From 26ec78ec00cc00f7d6c4c6edf94b8f558100d4b8 Mon Sep 17 00:00:00 2001
From: David Blaikie <dblaikie@gmail.com>
Date: Mon, 28 Jul 2025 14:06:57 -0700
Subject: [PATCH] Ensure vtable entries for generics are attached constants
 (#5853)

Otherwise these end up as unattached constants (see the baseline test
changes) and can't be resolved by `GetConstantValueInSpecific` in
lowering or in further derived vtables.

If the class is non-generic, then it's fine for the vtable entry for
some function inherited from a generic base is represented as an
unattached constant, since the specific in that specific_function is
already fully resolved.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>
---
 toolchain/check/class.cpp                     |  19 +-
 .../testdata/class/virtual_modifiers.carbon   | 439 ++++++++++++++++++
 toolchain/lower/file_context.cpp              |   2 +-
 toolchain/sem_ir/function.cpp                 |   6 +-
 toolchain/sem_ir/function.h                   |   4 +-
 5 files changed, 465 insertions(+), 5 deletions(-)

diff --git a/toolchain/check/class.cpp b/toolchain/check/class.cpp
index 007028d43965..508e8d470d8e 100644
--- a/toolchain/check/class.cpp
+++ b/toolchain/check/class.cpp
@@ -176,7 +176,8 @@ static auto BuildVtable(Context& context, Parse::ClassDefinitionId node_id,
     // TODO: Avoid quadratic search. Perhaps build a map from `NameId` to the
     // elements of the top of `vtable_stack`.
     for (auto base_vtable_entry_id : base_vtable_inst_block) {
-      auto [derived_vtable_entry_id, fn_id, specific_id] =
+      auto [derived_vtable_entry_id, derived_vtable_entry_const_id, fn_id,
+            specific_id] =
           DecomposeVirtualFunction(context.sem_ir(), base_vtable_entry_id,
                                    base_class_specific_id);
       const auto& fn = context.sem_ir().functions().Get(fn_id);
@@ -201,6 +202,22 @@ static auto BuildVtable(Context& context, Parse::ClassDefinitionId node_id,
         derived_vtable_entry_id = build_specific_function(*i);
         override_fn.virtual_index = vtable.size();
         CARBON_CHECK(override_fn.virtual_index == fn.virtual_index);
+      } else if (auto base_vtable_specific_function =
+                     context.sem_ir().insts().TryGetAs<SemIR::SpecificFunction>(
+                         derived_vtable_entry_id)) {
+        if (derived_vtable_entry_const_id.is_symbolic()) {
+          // Create a new instruction here that is otherwise identical to
+          // `derived_vtable_entry_id` but is dependent within the derived
+          // class. This ensures we can `GetConstantValueInSpecific` for it
+          // with the derived class's specific (when forming further derived
+          // classes, lowering the vtable, etc).
+          derived_vtable_entry_id = GetOrAddInst<SemIR::SpecificFunction>(
+              context, node_id,
+              {.type_id = GetSingletonType(
+                   context, SemIR::SpecificFunctionType::TypeInstId),
+               .callee_id = base_vtable_specific_function->callee_id,
+               .specific_id = base_vtable_specific_function->specific_id});
+        }
       }
       vtable.push_back(derived_vtable_entry_id);
     }
diff --git a/toolchain/check/testdata/class/virtual_modifiers.carbon b/toolchain/check/testdata/class/virtual_modifiers.carbon
index a0524cf6266d..08f3783ff12f 100644
--- a/toolchain/check/testdata/class/virtual_modifiers.carbon
+++ b/toolchain/check/testdata/class/virtual_modifiers.carbon
@@ -489,6 +489,33 @@ import library "generic_lib";
 class T1;
 var v: Base(T1) = {};
 
+// --- generic_derived_generic.carbon
+
+library "[[@TEST_NAME]]";
+
+base class T1(G1:! type) {
+  virtual fn F[self: Self]() { }
+}
+
+class T2(G2:! type) {
+  extend base: T1(G2);
+}
+
+// --- generic_derived_generic_context.carbon
+
+library "[[@TEST_NAME]]";
+
+base class T1(G1:! type) {
+  virtual fn F[self: Self]() { }
+}
+
+class T2(G2:! type) {
+  class T3 {
+    extend base : T1(G2);
+  }
+}
+
+
 // CHECK:STDOUT: --- modifiers.carbon
 // CHECK:STDOUT:
 // CHECK:STDOUT: constants {
@@ -4957,3 +4984,415 @@ var v: Base(T1) = {};
 // CHECK:STDOUT:   %require_complete => constants.%complete_type
 // CHECK:STDOUT: }
 // CHECK:STDOUT:
+// CHECK:STDOUT: --- generic_derived_generic.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %G1: type = bind_symbolic_name G1, 0 [symbolic]
+// CHECK:STDOUT:   %pattern_type.98f: type = pattern_type type [concrete]
+// CHECK:STDOUT:   %T1.type: type = generic_class_type @T1 [concrete]
+// CHECK:STDOUT:   %T1.generic: %T1.type = struct_value () [concrete]
+// CHECK:STDOUT:   %T1.18aea2.1: type = class_type @T1, @T1(%G1) [symbolic]
+// CHECK:STDOUT:   %pattern_type.48ecf1.1: type = pattern_type %T1.18aea2.1 [symbolic]
+// CHECK:STDOUT:   %T1.F.type.ebcc3f.1: type = fn_type @T1.F, @T1(%G1) [symbolic]
+// CHECK:STDOUT:   %T1.F.0df085.1: %T1.F.type.ebcc3f.1 = struct_value () [symbolic]
+// CHECK:STDOUT:   %ptr.454: type = ptr_type <vtable> [concrete]
+// CHECK:STDOUT:   %T1.F.specific_fn.1e2a8c.1: <specific function> = specific_function %T1.F.0df085.1, @T1.F(%G1) [symbolic]
+// CHECK:STDOUT:   %T1.vtable_ptr.b5fc91.1: ref %ptr.454 = vtable_ptr @T1.vtable, @T1(%G1) [symbolic]
+// CHECK:STDOUT:   %struct_type.vptr: type = struct_type {.<vptr>: %ptr.454} [concrete]
+// CHECK:STDOUT:   %complete_type.513: <witness> = complete_type_witness %struct_type.vptr [concrete]
+// CHECK:STDOUT:   %require_complete.86d019.1: <witness> = require_complete_type %T1.18aea2.1 [symbolic]
+// CHECK:STDOUT:   %G2: type = bind_symbolic_name G2, 0 [symbolic]
+// CHECK:STDOUT:   %T2.type: type = generic_class_type @T2 [concrete]
+// CHECK:STDOUT:   %T2.generic: %T2.type = struct_value () [concrete]
+// CHECK:STDOUT:   %T2: type = class_type @T2, @T2(%G2) [symbolic]
+// CHECK:STDOUT:   %T1.18aea2.2: type = class_type @T1, @T1(%G2) [symbolic]
+// CHECK:STDOUT:   %T1.F.type.ebcc3f.2: type = fn_type @T1.F, @T1(%G2) [symbolic]
+// CHECK:STDOUT:   %T1.F.0df085.2: %T1.F.type.ebcc3f.2 = struct_value () [symbolic]
+// CHECK:STDOUT:   %pattern_type.48ecf1.2: type = pattern_type %T1.18aea2.2 [symbolic]
+// CHECK:STDOUT:   %T1.F.specific_fn.1e2a8c.2: <specific function> = specific_function %T1.F.0df085.2, @T1.F(%G2) [symbolic]
+// CHECK:STDOUT:   %T1.vtable_ptr.b5fc91.2: ref %ptr.454 = vtable_ptr @T1.vtable, @T1(%G2) [symbolic]
+// CHECK:STDOUT:   %require_complete.86d019.2: <witness> = require_complete_type %T1.18aea2.2 [symbolic]
+// CHECK:STDOUT:   %T2.elem: type = unbound_element_type %T2, %T1.18aea2.2 [symbolic]
+// CHECK:STDOUT:   %T2.vtable_ptr: ref %ptr.454 = vtable_ptr @T2.vtable, @T2(%G2) [symbolic]
+// CHECK:STDOUT:   %struct_type.base: type = struct_type {.base: %T1.18aea2.2} [symbolic]
+// CHECK:STDOUT:   %complete_type.987: <witness> = complete_type_witness %struct_type.base [symbolic]
+// 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:     .T1 = %T1.decl
+// CHECK:STDOUT:     .T2 = %T2.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %T1.decl: %T1.type = class_decl @T1 [concrete = constants.%T1.generic] {
+// CHECK:STDOUT:     %G1.patt: %pattern_type.98f = symbolic_binding_pattern G1, 0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %G1.loc4_15.2: type = bind_symbolic_name G1, 0 [symbolic = %G1.loc4_15.1 (constants.%G1)]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %T2.decl: %T2.type = class_decl @T2 [concrete = constants.%T2.generic] {
+// CHECK:STDOUT:     %G2.patt: %pattern_type.98f = symbolic_binding_pattern G2, 0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %G2.loc8_10.2: type = bind_symbolic_name G2, 0 [symbolic = %G2.loc8_10.1 (constants.%G2)]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic class @T1(%G1.loc4_15.2: type) {
+// CHECK:STDOUT:   %G1.loc4_15.1: type = bind_symbolic_name G1, 0 [symbolic = %G1.loc4_15.1 (constants.%G1)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %T1.F.type: type = fn_type @T1.F, @T1(%G1.loc4_15.1) [symbolic = %T1.F.type (constants.%T1.F.type.ebcc3f.1)]
+// CHECK:STDOUT:   %T1.F: @T1.%T1.F.type (%T1.F.type.ebcc3f.1) = struct_value () [symbolic = %T1.F (constants.%T1.F.0df085.1)]
+// CHECK:STDOUT:   %T1.F.specific_fn.loc6_1.2: <specific function> = specific_function %T1.F, @T1.F(%G1.loc4_15.1) [symbolic = %T1.F.specific_fn.loc6_1.2 (constants.%T1.F.specific_fn.1e2a8c.1)]
+// CHECK:STDOUT:   %vtable_ptr.loc6_1.2: ref %ptr.454 = vtable_ptr @T1.vtable, @T1(%G1.loc4_15.1) [symbolic = %vtable_ptr.loc6_1.2 (constants.%T1.vtable_ptr.b5fc91.1)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   class {
+// CHECK:STDOUT:     %T1.F.decl: @T1.%T1.F.type (%T1.F.type.ebcc3f.1) = fn_decl @T1.F [symbolic = @T1.%T1.F (constants.%T1.F.0df085.1)] {
+// CHECK:STDOUT:       %self.patt: @T1.F.%pattern_type (%pattern_type.48ecf1.1) = binding_pattern self [concrete]
+// CHECK:STDOUT:       %self.param_patt: @T1.F.%pattern_type (%pattern_type.48ecf1.1) = value_param_pattern %self.patt, call_param0 [concrete]
+// CHECK:STDOUT:     } {
+// CHECK:STDOUT:       %self.param: @T1.F.%T1 (%T1.18aea2.1) = value_param call_param0
+// CHECK:STDOUT:       %.loc5_22.1: type = splice_block %Self.ref [symbolic = %T1 (constants.%T1.18aea2.1)] {
+// CHECK:STDOUT:         %.loc5_22.2: type = specific_constant constants.%T1.18aea2.1, @T1(constants.%G1) [symbolic = %T1 (constants.%T1.18aea2.1)]
+// CHECK:STDOUT:         %Self.ref: type = name_ref Self, %.loc5_22.2 [symbolic = %T1 (constants.%T1.18aea2.1)]
+// CHECK:STDOUT:       }
+// CHECK:STDOUT:       %self: @T1.F.%T1 (%T1.18aea2.1) = bind_name self, %self.param
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %T1.F.specific_fn.loc6_1.1: <specific function> = specific_function %T1.F.decl, @T1.F(constants.%G1) [symbolic = %T1.F.specific_fn.loc6_1.2 (constants.%T1.F.specific_fn.1e2a8c.1)]
+// CHECK:STDOUT:     %vtable_ptr.loc6_1.1: ref %ptr.454 = vtable_ptr @T1.vtable, @T1(constants.%G1) [symbolic = %vtable_ptr.loc6_1.2 (constants.%T1.vtable_ptr.b5fc91.1)]
+// CHECK:STDOUT:     %struct_type.vptr: type = struct_type {.<vptr>: %ptr.454} [concrete = constants.%struct_type.vptr]
+// CHECK:STDOUT:     %complete_type: <witness> = complete_type_witness %struct_type.vptr [concrete = constants.%complete_type.513]
+// CHECK:STDOUT:     complete_type_witness = %complete_type
+// CHECK:STDOUT:     vtable_ptr = %vtable_ptr.loc6_1.1
+// CHECK:STDOUT:
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .Self = constants.%T1.18aea2.1
+// CHECK:STDOUT:     .F = %T1.F.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic class @T2(%G2.loc8_10.2: type) {
+// CHECK:STDOUT:   %G2.loc8_10.1: type = bind_symbolic_name G2, 0 [symbolic = %G2.loc8_10.1 (constants.%G2)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %T1.loc9_21.2: type = class_type @T1, @T1(%G2.loc8_10.1) [symbolic = %T1.loc9_21.2 (constants.%T1.18aea2.2)]
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %T1.loc9_21.2 [symbolic = %require_complete (constants.%require_complete.86d019.2)]
+// CHECK:STDOUT:   %T2: type = class_type @T2, @T2(%G2.loc8_10.1) [symbolic = %T2 (constants.%T2)]
+// CHECK:STDOUT:   %T2.elem: type = unbound_element_type %T2, %T1.loc9_21.2 [symbolic = %T2.elem (constants.%T2.elem)]
+// CHECK:STDOUT:   %T1.F.type: type = fn_type @T1.F, @T1(%G2.loc8_10.1) [symbolic = %T1.F.type (constants.%T1.F.type.ebcc3f.2)]
+// CHECK:STDOUT:   %T1.F: @T2.%T1.F.type (%T1.F.type.ebcc3f.2) = struct_value () [symbolic = %T1.F (constants.%T1.F.0df085.2)]
+// CHECK:STDOUT:   %T1.F.specific_fn.loc10_1.2: <specific function> = specific_function %T1.F, @T1.F(%G2.loc8_10.1) [symbolic = %T1.F.specific_fn.loc10_1.2 (constants.%T1.F.specific_fn.1e2a8c.2)]
+// CHECK:STDOUT:   %vtable_ptr.loc10_1.2: ref %ptr.454 = vtable_ptr @T2.vtable, @T2(%G2.loc8_10.1) [symbolic = %vtable_ptr.loc10_1.2 (constants.%T2.vtable_ptr)]
+// CHECK:STDOUT:   %struct_type.base.loc10_1.2: type = struct_type {.base: @T2.%T1.loc9_21.2 (%T1.18aea2.2)} [symbolic = %struct_type.base.loc10_1.2 (constants.%struct_type.base)]
+// CHECK:STDOUT:   %complete_type.loc10_1.2: <witness> = complete_type_witness %struct_type.base.loc10_1.2 [symbolic = %complete_type.loc10_1.2 (constants.%complete_type.987)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   class {
+// CHECK:STDOUT:     %T1.ref: %T1.type = name_ref T1, file.%T1.decl [concrete = constants.%T1.generic]
+// CHECK:STDOUT:     %G2.ref: type = name_ref G2, %G2.loc8_10.2 [symbolic = %G2.loc8_10.1 (constants.%G2)]
+// CHECK:STDOUT:     %T1.loc9_21.1: type = class_type @T1, @T1(constants.%G2) [symbolic = %T1.loc9_21.2 (constants.%T1.18aea2.2)]
+// CHECK:STDOUT:     %.loc9: @T2.%T2.elem (%T2.elem) = base_decl %T1.loc9_21.1, element0 [concrete]
+// CHECK:STDOUT:     %T1.F.specific_fn.loc10_1.1: <specific function> = specific_function constants.%T1.F.0df085.2, @T1.F(constants.%G2) [symbolic = %T1.F.specific_fn.loc10_1.2 (constants.%T1.F.specific_fn.1e2a8c.2)]
+// CHECK:STDOUT:     %vtable_ptr.loc10_1.1: ref %ptr.454 = vtable_ptr @T2.vtable, @T2(constants.%G2) [symbolic = %vtable_ptr.loc10_1.2 (constants.%T2.vtable_ptr)]
+// CHECK:STDOUT:     %struct_type.base.loc10_1.1: type = struct_type {.base: %T1.18aea2.2} [symbolic = %struct_type.base.loc10_1.2 (constants.%struct_type.base)]
+// CHECK:STDOUT:     %complete_type.loc10_1.1: <witness> = complete_type_witness %struct_type.base.loc10_1.1 [symbolic = %complete_type.loc10_1.2 (constants.%complete_type.987)]
+// CHECK:STDOUT:     complete_type_witness = %complete_type.loc10_1.1
+// CHECK:STDOUT:     vtable_ptr = %vtable_ptr.loc10_1.1
+// CHECK:STDOUT:
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .Self = constants.%T2
+// CHECK:STDOUT:     .T1 = <poisoned>
+// CHECK:STDOUT:     .G2 = <poisoned>
+// CHECK:STDOUT:     .base = %.loc9
+// CHECK:STDOUT:     extend %T1.loc9_21.1
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: vtable @T1.vtable {
+// CHECK:STDOUT:   @T1.%T1.F.specific_fn.loc6_1.1
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: vtable @T2.vtable {
+// CHECK:STDOUT:   @T2.%T1.F.specific_fn.loc10_1.1
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic virtual fn @T1.F(@T1.%G1.loc4_15.2: type) {
+// CHECK:STDOUT:   %G1: type = bind_symbolic_name G1, 0 [symbolic = %G1 (constants.%G1)]
+// CHECK:STDOUT:   %T1: type = class_type @T1, @T1(%G1) [symbolic = %T1 (constants.%T1.18aea2.1)]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %T1 [symbolic = %pattern_type (constants.%pattern_type.48ecf1.1)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %T1 [symbolic = %require_complete (constants.%require_complete.86d019.1)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   virtual fn(%self.param: @T1.F.%T1 (%T1.18aea2.1)) {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T1(constants.%G1) {
+// CHECK:STDOUT:   %G1.loc4_15.1 => constants.%G1
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %T1.F.type => constants.%T1.F.type.ebcc3f.1
+// CHECK:STDOUT:   %T1.F => constants.%T1.F.0df085.1
+// CHECK:STDOUT:   %T1.F.specific_fn.loc6_1.2 => constants.%T1.F.specific_fn.1e2a8c.1
+// CHECK:STDOUT:   %vtable_ptr.loc6_1.2 => constants.%T1.vtable_ptr.b5fc91.1
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T1.F(constants.%G1) {
+// CHECK:STDOUT:   %G1 => constants.%G1
+// CHECK:STDOUT:   %T1 => constants.%T1.18aea2.1
+// CHECK:STDOUT:   %pattern_type => constants.%pattern_type.48ecf1.1
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete => constants.%require_complete.86d019.1
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T2(constants.%G2) {
+// CHECK:STDOUT:   %G2.loc8_10.1 => constants.%G2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T1(constants.%G2) {
+// CHECK:STDOUT:   %G1.loc4_15.1 => constants.%G2
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %T1.F.type => constants.%T1.F.type.ebcc3f.2
+// CHECK:STDOUT:   %T1.F => constants.%T1.F.0df085.2
+// CHECK:STDOUT:   %T1.F.specific_fn.loc6_1.2 => constants.%T1.F.specific_fn.1e2a8c.2
+// CHECK:STDOUT:   %vtable_ptr.loc6_1.2 => constants.%T1.vtable_ptr.b5fc91.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T1.F(constants.%G2) {
+// CHECK:STDOUT:   %G1 => constants.%G2
+// CHECK:STDOUT:   %T1 => constants.%T1.18aea2.2
+// CHECK:STDOUT:   %pattern_type => constants.%pattern_type.48ecf1.2
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete => constants.%require_complete.86d019.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- generic_derived_generic_context.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %G1: type = bind_symbolic_name G1, 0 [symbolic]
+// CHECK:STDOUT:   %pattern_type.98f: type = pattern_type type [concrete]
+// CHECK:STDOUT:   %T1.type: type = generic_class_type @T1 [concrete]
+// CHECK:STDOUT:   %T1.generic: %T1.type = struct_value () [concrete]
+// CHECK:STDOUT:   %T1.18aea2.1: type = class_type @T1, @T1(%G1) [symbolic]
+// CHECK:STDOUT:   %pattern_type.48ecf1.1: type = pattern_type %T1.18aea2.1 [symbolic]
+// CHECK:STDOUT:   %T1.F.type.ebcc3f.1: type = fn_type @T1.F, @T1(%G1) [symbolic]
+// CHECK:STDOUT:   %T1.F.0df085.1: %T1.F.type.ebcc3f.1 = struct_value () [symbolic]
+// CHECK:STDOUT:   %ptr.454: type = ptr_type <vtable> [concrete]
+// CHECK:STDOUT:   %T1.F.specific_fn.1e2a8c.1: <specific function> = specific_function %T1.F.0df085.1, @T1.F(%G1) [symbolic]
+// CHECK:STDOUT:   %T1.vtable_ptr.b5fc91.1: ref %ptr.454 = vtable_ptr @T1.vtable, @T1(%G1) [symbolic]
+// CHECK:STDOUT:   %struct_type.vptr: type = struct_type {.<vptr>: %ptr.454} [concrete]
+// CHECK:STDOUT:   %complete_type.513: <witness> = complete_type_witness %struct_type.vptr [concrete]
+// CHECK:STDOUT:   %require_complete.86d019.1: <witness> = require_complete_type %T1.18aea2.1 [symbolic]
+// CHECK:STDOUT:   %G2: type = bind_symbolic_name G2, 0 [symbolic]
+// CHECK:STDOUT:   %T2.type: type = generic_class_type @T2 [concrete]
+// CHECK:STDOUT:   %T2.generic: %T2.type = struct_value () [concrete]
+// CHECK:STDOUT:   %T2: type = class_type @T2, @T2(%G2) [symbolic]
+// CHECK:STDOUT:   %T3: type = class_type @T3, @T3(%G2) [symbolic]
+// CHECK:STDOUT:   %T1.18aea2.2: type = class_type @T1, @T1(%G2) [symbolic]
+// CHECK:STDOUT:   %T1.F.type.ebcc3f.2: type = fn_type @T1.F, @T1(%G2) [symbolic]
+// CHECK:STDOUT:   %T1.F.0df085.2: %T1.F.type.ebcc3f.2 = struct_value () [symbolic]
+// CHECK:STDOUT:   %pattern_type.48ecf1.2: type = pattern_type %T1.18aea2.2 [symbolic]
+// CHECK:STDOUT:   %T1.F.specific_fn.1e2a8c.2: <specific function> = specific_function %T1.F.0df085.2, @T1.F(%G2) [symbolic]
+// CHECK:STDOUT:   %T1.vtable_ptr.b5fc91.2: ref %ptr.454 = vtable_ptr @T1.vtable, @T1(%G2) [symbolic]
+// CHECK:STDOUT:   %require_complete.86d019.2: <witness> = require_complete_type %T1.18aea2.2 [symbolic]
+// CHECK:STDOUT:   %T3.elem: type = unbound_element_type %T3, %T1.18aea2.2 [symbolic]
+// CHECK:STDOUT:   %T3.vtable_ptr: ref %ptr.454 = vtable_ptr @T3.vtable, @T3(%G2) [symbolic]
+// CHECK:STDOUT:   %struct_type.base: type = struct_type {.base: %T1.18aea2.2} [symbolic]
+// CHECK:STDOUT:   %complete_type.987: <witness> = complete_type_witness %struct_type.base [symbolic]
+// CHECK:STDOUT:   %empty_struct_type: type = struct_type {} [concrete]
+// CHECK:STDOUT:   %complete_type.357: <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:     .T1 = %T1.decl
+// CHECK:STDOUT:     .T2 = %T2.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.import = import Core
+// CHECK:STDOUT:   %T1.decl: %T1.type = class_decl @T1 [concrete = constants.%T1.generic] {
+// CHECK:STDOUT:     %G1.patt: %pattern_type.98f = symbolic_binding_pattern G1, 0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %G1.loc4_15.2: type = bind_symbolic_name G1, 0 [symbolic = %G1.loc4_15.1 (constants.%G1)]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %T2.decl: %T2.type = class_decl @T2 [concrete = constants.%T2.generic] {
+// CHECK:STDOUT:     %G2.patt: %pattern_type.98f = symbolic_binding_pattern G2, 0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %G2.loc8_10.2: type = bind_symbolic_name G2, 0 [symbolic = %G2.loc8_10.1 (constants.%G2)]
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic class @T1(%G1.loc4_15.2: type) {
+// CHECK:STDOUT:   %G1.loc4_15.1: type = bind_symbolic_name G1, 0 [symbolic = %G1.loc4_15.1 (constants.%G1)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %T1.F.type: type = fn_type @T1.F, @T1(%G1.loc4_15.1) [symbolic = %T1.F.type (constants.%T1.F.type.ebcc3f.1)]
+// CHECK:STDOUT:   %T1.F: @T1.%T1.F.type (%T1.F.type.ebcc3f.1) = struct_value () [symbolic = %T1.F (constants.%T1.F.0df085.1)]
+// CHECK:STDOUT:   %T1.F.specific_fn.loc6_1.2: <specific function> = specific_function %T1.F, @T1.F(%G1.loc4_15.1) [symbolic = %T1.F.specific_fn.loc6_1.2 (constants.%T1.F.specific_fn.1e2a8c.1)]
+// CHECK:STDOUT:   %vtable_ptr.loc6_1.2: ref %ptr.454 = vtable_ptr @T1.vtable, @T1(%G1.loc4_15.1) [symbolic = %vtable_ptr.loc6_1.2 (constants.%T1.vtable_ptr.b5fc91.1)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   class {
+// CHECK:STDOUT:     %T1.F.decl: @T1.%T1.F.type (%T1.F.type.ebcc3f.1) = fn_decl @T1.F [symbolic = @T1.%T1.F (constants.%T1.F.0df085.1)] {
+// CHECK:STDOUT:       %self.patt: @T1.F.%pattern_type (%pattern_type.48ecf1.1) = binding_pattern self [concrete]
+// CHECK:STDOUT:       %self.param_patt: @T1.F.%pattern_type (%pattern_type.48ecf1.1) = value_param_pattern %self.patt, call_param0 [concrete]
+// CHECK:STDOUT:     } {
+// CHECK:STDOUT:       %self.param: @T1.F.%T1 (%T1.18aea2.1) = value_param call_param0
+// CHECK:STDOUT:       %.loc5_22.1: type = splice_block %Self.ref [symbolic = %T1 (constants.%T1.18aea2.1)] {
+// CHECK:STDOUT:         %.loc5_22.2: type = specific_constant constants.%T1.18aea2.1, @T1(constants.%G1) [symbolic = %T1 (constants.%T1.18aea2.1)]
+// CHECK:STDOUT:         %Self.ref: type = name_ref Self, %.loc5_22.2 [symbolic = %T1 (constants.%T1.18aea2.1)]
+// CHECK:STDOUT:       }
+// CHECK:STDOUT:       %self: @T1.F.%T1 (%T1.18aea2.1) = bind_name self, %self.param
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %T1.F.specific_fn.loc6_1.1: <specific function> = specific_function %T1.F.decl, @T1.F(constants.%G1) [symbolic = %T1.F.specific_fn.loc6_1.2 (constants.%T1.F.specific_fn.1e2a8c.1)]
+// CHECK:STDOUT:     %vtable_ptr.loc6_1.1: ref %ptr.454 = vtable_ptr @T1.vtable, @T1(constants.%G1) [symbolic = %vtable_ptr.loc6_1.2 (constants.%T1.vtable_ptr.b5fc91.1)]
+// CHECK:STDOUT:     %struct_type.vptr: type = struct_type {.<vptr>: %ptr.454} [concrete = constants.%struct_type.vptr]
+// CHECK:STDOUT:     %complete_type: <witness> = complete_type_witness %struct_type.vptr [concrete = constants.%complete_type.513]
+// CHECK:STDOUT:     complete_type_witness = %complete_type
+// CHECK:STDOUT:     vtable_ptr = %vtable_ptr.loc6_1.1
+// CHECK:STDOUT:
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .Self = constants.%T1.18aea2.1
+// CHECK:STDOUT:     .F = %T1.F.decl
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic class @T2(%G2.loc8_10.2: type) {
+// CHECK:STDOUT:   %G2.loc8_10.1: type = bind_symbolic_name G2, 0 [symbolic = %G2.loc8_10.1 (constants.%G2)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %T3: type = class_type @T3, @T3(%G2.loc8_10.1) [symbolic = %T3 (constants.%T3)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   class {
+// CHECK:STDOUT:     %T3.decl: type = class_decl @T3 [symbolic = @T2.%T3 (constants.%T3)] {} {}
+// CHECK:STDOUT:     %empty_struct_type: type = struct_type {} [concrete = constants.%empty_struct_type]
+// CHECK:STDOUT:     %complete_type: <witness> = complete_type_witness %empty_struct_type [concrete = constants.%complete_type.357]
+// CHECK:STDOUT:     complete_type_witness = %complete_type
+// CHECK:STDOUT:
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .Self = constants.%T2
+// CHECK:STDOUT:     .T3 = %T3.decl
+// CHECK:STDOUT:     .T1 = <poisoned>
+// CHECK:STDOUT:     .G2 = <poisoned>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic class @T3(@T2.%G2.loc8_10.2: type) {
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %G2: type = bind_symbolic_name G2, 0 [symbolic = %G2 (constants.%G2)]
+// CHECK:STDOUT:   %T1.loc10_24.2: type = class_type @T1, @T1(%G2) [symbolic = %T1.loc10_24.2 (constants.%T1.18aea2.2)]
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %T1.loc10_24.2 [symbolic = %require_complete (constants.%require_complete.86d019.2)]
+// CHECK:STDOUT:   %T3: type = class_type @T3, @T3(%G2) [symbolic = %T3 (constants.%T3)]
+// CHECK:STDOUT:   %T3.elem: type = unbound_element_type %T3, %T1.loc10_24.2 [symbolic = %T3.elem (constants.%T3.elem)]
+// CHECK:STDOUT:   %T1.F.type: type = fn_type @T1.F, @T1(%G2) [symbolic = %T1.F.type (constants.%T1.F.type.ebcc3f.2)]
+// CHECK:STDOUT:   %T1.F: @T3.%T1.F.type (%T1.F.type.ebcc3f.2) = struct_value () [symbolic = %T1.F (constants.%T1.F.0df085.2)]
+// CHECK:STDOUT:   %T1.F.specific_fn.loc11_3.2: <specific function> = specific_function %T1.F, @T1.F(%G2) [symbolic = %T1.F.specific_fn.loc11_3.2 (constants.%T1.F.specific_fn.1e2a8c.2)]
+// CHECK:STDOUT:   %vtable_ptr.loc11_3.2: ref %ptr.454 = vtable_ptr @T3.vtable, @T3(%G2) [symbolic = %vtable_ptr.loc11_3.2 (constants.%T3.vtable_ptr)]
+// CHECK:STDOUT:   %struct_type.base.loc11_3.2: type = struct_type {.base: @T3.%T1.loc10_24.2 (%T1.18aea2.2)} [symbolic = %struct_type.base.loc11_3.2 (constants.%struct_type.base)]
+// CHECK:STDOUT:   %complete_type.loc11_3.2: <witness> = complete_type_witness %struct_type.base.loc11_3.2 [symbolic = %complete_type.loc11_3.2 (constants.%complete_type.987)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   class {
+// CHECK:STDOUT:     %T1.ref: %T1.type = name_ref T1, file.%T1.decl [concrete = constants.%T1.generic]
+// CHECK:STDOUT:     %G2.ref: type = name_ref G2, @T2.%G2.loc8_10.2 [symbolic = %G2 (constants.%G2)]
+// CHECK:STDOUT:     %T1.loc10_24.1: type = class_type @T1, @T1(constants.%G2) [symbolic = %T1.loc10_24.2 (constants.%T1.18aea2.2)]
+// CHECK:STDOUT:     %.loc10: @T3.%T3.elem (%T3.elem) = base_decl %T1.loc10_24.1, element0 [concrete]
+// CHECK:STDOUT:     %T1.F.specific_fn.loc11_3.1: <specific function> = specific_function constants.%T1.F.0df085.2, @T1.F(constants.%G2) [symbolic = %T1.F.specific_fn.loc11_3.2 (constants.%T1.F.specific_fn.1e2a8c.2)]
+// CHECK:STDOUT:     %vtable_ptr.loc11_3.1: ref %ptr.454 = vtable_ptr @T3.vtable, @T3(constants.%G2) [symbolic = %vtable_ptr.loc11_3.2 (constants.%T3.vtable_ptr)]
+// CHECK:STDOUT:     %struct_type.base.loc11_3.1: type = struct_type {.base: %T1.18aea2.2} [symbolic = %struct_type.base.loc11_3.2 (constants.%struct_type.base)]
+// CHECK:STDOUT:     %complete_type.loc11_3.1: <witness> = complete_type_witness %struct_type.base.loc11_3.1 [symbolic = %complete_type.loc11_3.2 (constants.%complete_type.987)]
+// CHECK:STDOUT:     complete_type_witness = %complete_type.loc11_3.1
+// CHECK:STDOUT:     vtable_ptr = %vtable_ptr.loc11_3.1
+// CHECK:STDOUT:
+// CHECK:STDOUT:   !members:
+// CHECK:STDOUT:     .Self = constants.%T3
+// CHECK:STDOUT:     .T1 = <poisoned>
+// CHECK:STDOUT:     .G2 = <poisoned>
+// CHECK:STDOUT:     .base = %.loc10
+// CHECK:STDOUT:     extend %T1.loc10_24.1
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: vtable @T1.vtable {
+// CHECK:STDOUT:   @T1.%T1.F.specific_fn.loc6_1.1
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: vtable @T3.vtable {
+// CHECK:STDOUT:   @T3.%T1.F.specific_fn.loc11_3.1
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: generic virtual fn @T1.F(@T1.%G1.loc4_15.2: type) {
+// CHECK:STDOUT:   %G1: type = bind_symbolic_name G1, 0 [symbolic = %G1 (constants.%G1)]
+// CHECK:STDOUT:   %T1: type = class_type @T1, @T1(%G1) [symbolic = %T1 (constants.%T1.18aea2.1)]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %T1 [symbolic = %pattern_type (constants.%pattern_type.48ecf1.1)]
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete: <witness> = require_complete_type %T1 [symbolic = %require_complete (constants.%require_complete.86d019.1)]
+// CHECK:STDOUT:
+// CHECK:STDOUT:   virtual fn(%self.param: @T1.F.%T1 (%T1.18aea2.1)) {
+// CHECK:STDOUT:   !entry:
+// CHECK:STDOUT:     return
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T1(constants.%G1) {
+// CHECK:STDOUT:   %G1.loc4_15.1 => constants.%G1
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %T1.F.type => constants.%T1.F.type.ebcc3f.1
+// CHECK:STDOUT:   %T1.F => constants.%T1.F.0df085.1
+// CHECK:STDOUT:   %T1.F.specific_fn.loc6_1.2 => constants.%T1.F.specific_fn.1e2a8c.1
+// CHECK:STDOUT:   %vtable_ptr.loc6_1.2 => constants.%T1.vtable_ptr.b5fc91.1
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T1.F(constants.%G1) {
+// CHECK:STDOUT:   %G1 => constants.%G1
+// CHECK:STDOUT:   %T1 => constants.%T1.18aea2.1
+// CHECK:STDOUT:   %pattern_type => constants.%pattern_type.48ecf1.1
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete => constants.%require_complete.86d019.1
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T2(constants.%G2) {
+// CHECK:STDOUT:   %G2.loc8_10.1 => constants.%G2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T3(constants.%G2) {}
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T1(constants.%G2) {
+// CHECK:STDOUT:   %G1.loc4_15.1 => constants.%G2
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %T1.F.type => constants.%T1.F.type.ebcc3f.2
+// CHECK:STDOUT:   %T1.F => constants.%T1.F.0df085.2
+// CHECK:STDOUT:   %T1.F.specific_fn.loc6_1.2 => constants.%T1.F.specific_fn.1e2a8c.2
+// CHECK:STDOUT:   %vtable_ptr.loc6_1.2 => constants.%T1.vtable_ptr.b5fc91.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: specific @T1.F(constants.%G2) {
+// CHECK:STDOUT:   %G1 => constants.%G2
+// CHECK:STDOUT:   %T1 => constants.%T1.18aea2.2
+// CHECK:STDOUT:   %pattern_type => constants.%pattern_type.48ecf1.2
+// CHECK:STDOUT:
+// CHECK:STDOUT: !definition:
+// CHECK:STDOUT:   %require_complete => constants.%require_complete.86d019.2
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
diff --git a/toolchain/lower/file_context.cpp b/toolchain/lower/file_context.cpp
index 89854c9721da..be1171d95b4a 100644
--- a/toolchain/lower/file_context.cpp
+++ b/toolchain/lower/file_context.cpp
@@ -978,7 +978,7 @@ auto FileContext::BuildVtable(const SemIR::Vtable& vtable,
   vfuncs.reserve(vtable_inst_block.size());
 
   for (auto fn_decl_id : vtable_inst_block) {
-    auto [fn_decl, fn_id, fn_specific_id] =
+    auto [_1, _2, fn_id, fn_specific_id] =
         DecomposeVirtualFunction(sem_ir(), fn_decl_id, specific_id);
 
     vfuncs.push_back(llvm::ConstantExpr::getTrunc(
diff --git a/toolchain/sem_ir/function.cpp b/toolchain/sem_ir/function.cpp
index c76581b7628d..c073eb422f9b 100644
--- a/toolchain/sem_ir/function.cpp
+++ b/toolchain/sem_ir/function.cpp
@@ -70,8 +70,9 @@ auto DecomposeVirtualFunction(const File& sem_ir, InstId fn_decl_id,
   // Remap the base's vtable entry to the appropriate constant usable in
   // the context of the derived class (for the specific for the base
   // class, for instance).
-  fn_decl_id = sem_ir.constant_values().GetInstId(
-      GetConstantValueInSpecific(sem_ir, base_class_specific_id, fn_decl_id));
+  auto fn_decl_const_id =
+      GetConstantValueInSpecific(sem_ir, base_class_specific_id, fn_decl_id);
+  fn_decl_id = sem_ir.constant_values().GetInstId(fn_decl_const_id);
   auto specific_id = SemIR::SpecificId::None;
   auto callee_id = fn_decl_id;
   if (auto specific_function =
@@ -85,6 +86,7 @@ auto DecomposeVirtualFunction(const File& sem_ir, InstId fn_decl_id,
       sem_ir.types().GetAsInst(sem_ir.insts().Get(callee_id).type_id());
 
   return {.fn_decl_id = fn_decl_id,
+          .fn_decl_const_id = fn_decl_const_id,
           .function_id = fn_type_inst.As<FunctionType>().function_id,
           .specific_id = specific_id};
 }
diff --git a/toolchain/sem_ir/function.h b/toolchain/sem_ir/function.h
index cb9921883247..2a1fa9fe7742 100644
--- a/toolchain/sem_ir/function.h
+++ b/toolchain/sem_ir/function.h
@@ -192,8 +192,10 @@ auto GetCalleeFunction(const File& sem_ir, InstId callee_id,
     -> CalleeFunction;
 
 struct DecomposedVirtualFunction {
-  // The underlying `FunctionDecl`.
+  // The canonical instruction from the `fn_decl_const_id`.
   InstId fn_decl_id;
+  // The constant for the underlying instruction.
+  ConstantId fn_decl_const_id;
   // The function.
   FunctionId function_id;
   // The specific for the function.