}
+
+class E {
+  extend impl as J {
+    fn F[self: Self]() {}
+  }
+
+  fn G[self: Self]() { self.F(); }
+}
+
+// --- fail_todo_use_extend_constant.carbon
+library "[[@TEST_NAME]]";
+
+interface J {
+  let X:! type;
+}
+
+class E {
+  extend impl as J where .X = () {}
+
+  // TODO: `X` should be in scope and directly usable.
+  // CHECK:STDERR: fail_todo_use_extend_constant.carbon:[[@LINE+7]]:13: error: cannot implicitly convert non-type value of type `<associated entity in J>` to `type` [ConversionFailureNonTypeToFacet]
+  // CHECK:STDERR:   fn G() -> X { return (); }
+  // CHECK:STDERR:             ^
+  // CHECK:STDERR: fail_todo_use_extend_constant.carbon:[[@LINE+4]]:13: note: type `<associated entity in J>` does not implement interface `Core.ImplicitAs(type)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   fn G() -> X { return (); }
+  // CHECK:STDERR:             ^
+  // CHECK:STDERR:
+  fn G() -> X { return (); }
+}
+
 // --- fail_todo_self_period_associated_type.carbon
 library "[[@TEST_NAME]]";
 
@@ -2309,11 +2367,11 @@ fn F() {
 // CHECK:STDOUT:   %J.impl_witness: <witness> = impl_witness @E.%J.impl_witness_table [concrete]
 // CHECK:STDOUT:   %ImplicitAs.type.cc7: type = generic_interface_type @ImplicitAs [concrete]
 // CHECK:STDOUT:   %ImplicitAs.generic: %ImplicitAs.type.cc7 = struct_value () [concrete]
-// CHECK:STDOUT:   %E.as.J.impl.F.type.3487c0.1: type = fn_type @E.as.J.impl.F.loc24_21.1 [concrete]
+// CHECK:STDOUT:   %E.as.J.impl.F.type.3487c0.1: type = fn_type @E.as.J.impl.F.loc27_21.1 [concrete]
 // CHECK:STDOUT:   %E.as.J.impl.F.1487eb.1: %E.as.J.impl.F.type.3487c0.1 = struct_value () [concrete]
 // CHECK:STDOUT:   %J.facet: %J.type = facet_value %E, (%J.impl_witness) [concrete]
 // CHECK:STDOUT:   %pattern_type.7ce: type = pattern_type %i32 [concrete]
-// CHECK:STDOUT:   %E.as.J.impl.F.type.3487c0.2: type = fn_type @E.as.J.impl.F.loc24_21.2 [concrete]
+// CHECK:STDOUT:   %E.as.J.impl.F.type.3487c0.2: type = fn_type @E.as.J.impl.F.loc27_21.2 [concrete]
 // CHECK:STDOUT:   %E.as.J.impl.F.1487eb.2: %E.as.J.impl.F.type.3487c0.2 = struct_value () [concrete]
 // CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
 // CHECK:STDOUT:   %complete_type.357: <witness> = complete_type_witness %empty_struct_type [concrete]
@@ -2379,24 +2437,24 @@ fn F() {
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
 // CHECK:STDOUT: impl @E.as.J.impl: %Self.ref as %.loc9_20 {
-// CHECK:STDOUT:   %E.as.J.impl.F.decl.loc24_21.1: %E.as.J.impl.F.type.3487c0.1 = fn_decl @E.as.J.impl.F.loc24_21.1 [concrete = constants.%E.as.J.impl.F.1487eb.1] {
+// CHECK:STDOUT:   %E.as.J.impl.F.decl.loc27_21.1: %E.as.J.impl.F.type.3487c0.1 = fn_decl @E.as.J.impl.F.loc27_21.1 [concrete = constants.%E.as.J.impl.F.1487eb.1] {
 // CHECK:STDOUT:     %u.patt: <error> = value_binding_pattern u [concrete]
 // CHECK:STDOUT:     %u.param_patt: <error> = value_param_pattern %u.patt, call_param0 [concrete]
 // CHECK:STDOUT:     %return.patt: <error> = return_slot_pattern [concrete]
 // CHECK:STDOUT:     %return.param_patt: <error> = out_param_pattern %return.patt, call_param1 [concrete]
 // CHECK:STDOUT:   } {
-// CHECK:STDOUT:     %U.ref.loc24_19: %J.assoc_type = name_ref U, @U.%assoc0 [concrete = constants.%assoc0.411]
-// CHECK:STDOUT:     %.loc24_19: type = converted %U.ref.loc24_19, <error> [concrete = <error>]
+// CHECK:STDOUT:     %U.ref.loc27_19: %J.assoc_type = name_ref U, @U.%assoc0 [concrete = constants.%assoc0.411]
+// CHECK:STDOUT:     %.loc27_19: type = converted %U.ref.loc27_19, <error> [concrete = <error>]
 // CHECK:STDOUT:     %u.param: <error> = value_param call_param0
 // CHECK:STDOUT:     %.1: <error> = splice_block <error> [concrete = <error>] {
-// CHECK:STDOUT:       %U.ref.loc24_13: %J.assoc_type = name_ref U, @U.%assoc0 [concrete = constants.%assoc0.411]
-// CHECK:STDOUT:       %.loc24_13: type = converted %U.ref.loc24_13, <error> [concrete = <error>]
+// CHECK:STDOUT:       %U.ref.loc27_13: %J.assoc_type = name_ref U, @U.%assoc0 [concrete = constants.%assoc0.411]
+// CHECK:STDOUT:       %.loc27_13: type = converted %U.ref.loc27_13, <error> [concrete = <error>]
 // CHECK:STDOUT:     }
 // CHECK:STDOUT:     %u: <error> = value_binding u, %u.param
 // CHECK:STDOUT:     %return.param: ref <error> = out_param call_param1
 // CHECK:STDOUT:     %return: ref <error> = return_slot %return.param
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %E.as.J.impl.F.decl.loc24_21.2: %E.as.J.impl.F.type.3487c0.2 = fn_decl @E.as.J.impl.F.loc24_21.2 [concrete = constants.%E.as.J.impl.F.1487eb.2] {
+// CHECK:STDOUT:   %E.as.J.impl.F.decl.loc27_21.2: %E.as.J.impl.F.type.3487c0.2 = fn_decl @E.as.J.impl.F.loc27_21.2 [concrete = constants.%E.as.J.impl.F.1487eb.2] {
 // CHECK:STDOUT:     %u.patt: %pattern_type.7ce = value_binding_pattern u [concrete]
 // CHECK:STDOUT:     %u.param_patt: %pattern_type.7ce = value_param_pattern %u.patt, call_param0 [concrete]
 // CHECK:STDOUT:     %return.patt: %pattern_type.7ce = return_slot_pattern [concrete]
@@ -2410,7 +2468,7 @@ fn F() {
 // CHECK:STDOUT:
 // CHECK:STDOUT: !members:
 // CHECK:STDOUT:   .U = <poisoned>
-// CHECK:STDOUT:   .F = %E.as.J.impl.F.decl.loc24_21.1
+// CHECK:STDOUT:   .F = %E.as.J.impl.F.decl.loc27_21.1
 // CHECK:STDOUT:   witness = @E.%J.impl_witness
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
@@ -2431,7 +2489,7 @@ fn F() {
 // CHECK:STDOUT:       requirement_rewrite %impl.elem0, %i32
 // CHECK:STDOUT:     }
 // CHECK:STDOUT:   }
-// CHECK:STDOUT:   %J.impl_witness_table = impl_witness_table (%impl_witness_assoc_constant, @E.as.J.impl.%E.as.J.impl.F.decl.loc24_21.2), @E.as.J.impl [concrete]
+// CHECK:STDOUT:   %J.impl_witness_table = impl_witness_table (%impl_witness_assoc_constant, @E.as.J.impl.%E.as.J.impl.F.decl.loc27_21.2), @E.as.J.impl [concrete]
 // CHECK:STDOUT:   %J.impl_witness: <witness> = impl_witness %J.impl_witness_table [concrete = constants.%J.impl_witness]
 // CHECK:STDOUT:   %impl_witness_assoc_constant: type = impl_witness_assoc_constant constants.%i32 [concrete = constants.%i32]
 // CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness constants.%empty_struct_type [concrete = constants.%complete_type.357]
@@ -2453,15 +2511,15 @@ fn F() {
 // CHECK:STDOUT:   fn(%u.param: @J.F.%impl.elem0.loc5_11.1 (%impl.elem0.fa9)) -> @J.F.%impl.elem0.loc5_11.1 (%impl.elem0.fa9);
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @E.as.J.impl.F.loc24_21.1(%u.param: <error>) -> <error> {
+// CHECK:STDOUT: fn @E.as.J.impl.F.loc27_21.1(%u.param: <error>) -> <error> {
 // CHECK:STDOUT: !entry:
 // CHECK:STDOUT:   %u.ref: <error> = name_ref u, %u
 // CHECK:STDOUT:   return <error> to %return
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
-// CHECK:STDOUT: fn @E.as.J.impl.F.loc24_21.2(%u.param: %i32) -> %i32 [thunk @E.as.J.impl.%E.as.J.impl.F.decl.loc24_21.1] {
+// CHECK:STDOUT: fn @E.as.J.impl.F.loc27_21.2(%u.param: %i32) -> %i32 [thunk @E.as.J.impl.%E.as.J.impl.F.decl.loc27_21.1] {
 // CHECK:STDOUT: !entry:
-// CHECK:STDOUT:   %F.ref: %E.as.J.impl.F.type.3487c0.1 = name_ref F, @E.as.J.impl.%E.as.J.impl.F.decl.loc24_21.1 [concrete = constants.%E.as.J.impl.F.1487eb.1]
+// CHECK:STDOUT:   %F.ref: %E.as.J.impl.F.type.3487c0.1 = name_ref F, @E.as.J.impl.%E.as.J.impl.F.decl.loc27_21.1 [concrete = constants.%E.as.J.impl.F.1487eb.1]
 // CHECK:STDOUT:   %u.ref: %i32 = name_ref u, %u.param
 // CHECK:STDOUT:   %return.ref: ref %i32 = name_ref <return slot>, %return.param
 // CHECK:STDOUT:   %E.as.J.impl.F.call: init <error> = call %F.ref(<error>)
@@ -2486,6 +2544,375 @@ fn F() {
 // CHECK:STDOUT:   %pattern_type => constants.%pattern_type.7ce
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_todo_call_extend_fn.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %J.type: type = facet_type <@J> [concrete]
+// CHECK:STDOUT:   %Self: %J.type = symbolic_binding Self, 0 [symbolic]
+// CHECK:STDOUT:   %J.F.type: type = fn_type @J.F [concrete]
+// CHECK:STDOUT:   %J.F: %J.F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %J.assoc_type: type = assoc_entity_type @J [concrete]
+// CHECK:STDOUT:   %assoc0: %J.assoc_type = assoc_entity element0, @J.%J.F.decl [concrete]
+// CHECK:STDOUT:   %E: type = class_type @E [concrete]
+// CHECK:STDOUT:   %J.impl_witness: <witness> = impl_witness @E.%J.impl_witness_table [concrete]
+// CHECK:STDOUT:   %E.as.J.impl.F.type: type = fn_type @E.as.J.impl.F [concrete]
+// CHECK:STDOUT:   %E.as.J.impl.F: %E.as.J.impl.F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %J.facet: %J.type = facet_value %E, (%J.impl_witness) [concrete]
+// CHECK:STDOUT:   %E.G.type: type = fn_type @E.G [concrete]
+// CHECK:STDOUT:   %E.G: %E.G.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .J = %J.decl
+// CHECK:STDOUT:     .E = %E.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %J.decl: type = interface_decl @J [concrete = constants.%J.type] {} {}
+// CHECK:STDOUT:   %E.decl: type = class_decl @E [concrete = constants.%E] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @J {
+// CHECK:STDOUT:   %Self: %J.type = symbolic_binding Self, 0 [symbolic = constants.%Self]
+// CHECK:STDOUT:   %J.F.decl: %J.F.type = fn_decl @J.F [concrete = constants.%J.F] {} {}
+// CHECK:STDOUT:   %assoc0: %J.assoc_type = assoc_entity element0, %J.F.decl [concrete = constants.%assoc0]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   .F = %assoc0
+// CHECK:STDOUT:   witness = (%J.F.decl)
+// CHECK:STDOUT:
+// CHECK:STDOUT: !requires:
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @E.as.J.impl: %Self.ref as %J.ref {
+// CHECK:STDOUT:   %E.as.J.impl.F.decl: %E.as.J.impl.F.type = fn_decl @E.as.J.impl.F [concrete = constants.%E.as.J.impl.F] {} {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .F = %E.as.J.impl.F.decl
+// CHECK:STDOUT:   witness = @E.%J.impl_witness
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @E {
+// CHECK:STDOUT:   impl_decl @E.as.J.impl [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%E [concrete = constants.%E]
+// CHECK:STDOUT:     %J.ref: type = name_ref J, file.%J.decl [concrete = constants.%J.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %J.impl_witness_table = impl_witness_table (@E.as.J.impl.%E.as.J.impl.F.decl), @E.as.J.impl [concrete]
+// CHECK:STDOUT:   %J.impl_witness: <witness> = impl_witness %J.impl_witness_table [concrete = constants.%J.impl_witness]
+// CHECK:STDOUT:   %E.G.decl: %E.G.type = fn_decl @E.G [concrete = constants.%E.G] {} {}
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness constants.%empty_struct_type [concrete = constants.%complete_type]
+// CHECK:STDOUT:   complete_type_witness = %complete_type
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%E
+// CHECK:STDOUT:   .J = <poisoned>
+// CHECK:STDOUT:   .G = %E.G.decl
+// CHECK:STDOUT:   .F = <poisoned>
+// CHECK:STDOUT:   extend @E.as.J.impl.%J.ref
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @J.F(@J.%Self: %J.type) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn() {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @E.as.J.impl.F() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @E.G() {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %F.ref: %J.assoc_type = name_ref F, @J.%assoc0 [concrete = constants.%assoc0]
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @J.F(constants.%Self) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @J.F(constants.%J.facet) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- call_extend_method.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %J.type: type = facet_type <@J> [concrete]
+// CHECK:STDOUT:   %Self: %J.type = symbolic_binding Self, 0 [symbolic]
+// CHECK:STDOUT:   %Self.binding.as_type: type = symbolic_binding_type Self, 0, %Self [symbolic]
+// CHECK:STDOUT:   %pattern_type.4c4: type = pattern_type %Self.binding.as_type [symbolic]
+// CHECK:STDOUT:   %J.F.type: type = fn_type @J.F [concrete]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %J.F: %J.F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %J.assoc_type: type = assoc_entity_type @J [concrete]
+// CHECK:STDOUT:   %assoc0: %J.assoc_type = assoc_entity element0, @J.%J.F.decl [concrete]
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %Self.binding.as_type [symbolic]
+// CHECK:STDOUT:   %E: type = class_type @E [concrete]
+// CHECK:STDOUT:   %J.impl_witness: <witness> = impl_witness @E.%J.impl_witness_table [concrete]
+// CHECK:STDOUT:   %pattern_type.a4a: type = pattern_type %E [concrete]
+// CHECK:STDOUT:   %E.as.J.impl.F.type: type = fn_type @E.as.J.impl.F [concrete]
+// CHECK:STDOUT:   %E.as.J.impl.F: %E.as.J.impl.F.type = struct_value () [concrete]
+// CHECK:STDOUT:   %J.facet: %J.type = facet_value %E, (%J.impl_witness) [concrete]
+// CHECK:STDOUT:   %E.G.type: type = fn_type @E.G [concrete]
+// CHECK:STDOUT:   %E.G: %E.G.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete]
+// CHECK:STDOUT:   %.225: type = fn_type_with_self_type %J.F.type, %J.facet [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .J = %J.decl
+// CHECK:STDOUT:     .E = %E.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %J.decl: type = interface_decl @J [concrete = constants.%J.type] {} {}
+// CHECK:STDOUT:   %E.decl: type = class_decl @E [concrete = constants.%E] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @J {
+// CHECK:STDOUT:   %Self: %J.type = symbolic_binding Self, 0 [symbolic = constants.%Self]
+// CHECK:STDOUT:   %J.F.decl: %J.F.type = fn_decl @J.F [concrete = constants.%J.F] {
+// CHECK:STDOUT:     %self.patt: @J.F.%pattern_type (%pattern_type.4c4) = value_binding_pattern self [concrete]
+// CHECK:STDOUT:     %self.param_patt: @J.F.%pattern_type (%pattern_type.4c4) = value_param_pattern %self.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %self.param: @J.F.%Self.binding.as_type (%Self.binding.as_type) = value_param call_param0
+// CHECK:STDOUT:     %.loc4_14.1: type = splice_block %.loc4_14.2 [symbolic = %Self.binding.as_type (constants.%Self.binding.as_type)] {
+// CHECK:STDOUT:       %Self.ref: %J.type = name_ref Self, @J.%Self [symbolic = %Self (constants.%Self)]
+// CHECK:STDOUT:       %Self.as_type: type = facet_access_type %Self.ref [symbolic = %Self.binding.as_type (constants.%Self.binding.as_type)]
+// CHECK:STDOUT:       %.loc4_14.2: type = converted %Self.ref, %Self.as_type [symbolic = %Self.binding.as_type (constants.%Self.binding.as_type)]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %self: @J.F.%Self.binding.as_type (%Self.binding.as_type) = value_binding self, %self.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %assoc0: %J.assoc_type = assoc_entity element0, %J.F.decl [concrete = constants.%assoc0]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   .F = %assoc0
+// CHECK:STDOUT:   witness = (%J.F.decl)
+// CHECK:STDOUT:
+// CHECK:STDOUT: !requires:
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @E.as.J.impl: %Self.ref as %J.ref {
+// CHECK:STDOUT:   %E.as.J.impl.F.decl: %E.as.J.impl.F.type = fn_decl @E.as.J.impl.F [concrete = constants.%E.as.J.impl.F] {
+// CHECK:STDOUT:     %self.patt: %pattern_type.a4a = value_binding_pattern self [concrete]
+// CHECK:STDOUT:     %self.param_patt: %pattern_type.a4a = value_param_pattern %self.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %self.param: %E = value_param call_param0
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%E [concrete = constants.%E]
+// CHECK:STDOUT:     %self: %E = value_binding self, %self.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .F = %E.as.J.impl.F.decl
+// CHECK:STDOUT:   witness = @E.%J.impl_witness
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @E {
+// CHECK:STDOUT:   impl_decl @E.as.J.impl [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%E [concrete = constants.%E]
+// CHECK:STDOUT:     %J.ref: type = name_ref J, file.%J.decl [concrete = constants.%J.type]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %J.impl_witness_table = impl_witness_table (@E.as.J.impl.%E.as.J.impl.F.decl), @E.as.J.impl [concrete]
+// CHECK:STDOUT:   %J.impl_witness: <witness> = impl_witness %J.impl_witness_table [concrete = constants.%J.impl_witness]
+// CHECK:STDOUT:   %E.G.decl: %E.G.type = fn_decl @E.G [concrete = constants.%E.G] {
+// CHECK:STDOUT:     %self.patt: %pattern_type.a4a = value_binding_pattern self [concrete]
+// CHECK:STDOUT:     %self.param_patt: %pattern_type.a4a = value_param_pattern %self.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %self.param: %E = value_param call_param0
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%E [concrete = constants.%E]
+// CHECK:STDOUT:     %self: %E = value_binding self, %self.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness constants.%empty_struct_type [concrete = constants.%complete_type]
+// CHECK:STDOUT:   complete_type_witness = %complete_type
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%E
+// CHECK:STDOUT:   .J = <poisoned>
+// CHECK:STDOUT:   .G = %E.G.decl
+// CHECK:STDOUT:   .F = <poisoned>
+// CHECK:STDOUT:   extend @E.as.J.impl.%J.ref
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic fn @J.F(@J.%Self: %J.type) {
+// CHECK:STDOUT:   %Self: %J.type = symbolic_binding Self, 0 [symbolic = %Self (constants.%Self)]
+// CHECK:STDOUT:   %Self.binding.as_type: type = symbolic_binding_type Self, 0, %Self [symbolic = %Self.binding.as_type (constants.%Self.binding.as_type)]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %Self.binding.as_type [symbolic = %pattern_type (constants.%pattern_type.4c4)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %Self.binding.as_type [symbolic = %require_complete (constants.%require_complete)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   fn(%self.param: @J.F.%Self.binding.as_type (%Self.binding.as_type)) {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @E.as.J.impl.F(%self.param: %E) {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @E.G(%self.param: %E) {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %self.ref: %E = name_ref self, %self
+// CHECK:STDOUT:   %F.ref: %J.assoc_type = name_ref F, @J.%assoc0 [concrete = constants.%assoc0]
+// CHECK:STDOUT:   %impl.elem0: %.225 = impl_witness_access constants.%J.impl_witness, element0 [concrete = constants.%E.as.J.impl.F]
+// CHECK:STDOUT:   %bound_method: <bound method> = bound_method %self.ref, %impl.elem0
+// CHECK:STDOUT:   %E.as.J.impl.F.call: init %empty_tuple.type = call %bound_method(%self.ref)
+// CHECK:STDOUT:   return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @J.F(constants.%Self) {
+// CHECK:STDOUT:   %Self => constants.%Self
+// CHECK:STDOUT:   %Self.binding.as_type => constants.%Self.binding.as_type
+// CHECK:STDOUT:   %pattern_type => constants.%pattern_type.4c4
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @J.F(constants.%J.facet) {
+// CHECK:STDOUT:   %Self => constants.%J.facet
+// CHECK:STDOUT:   %Self.binding.as_type => constants.%E
+// CHECK:STDOUT:   %pattern_type => constants.%pattern_type.a4a
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- fail_todo_use_extend_constant.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %J.type: type = facet_type <@J> [concrete]
+// CHECK:STDOUT:   %Self.dc6: %J.type = symbolic_binding Self, 0 [symbolic]
+// CHECK:STDOUT:   %J.assoc_type: type = assoc_entity_type @J [concrete]
+// CHECK:STDOUT:   %assoc0.8fd: %J.assoc_type = assoc_entity element0, @J.%X [concrete]
+// CHECK:STDOUT:   %E: type = class_type @E [concrete]
+// CHECK:STDOUT:   %.Self: %J.type = symbolic_binding .Self [symbolic_self]
+// CHECK:STDOUT:   %empty_tuple.type: type = tuple_type () [concrete]
+// CHECK:STDOUT:   %.Self.binding.as_type: type = symbolic_binding_type .Self, %.Self [symbolic_self]
+// CHECK:STDOUT:   %J.lookup_impl_witness: <witness> = lookup_impl_witness %.Self, @J [symbolic_self]
+// CHECK:STDOUT:   %impl.elem0: type = impl_witness_access %J.lookup_impl_witness, element0 [symbolic_self]
+// CHECK:STDOUT:   %empty_tuple: %empty_tuple.type = tuple_value () [concrete]
+// CHECK:STDOUT:   %J_where.type: type = facet_type <@J where %impl.elem0 = %empty_tuple.type> [concrete]
+// CHECK:STDOUT:   %J.impl_witness: <witness> = impl_witness @E.%J.impl_witness_table [concrete]
+// CHECK:STDOUT:   %ImplicitAs.type.cc7: type = generic_interface_type @ImplicitAs [concrete]
+// CHECK:STDOUT:   %ImplicitAs.generic: %ImplicitAs.type.cc7 = struct_value () [concrete]
+// CHECK:STDOUT:   %E.G.type: type = fn_type @E.G [concrete]
+// CHECK:STDOUT:   %E.G: %E.G.type = struct_value () [concrete]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     .ImplicitAs = %Core.ImplicitAs
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.ImplicitAs: %ImplicitAs.type.cc7 = import_ref Core//prelude/parts/as, ImplicitAs, loaded [concrete = constants.%ImplicitAs.generic]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   package: <namespace> = namespace [concrete] {
+// CHECK:STDOUT:     .Core = imports.%Core
+// CHECK:STDOUT:     .J = %J.decl
+// CHECK:STDOUT:     .E = %E.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %J.decl: type = interface_decl @J [concrete = constants.%J.type] {} {}
+// CHECK:STDOUT:   %E.decl: type = class_decl @E [concrete = constants.%E] {} {}
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: interface @J {
+// CHECK:STDOUT:   %Self: %J.type = symbolic_binding Self, 0 [symbolic = constants.%Self.dc6]
+// CHECK:STDOUT:   %X: type = assoc_const_decl @X [concrete] {
+// CHECK:STDOUT:     %assoc0: %J.assoc_type = assoc_entity element0, @J.%X [concrete = constants.%assoc0.8fd]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = %Self
+// CHECK:STDOUT:   .X = @X.%assoc0
+// CHECK:STDOUT:   witness = (%X)
+// CHECK:STDOUT:
+// CHECK:STDOUT: !requires:
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic assoc_const @X(@J.%Self: %J.type) {
+// CHECK:STDOUT:   assoc_const X:! type;
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: impl @E.as.J.impl: %Self.ref as %.loc8_20 {
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   witness = @E.%J.impl_witness
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: class @E {
+// CHECK:STDOUT:   impl_decl @E.as.J.impl [concrete] {} {
+// CHECK:STDOUT:     %Self.ref: type = name_ref Self, constants.%E [concrete = constants.%E]
+// CHECK:STDOUT:     %J.ref: type = name_ref J, file.%J.decl [concrete = constants.%J.type]
+// CHECK:STDOUT:     %.Self: %J.type = symbolic_binding .Self [symbolic_self = constants.%.Self]
+// CHECK:STDOUT:     %.Self.ref: %J.type = name_ref .Self, %.Self [symbolic_self = constants.%.Self]
+// CHECK:STDOUT:     %X.ref: %J.assoc_type = name_ref X, @X.%assoc0 [concrete = constants.%assoc0.8fd]
+// CHECK:STDOUT:     %.Self.as_type: type = facet_access_type %.Self.ref [symbolic_self = constants.%.Self.binding.as_type]
+// CHECK:STDOUT:     %.loc8_26: type = converted %.Self.ref, %.Self.as_type [symbolic_self = constants.%.Self.binding.as_type]
+// CHECK:STDOUT:     %impl.elem0: type = impl_witness_access constants.%J.lookup_impl_witness, element0 [symbolic_self = constants.%impl.elem0]
+// CHECK:STDOUT:     %.loc8_32.1: %empty_tuple.type = tuple_literal () [concrete = constants.%empty_tuple]
+// CHECK:STDOUT:     %.loc8_32.2: type = converted %.loc8_32.1, constants.%empty_tuple.type [concrete = constants.%empty_tuple.type]
+// CHECK:STDOUT:     %.loc8_20: type = where_expr %.Self [concrete = constants.%J_where.type] {
+// CHECK:STDOUT:       requirement_base_facet_type constants.%J.type
+// CHECK:STDOUT:       requirement_rewrite %impl.elem0, %.loc8_32.2
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %J.impl_witness_table = impl_witness_table (%impl_witness_assoc_constant), @E.as.J.impl [concrete]
+// CHECK:STDOUT:   %J.impl_witness: <witness> = impl_witness %J.impl_witness_table [concrete = constants.%J.impl_witness]
+// CHECK:STDOUT:   %impl_witness_assoc_constant: type = impl_witness_assoc_constant constants.%empty_tuple.type [concrete = constants.%empty_tuple.type]
+// CHECK:STDOUT:   %E.G.decl: %E.G.type = fn_decl @E.G [concrete = constants.%E.G] {
+// CHECK:STDOUT:     %return.patt: <error> = return_slot_pattern [concrete]
+// CHECK:STDOUT:     %return.param_patt: <error> = out_param_pattern %return.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %X.ref: %J.assoc_type = name_ref X, @X.%assoc0 [concrete = constants.%assoc0.8fd]
+// CHECK:STDOUT:     %.loc18_13: type = converted %X.ref, <error> [concrete = <error>]
+// CHECK:STDOUT:     %return.param: ref <error> = out_param call_param0
+// CHECK:STDOUT:     %return: ref <error> = return_slot %return.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %complete_type: <witness> = complete_type_witness constants.%empty_struct_type [concrete = constants.%complete_type]
+// CHECK:STDOUT:   complete_type_witness = %complete_type
+// CHECK:STDOUT:
+// CHECK:STDOUT: !members:
+// CHECK:STDOUT:   .Self = constants.%E
+// CHECK:STDOUT:   .J = <poisoned>
+// CHECK:STDOUT:   .X = <poisoned>
+// CHECK:STDOUT:   .G = %E.G.decl
+// CHECK:STDOUT:   extend @E.as.J.impl.%.loc8_20
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @E.G() -> <error> {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %.loc18_25: %empty_tuple.type = tuple_literal () [concrete = constants.%empty_tuple]
+// CHECK:STDOUT:   return <error> to %return
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @X(constants.%Self.dc6) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @X(constants.%.Self) {}
+// CHECK:STDOUT:
 // CHECK:STDOUT: --- fail_todo_self_period_associated_type.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {