adds witness support for associated types (#6937)

This commit creates an instance for any associated types in an interface
with a custom witness table. This unlocks interfaces designed for C++
interop that rely on arbitrary return types. For example,
`CppUnsafeDeref` becomes usable as of this commit.

This commit may have also implemented support for non-type associated
constants, but since we're lacking a practical test case, they're still
marked as TODO for the time being.

---------

Co-authored-by: Dana Jansens <danakj@orodu.net>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>

core/prelude/operators/deref.carbon

@@ -7,5 +7,5 @@ package Core library "prelude/operators/deref";
 // TODO: Align with https://docs.carbon-lang.dev/docs/design/values.html#dereferencing-customization.
 interface CppUnsafeDeref {
   let Result:! type;
-  fn Op[self: Self]() -> ref Result;
+  fn Op[ref self: Self]() -> ref Result;
 }

toolchain/check/custom_witness.cpp

@@ -15,6 +15,7 @@
 #include "toolchain/check/name_lookup.h"
 #include "toolchain/check/type.h"
 #include "toolchain/check/type_completion.h"
+#include "toolchain/sem_ir/associated_constant.h"
 #include "toolchain/sem_ir/builtin_function_kind.h"
 #include "toolchain/sem_ir/ids.h"
 #include "toolchain/sem_ir/typed_insts.h"
@@ -247,10 +248,29 @@ auto BuildCustomWitness(Context& context, SemIR::LocId loc_id,
             /*defer_thunk_definition=*/false));
         break;
       }
-      case SemIR::AssociatedConstantDecl::Kind: {
-        context.TODO(loc_id,
-                     "Associated constant in interface with synthesized impl");
-        return SemIR::ErrorInst::InstId;
+      case CARBON_KIND(SemIR::AssociatedConstantDecl decl): {
+        if (decl.type_id == SemIR::ErrorInst::TypeId) {
+          return SemIR::ErrorInst::InstId;
+        }
+
+        // TODO: remove once we have a test-case for all associated constants.
+        // Special-case the ones we want to support in this if-statement, until
+        // we're able to account for everything.
+        if (decl.type_id != SemIR::TypeType::TypeId) {
+          context.TODO(loc_id,
+                       "Associated constant of type other than `TypeType` in "
+                       "synthesized impl");
+          return SemIR::ErrorInst::InstId;
+        }
+
+        auto type_id = context.insts().Get(value_id).type_id();
+        CARBON_CHECK(type_id == SemIR::TypeType::TypeId ||
+                     type_id == SemIR::ErrorInst::TypeId);
+        auto impl_witness_associated_constant =
+            AddInst<SemIR::ImplWitnessAssociatedConstant>(
+                context, loc_id, {.type_id = type_id, .inst_id = value_id});
+        entries.push_back(impl_witness_associated_constant);
+        break;
       }
       default:
         CARBON_CHECK(decl_id == SemIR::ErrorInst::InstId,
@@ -263,7 +283,10 @@ auto BuildCustomWitness(Context& context, SemIR::LocId loc_id,
   // then a second after all associated functions, rather than building one in
   // each `StructValue`. Right now the code is written assuming at most one
   // function, though this CHECK can be removed as a temporary workaround.
-  CARBON_CHECK(entries.size() <= 1,
+  auto associated_functions = llvm::count_if(entries, [&](SemIR::InstId id) {
+    return context.insts().Get(id).kind() == SemIR::InstKind::FunctionDecl;
+  });
+  CARBON_CHECK(associated_functions <= 1,
                "TODO: Support multiple associated functions");
 
   return MakeCustomWitnessConstantInst(context, loc_id,

toolchain/check/impl_lookup.cpp

@@ -380,7 +380,8 @@ static auto LookupImplWitnessInSelfFacetValue(
           self_facet_value_inst_id)) {
     auto witness_id =
         context.inst_blocks().Get(facet_value->witnesses_block_id)[index];
-    if (context.insts().Is<SemIR::ImplWitness>(witness_id)) {
+    if (context.insts().IsOneOf<SemIR::ImplWitness, SemIR::CustomWitness>(
+            witness_id)) {
       return EvalImplLookupResult::MakeFinal(witness_id);
     }
   }

toolchain/check/testdata/interop/cpp/deref.carbon

@@ -4,7 +4,7 @@
 //
 // INCLUDE-FILE: toolchain/testing/testdata/min_prelude/full.carbon
 //
-// EXTRA-ARGS: --target=aarch64-unknown-linux --clang-arg=--std=c++20
+// EXTRA-ARGS: --clang-arg=-std=c++20
 //
 // AUTOUPDATE
 // TIP: To test this file alone, run:
@@ -12,7 +12,7 @@
 // TIP: To dump output, run:
 // TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/deref.carbon
 
-// --- fail_todo_const_deref.carbon
+// --- const_deref.carbon
 
 library "[[@TEST_NAME]]";
 
@@ -24,14 +24,12 @@ public:
 ''';
 
 fn TestDerefPass(var int_ptr: Cpp.ConstDeref) -> ref i32 {
-  // CHECK:STDERR: fail_todo_const_deref.carbon:[[@LINE+4]]:10: error: semantics TODO: `Associated constant in interface with synthesized impl` [SemanticsTodo]
-  // CHECK:STDERR:   return int_ptr.(Core.CppUnsafeDeref.Op)();
-  // CHECK:STDERR:          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-  // CHECK:STDERR:
+  //@dump-sem-ir-begin
   return int_ptr.(Core.CppUnsafeDeref.Op)();
+  //@dump-sem-ir-end
 }
 
-// --- fail_todo_mutable_deref.carbon
+// --- mutable_deref.carbon
 
 library "[[@TEST_NAME]]";
 
@@ -43,11 +41,9 @@ public:
 ''';
 
 fn TestDerefPass(var int_ptr: Cpp.MutableDeref) -> ref i32 {
-  // CHECK:STDERR: fail_todo_mutable_deref.carbon:[[@LINE+4]]:10: error: semantics TODO: `Associated constant in interface with synthesized impl` [SemanticsTodo]
-  // CHECK:STDERR:   return int_ptr.(Core.CppUnsafeDeref.Op)();
-  // CHECK:STDERR:          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-  // CHECK:STDERR:
+  //@dump-sem-ir-begin
   return int_ptr.(Core.CppUnsafeDeref.Op)();
+  //@dump-sem-ir-end
 }
 
 // --- fail_todo_multi_deref.carbon
@@ -117,3 +113,122 @@ fn TestDerefFail(not_ptr: Cpp.NotAPtr) {
   // CHECK:STDERR:
   not_ptr.(Core.CppUnsafeDeref.Op)();
 }
+
+// CHECK:STDOUT: --- const_deref.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %ConstDeref: type = class_type @ConstDeref [concrete]
+// CHECK:STDOUT:   %pattern_type.3fb: type = pattern_type %ConstDeref [concrete]
+// CHECK:STDOUT:   %int_32: Core.IntLiteral = int_value 32 [concrete]
+// CHECK:STDOUT:   %Int.type: type = generic_class_type @Int [concrete]
+// CHECK:STDOUT:   %Int.generic: %Int.type = struct_value () [concrete]
+// CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
+// CHECK:STDOUT:   %CppUnsafeDeref.type: type = facet_type <@CppUnsafeDeref> [concrete]
+// CHECK:STDOUT:   %Self: %CppUnsafeDeref.type = symbolic_binding Self, 0 [symbolic]
+// CHECK:STDOUT:   %CppUnsafeDeref.WithSelf.Op.type.792: type = fn_type @CppUnsafeDeref.WithSelf.Op, @CppUnsafeDeref.WithSelf(%Self) [symbolic]
+// CHECK:STDOUT:   %CppUnsafeDeref.WithSelf.Op.a80: %CppUnsafeDeref.WithSelf.Op.type.792 = struct_value () [symbolic]
+// CHECK:STDOUT:   %CppUnsafeDeref.assoc_type: type = assoc_entity_type @CppUnsafeDeref [concrete]
+// CHECK:STDOUT:   %assoc1: %CppUnsafeDeref.assoc_type = assoc_entity element1, imports.%Core.import_ref.ad7 [concrete]
+// CHECK:STDOUT:   %ConstDeref.cpp_operator.type: type = fn_type @ConstDeref.cpp_operator [concrete]
+// CHECK:STDOUT:   %ConstDeref.cpp_operator: %ConstDeref.cpp_operator.type = struct_value () [concrete]
+// CHECK:STDOUT:   %operator_Star__carbon_thunk.type: type = fn_type @operator_Star__carbon_thunk [concrete]
+// CHECK:STDOUT:   %operator_Star__carbon_thunk: %operator_Star__carbon_thunk.type = struct_value () [concrete]
+// CHECK:STDOUT:   %ConstDeref.Op.type: type = fn_type @ConstDeref.Op [concrete]
+// CHECK:STDOUT:   %ConstDeref.Op: %ConstDeref.Op.type = struct_value () [concrete]
+// CHECK:STDOUT:   %custom_witness.5a5: <witness> = custom_witness (%i32, %ConstDeref.Op), @CppUnsafeDeref [concrete]
+// CHECK:STDOUT:   %CppUnsafeDeref.facet.5c0: %CppUnsafeDeref.type = facet_value %ConstDeref, (%custom_witness.5a5) [concrete]
+// CHECK:STDOUT:   %CppUnsafeDeref.WithSelf.Op.type.d8f: type = fn_type @CppUnsafeDeref.WithSelf.Op, @CppUnsafeDeref.WithSelf(%CppUnsafeDeref.facet.5c0) [concrete]
+// CHECK:STDOUT:   %.d81: type = fn_type_with_self_type %CppUnsafeDeref.WithSelf.Op.type.d8f, %CppUnsafeDeref.facet.5c0 [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     .Int = %Core.Int
+// CHECK:STDOUT:     .CppUnsafeDeref = %Core.CppUnsafeDeref
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.Int: %Int.type = import_ref Core//prelude/types/int, Int, loaded [concrete = constants.%Int.generic]
+// CHECK:STDOUT:   %Core.CppUnsafeDeref: type = import_ref Core//prelude/operators/deref, CppUnsafeDeref, loaded [concrete = constants.%CppUnsafeDeref.type]
+// CHECK:STDOUT:   %Core.import_ref.83d: %CppUnsafeDeref.assoc_type = import_ref Core//prelude/operators/deref, loc{{\d+_\d+}}, loaded [concrete = constants.%assoc1]
+// CHECK:STDOUT:   %Core.import_ref.ad7: @CppUnsafeDeref.WithSelf.%CppUnsafeDeref.WithSelf.Op.type (%CppUnsafeDeref.WithSelf.Op.type.792) = import_ref Core//prelude/operators/deref, loc{{\d+_\d+}}, loaded [symbolic = @CppUnsafeDeref.WithSelf.%CppUnsafeDeref.WithSelf.Op (constants.%CppUnsafeDeref.WithSelf.Op.a80)]
+// CHECK:STDOUT:   %ConstDeref.cpp_operator.decl: %ConstDeref.cpp_operator.type = fn_decl @ConstDeref.cpp_operator [concrete = constants.%ConstDeref.cpp_operator] {
+// CHECK:STDOUT:     %self.patt: %pattern_type.3fb = value_binding_pattern self [concrete]
+// CHECK:STDOUT:     %self.param_patt: %pattern_type.3fb = value_param_pattern %self.patt [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:     %self.param: %ConstDeref = value_param call_param0
+// CHECK:STDOUT:     %self: %ConstDeref = value_binding self, %self.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %operator_Star__carbon_thunk.decl: %operator_Star__carbon_thunk.type = fn_decl @operator_Star__carbon_thunk [concrete = constants.%operator_Star__carbon_thunk] {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     <elided>
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @TestDerefPass(%int_ptr.param: ref %ConstDeref) -> ref %i32 {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %int_ptr.ref: ref %ConstDeref = name_ref int_ptr, %int_ptr
+// CHECK:STDOUT:   %Core.ref: <namespace> = name_ref Core, imports.%Core [concrete = imports.%Core]
+// CHECK:STDOUT:   %CppUnsafeDeref.ref: type = name_ref CppUnsafeDeref, imports.%Core.CppUnsafeDeref [concrete = constants.%CppUnsafeDeref.type]
+// CHECK:STDOUT:   %Op.ref.loc13_38: %CppUnsafeDeref.assoc_type = name_ref Op, imports.%Core.import_ref.83d [concrete = constants.%assoc1]
+// CHECK:STDOUT:   %impl_witness_assoc_constant: type = impl_witness_assoc_constant @ConstDeref.cpp_operator.%i32.2 [concrete = constants.%i32]
+// CHECK:STDOUT:   <elided>
+// CHECK:STDOUT:   %impl.elem1: %.d81 = impl_witness_access constants.%custom_witness.5a5, element1 [concrete = constants.%ConstDeref.Op]
+// CHECK:STDOUT:   %bound_method: <bound method> = bound_method %int_ptr.ref, %impl.elem1
+// CHECK:STDOUT:   %Op.ref.loc13_43: %ConstDeref.cpp_operator.type = name_ref Op, imports.%ConstDeref.cpp_operator.decl [concrete = constants.%ConstDeref.cpp_operator]
+// CHECK:STDOUT:   %ConstDeref.cpp_operator.bound: <bound method> = bound_method %int_ptr.ref, %Op.ref.loc13_43
+// CHECK:STDOUT:   %.loc13: %ConstDeref = acquire_value %int_ptr.ref
+// CHECK:STDOUT:   %operator_Star__carbon_thunk.call: ref %i32 = call imports.%operator_Star__carbon_thunk.decl(%.loc13)
+// CHECK:STDOUT:   return %operator_Star__carbon_thunk.call
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- mutable_deref.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %MutableDeref: type = class_type @MutableDeref [concrete]
+// CHECK:STDOUT:   %int_32: Core.IntLiteral = int_value 32 [concrete]
+// CHECK:STDOUT:   %Int.type: type = generic_class_type @Int [concrete]
+// CHECK:STDOUT:   %Int.generic: %Int.type = struct_value () [concrete]
+// CHECK:STDOUT:   %i32: type = class_type @Int, @Int(%int_32) [concrete]
+// CHECK:STDOUT:   %CppUnsafeDeref.type: type = facet_type <@CppUnsafeDeref> [concrete]
+// CHECK:STDOUT:   %Self: %CppUnsafeDeref.type = symbolic_binding Self, 0 [symbolic]
+// CHECK:STDOUT:   %CppUnsafeDeref.WithSelf.Op.type.792: type = fn_type @CppUnsafeDeref.WithSelf.Op, @CppUnsafeDeref.WithSelf(%Self) [symbolic]
+// CHECK:STDOUT:   %CppUnsafeDeref.WithSelf.Op.a80: %CppUnsafeDeref.WithSelf.Op.type.792 = struct_value () [symbolic]
+// CHECK:STDOUT:   %CppUnsafeDeref.assoc_type: type = assoc_entity_type @CppUnsafeDeref [concrete]
+// CHECK:STDOUT:   %assoc1: %CppUnsafeDeref.assoc_type = assoc_entity element1, imports.%Core.import_ref.ad7 [concrete]
+// CHECK:STDOUT:   %MutableDeref.cpp_operator.type: type = fn_type @MutableDeref.cpp_operator [concrete]
+// CHECK:STDOUT:   %MutableDeref.cpp_operator: %MutableDeref.cpp_operator.type = struct_value () [concrete]
+// CHECK:STDOUT:   %custom_witness.e20: <witness> = custom_witness (%i32, %MutableDeref.cpp_operator), @CppUnsafeDeref [concrete]
+// CHECK:STDOUT:   %CppUnsafeDeref.facet.3f6: %CppUnsafeDeref.type = facet_value %MutableDeref, (%custom_witness.e20) [concrete]
+// CHECK:STDOUT:   %CppUnsafeDeref.WithSelf.Op.type.cfd: type = fn_type @CppUnsafeDeref.WithSelf.Op, @CppUnsafeDeref.WithSelf(%CppUnsafeDeref.facet.3f6) [concrete]
+// CHECK:STDOUT:   %.c6d: type = fn_type_with_self_type %CppUnsafeDeref.WithSelf.Op.type.cfd, %CppUnsafeDeref.facet.3f6 [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT:   %Core: <namespace> = namespace file.%Core.import, [concrete] {
+// CHECK:STDOUT:     .Int = %Core.Int
+// CHECK:STDOUT:     .CppUnsafeDeref = %Core.CppUnsafeDeref
+// CHECK:STDOUT:     import Core//prelude
+// CHECK:STDOUT:     import Core//prelude/...
+// CHECK:STDOUT:   }
+// CHECK:STDOUT:   %Core.Int: %Int.type = import_ref Core//prelude/types/int, Int, loaded [concrete = constants.%Int.generic]
+// CHECK:STDOUT:   %Core.CppUnsafeDeref: type = import_ref Core//prelude/operators/deref, CppUnsafeDeref, loaded [concrete = constants.%CppUnsafeDeref.type]
+// CHECK:STDOUT:   %Core.import_ref.83d: %CppUnsafeDeref.assoc_type = import_ref Core//prelude/operators/deref, loc{{\d+_\d+}}, loaded [concrete = constants.%assoc1]
+// CHECK:STDOUT:   %Core.import_ref.ad7: @CppUnsafeDeref.WithSelf.%CppUnsafeDeref.WithSelf.Op.type (%CppUnsafeDeref.WithSelf.Op.type.792) = import_ref Core//prelude/operators/deref, loc{{\d+_\d+}}, loaded [symbolic = @CppUnsafeDeref.WithSelf.%CppUnsafeDeref.WithSelf.Op (constants.%CppUnsafeDeref.WithSelf.Op.a80)]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @TestDerefPass(%int_ptr.param: ref %MutableDeref) -> ref %i32 {
+// CHECK:STDOUT: !entry:
+// CHECK:STDOUT:   %int_ptr.ref: ref %MutableDeref = name_ref int_ptr, %int_ptr
+// CHECK:STDOUT:   %Core.ref: <namespace> = name_ref Core, imports.%Core [concrete = imports.%Core]
+// CHECK:STDOUT:   %CppUnsafeDeref.ref: type = name_ref CppUnsafeDeref, imports.%Core.CppUnsafeDeref [concrete = constants.%CppUnsafeDeref.type]
+// CHECK:STDOUT:   %Op.ref: %CppUnsafeDeref.assoc_type = name_ref Op, imports.%Core.import_ref.83d [concrete = constants.%assoc1]
+// CHECK:STDOUT:   %impl_witness_assoc_constant: type = impl_witness_assoc_constant @MutableDeref.cpp_operator.%i32.2 [concrete = constants.%i32]
+// CHECK:STDOUT:   %impl.elem1: %.c6d = impl_witness_access constants.%custom_witness.e20, element1 [concrete = constants.%MutableDeref.cpp_operator]
+// CHECK:STDOUT:   %bound_method: <bound method> = bound_method %int_ptr.ref, %impl.elem1
+// CHECK:STDOUT:   %MutableDeref.cpp_operator.call: ref %i32 = call %bound_method(%int_ptr.ref)
+// CHECK:STDOUT:   return %MutableDeref.cpp_operator.call
+// CHECK:STDOUT: }
+// CHECK:STDOUT: