Basic SemIR `partial` support (#5736)

This adds something similar to the level of `const` support - that it's
a type, but not the conversions and limitations on usage that are
needed.

---------

Co-authored-by: Chandler Carruth <chandlerc@gmail.com>

toolchain/check/eval_inst.cpp

@@ -167,6 +167,11 @@ auto EvalConstantInst(Context& context, SemIR::ConstType inst)
   return ConstantEvalResult::NewSamePhase(inst);
 }
 
+auto EvalConstantInst(Context& /*context*/, SemIR::PartialType inst)
+    -> ConstantEvalResult {
+  return ConstantEvalResult::NewSamePhase(inst);
+}
+
 auto EvalConstantInst(Context& context, SemIR::Converted inst)
     -> ConstantEvalResult {
   // A conversion evaluates to the result of the conversion.

toolchain/check/handle_operator.cpp

@@ -312,7 +312,13 @@ auto HandleParseNode(Context& context, Parse::PrefixOperatorNotId node_id)
 
 auto HandleParseNode(Context& context, Parse::PrefixOperatorPartialId node_id)
     -> bool {
-  return context.TODO(node_id, "partial operator");
+  auto value_id = context.node_stack().PopExpr();
+  auto inner_type = ExprAsType(context, node_id, value_id);
+  // TODO: Add diagnostics for partial applied to non-base/abstract types.
+  AddInstAndPush<SemIR::PartialType>(
+      context, node_id,
+      {.type_id = SemIR::TypeType::TypeId, .inner_id = inner_type.inst_id});
+  return true;
 }
 
 auto HandleParseNode(Context& context, Parse::PrefixOperatorPlusPlusId node_id)

toolchain/check/testdata/class/partial.carbon

@@ -0,0 +1,344 @@
+// 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/class/partial.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/class/partial.carbon
+
+// --- base.carbon
+library "[[@TEST_NAME]]";
+
+base class C { }
+
+//@dump-sem-ir-begin
+fn A(p: partial C);
+//@dump-sem-ir-end
+
+// --- abstract.carbon
+library "[[@TEST_NAME]]";
+
+abstract class C { }
+
+//@dump-sem-ir-begin
+fn A(p: partial C);
+//@dump-sem-ir-end
+
+// --- todo_fail_partial_nondynamic.carbon
+library "[[@TEST_NAME]]";
+
+class C { }
+
+//@dump-sem-ir-begin
+fn G(p: partial C);
+//@dump-sem-ir-end
+
+// --- todo_fail_partial_final.carbon
+library "[[@TEST_NAME]]";
+
+base class Base { }
+class Derived {
+  extend base: Base;
+}
+
+//@dump-sem-ir-begin
+fn G(p: partial Derived);
+//@dump-sem-ir-end
+
+// --- todo_fail_partial_decl.carbon
+library "[[@TEST_NAME]]";
+
+class C;
+
+//@dump-sem-ir-begin
+fn G(p: partial C);
+//@dump-sem-ir-end
+
+// --- todo_fail_partial_tuple.carbon
+library "[[@TEST_NAME]]";
+
+class C;
+
+//@dump-sem-ir-begin
+fn G(p: partial (C, C));
+//@dump-sem-ir-end
+
+// --- todo_fail_partial_struct.carbon
+library "[[@TEST_NAME]]";
+
+class C;
+
+//@dump-sem-ir-begin
+fn G(p: partial {.x: C});
+//@dump-sem-ir-end
+
+// --- todo_fail_duplicate.carbon
+library "[[@TEST_NAME]]";
+
+base class C { }
+
+//@dump-sem-ir-begin
+fn F(p: partial (partial C));
+//@dump-sem-ir-end
+
+// --- fail_convert_to_nonpartial.carbon
+library "[[@TEST_NAME]]";
+
+base class C { }
+
+fn G(p: partial C*) -> C* {
+  // CHECK:STDERR: fail_convert_to_nonpartial.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `partial C*` to `C*` [ConversionFailure]
+  // CHECK:STDERR:   return p;
+  // CHECK:STDERR:   ^~~~~~~~~
+  // CHECK:STDERR: fail_convert_to_nonpartial.carbon:[[@LINE+4]]:3: note: type `partial C*` does not implement interface `Core.ImplicitAs(C*)` [MissingImplInMemberAccessNote]
+  // CHECK:STDERR:   return p;
+  // CHECK:STDERR:   ^~~~~~~~~
+  // CHECK:STDERR:
+  return p;
+}
+
+// CHECK:STDOUT: --- base.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %C: type = class_type @C [concrete]
+// CHECK:STDOUT:   %.43f: type = partial_type %C [concrete]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %.43f [concrete]
+// CHECK:STDOUT:   %A.type: type = fn_type @A [concrete]
+// CHECK:STDOUT:   %A: %A.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   %A.decl: %A.type = fn_decl @A [concrete = constants.%A] {
+// CHECK:STDOUT:     %p.patt: %pattern_type = binding_pattern p [concrete]
+// CHECK:STDOUT:     %p.param_patt: %pattern_type = value_param_pattern %p.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %p.param: %.43f = value_param call_param0
+// CHECK:STDOUT:     %.loc6_9.1: type = splice_block %.loc6_9.2 [concrete = constants.%.43f] {
+// CHECK:STDOUT:       %C.ref: type = name_ref C, file.%C.decl [concrete = constants.%C]
+// CHECK:STDOUT:       %.loc6_9.2: type = partial_type %C.ref [concrete = constants.%.43f]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %p: %.43f = bind_name p, %p.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @A(%p.param: %.43f);
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- abstract.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %C: type = class_type @C [concrete]
+// CHECK:STDOUT:   %.43f: type = partial_type %C [concrete]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %.43f [concrete]
+// CHECK:STDOUT:   %A.type: type = fn_type @A [concrete]
+// CHECK:STDOUT:   %A: %A.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   %A.decl: %A.type = fn_decl @A [concrete = constants.%A] {
+// CHECK:STDOUT:     %p.patt: %pattern_type = binding_pattern p [concrete]
+// CHECK:STDOUT:     %p.param_patt: %pattern_type = value_param_pattern %p.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %p.param: %.43f = value_param call_param0
+// CHECK:STDOUT:     %.loc6_9.1: type = splice_block %.loc6_9.2 [concrete = constants.%.43f] {
+// CHECK:STDOUT:       %C.ref: type = name_ref C, file.%C.decl [concrete = constants.%C]
+// CHECK:STDOUT:       %.loc6_9.2: type = partial_type %C.ref [concrete = constants.%.43f]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %p: %.43f = bind_name p, %p.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @A(%p.param: %.43f);
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- todo_fail_partial_nondynamic.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %C: type = class_type @C [concrete]
+// CHECK:STDOUT:   %.43f: type = partial_type %C [concrete]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %.43f [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:   %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {
+// CHECK:STDOUT:     %p.patt: %pattern_type = binding_pattern p [concrete]
+// CHECK:STDOUT:     %p.param_patt: %pattern_type = value_param_pattern %p.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %p.param: %.43f = value_param call_param0
+// CHECK:STDOUT:     %.loc6_9.1: type = splice_block %.loc6_9.2 [concrete = constants.%.43f] {
+// CHECK:STDOUT:       %C.ref: type = name_ref C, file.%C.decl [concrete = constants.%C]
+// CHECK:STDOUT:       %.loc6_9.2: type = partial_type %C.ref [concrete = constants.%.43f]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %p: %.43f = bind_name p, %p.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @G(%p.param: %.43f);
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- todo_fail_partial_final.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %Derived: type = class_type @Derived [concrete]
+// CHECK:STDOUT:   %.604: type = partial_type %Derived [concrete]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %.604 [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:   %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {
+// CHECK:STDOUT:     %p.patt: %pattern_type = binding_pattern p [concrete]
+// CHECK:STDOUT:     %p.param_patt: %pattern_type = value_param_pattern %p.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %p.param: %.604 = value_param call_param0
+// CHECK:STDOUT:     %.loc9_9.1: type = splice_block %.loc9_9.2 [concrete = constants.%.604] {
+// CHECK:STDOUT:       %Derived.ref: type = name_ref Derived, file.%Derived.decl [concrete = constants.%Derived]
+// CHECK:STDOUT:       %.loc9_9.2: type = partial_type %Derived.ref [concrete = constants.%.604]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %p: %.604 = bind_name p, %p.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @G(%p.param: %.604);
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- todo_fail_partial_decl.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %C: type = class_type @C [concrete]
+// CHECK:STDOUT:   %.43f: type = partial_type %C [concrete]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %.43f [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:   %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {
+// CHECK:STDOUT:     %p.patt: %pattern_type = binding_pattern p [concrete]
+// CHECK:STDOUT:     %p.param_patt: %pattern_type = value_param_pattern %p.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %p.param: %.43f = value_param call_param0
+// CHECK:STDOUT:     %.loc6_9.1: type = splice_block %.loc6_9.2 [concrete = constants.%.43f] {
+// CHECK:STDOUT:       %C.ref: type = name_ref C, file.%C.decl [concrete = constants.%C]
+// CHECK:STDOUT:       %.loc6_9.2: type = partial_type %C.ref [concrete = constants.%.43f]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %p: %.43f = bind_name p, %p.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @G(%p.param: %.43f);
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- todo_fail_partial_tuple.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %C: type = class_type @C [concrete]
+// CHECK:STDOUT:   %tuple.type.24b: type = tuple_type (type, type) [concrete]
+// CHECK:STDOUT:   %tuple.type.56b: type = tuple_type (%C, %C) [concrete]
+// CHECK:STDOUT:   %.62c: type = partial_type %tuple.type.56b [concrete]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %.62c [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:   %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {
+// CHECK:STDOUT:     %p.patt: %pattern_type = binding_pattern p [concrete]
+// CHECK:STDOUT:     %p.param_patt: %pattern_type = value_param_pattern %p.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %p.param: %.62c = value_param call_param0
+// CHECK:STDOUT:     %.loc6_9.1: type = splice_block %.loc6_9.3 [concrete = constants.%.62c] {
+// CHECK:STDOUT:       %C.ref.loc6_18: type = name_ref C, file.%C.decl [concrete = constants.%C]
+// CHECK:STDOUT:       %C.ref.loc6_21: type = name_ref C, file.%C.decl [concrete = constants.%C]
+// CHECK:STDOUT:       %.loc6_22: %tuple.type.24b = tuple_literal (%C.ref.loc6_18, %C.ref.loc6_21)
+// CHECK:STDOUT:       %.loc6_9.2: type = converted %.loc6_22, constants.%tuple.type.56b [concrete = constants.%tuple.type.56b]
+// CHECK:STDOUT:       %.loc6_9.3: type = partial_type %.loc6_9.2 [concrete = constants.%.62c]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %p: %.62c = bind_name p, %p.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @G(%p.param: %.62c);
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- todo_fail_partial_struct.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %C: type = class_type @C [concrete]
+// CHECK:STDOUT:   %struct_type.x: type = struct_type {.x: %C} [concrete]
+// CHECK:STDOUT:   %.1ed: type = partial_type %struct_type.x [concrete]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %.1ed [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:   %G.decl: %G.type = fn_decl @G [concrete = constants.%G] {
+// CHECK:STDOUT:     %p.patt: %pattern_type = binding_pattern p [concrete]
+// CHECK:STDOUT:     %p.param_patt: %pattern_type = value_param_pattern %p.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %p.param: %.1ed = value_param call_param0
+// CHECK:STDOUT:     %.loc6_9.1: type = splice_block %.loc6_9.2 [concrete = constants.%.1ed] {
+// CHECK:STDOUT:       %C.ref: type = name_ref C, file.%C.decl [concrete = constants.%C]
+// CHECK:STDOUT:       %struct_type.x: type = struct_type {.x: %C} [concrete = constants.%struct_type.x]
+// CHECK:STDOUT:       %.loc6_9.2: type = partial_type %struct_type.x [concrete = constants.%.1ed]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %p: %.1ed = bind_name p, %p.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @G(%p.param: %.1ed);
+// CHECK:STDOUT:
+// CHECK:STDOUT: --- todo_fail_duplicate.carbon
+// CHECK:STDOUT:
+// CHECK:STDOUT: constants {
+// CHECK:STDOUT:   %C: type = class_type @C [concrete]
+// CHECK:STDOUT:   %.43f: type = partial_type %C [concrete]
+// CHECK:STDOUT:   %.a34: type = partial_type %.43f [concrete]
+// CHECK:STDOUT:   %pattern_type: type = pattern_type %.a34 [concrete]
+// CHECK:STDOUT:   %F.type: type = fn_type @F [concrete]
+// CHECK:STDOUT:   %F: %F.type = struct_value () [concrete]
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: imports {
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: file {
+// CHECK:STDOUT:   %F.decl: %F.type = fn_decl @F [concrete = constants.%F] {
+// CHECK:STDOUT:     %p.patt: %pattern_type = binding_pattern p [concrete]
+// CHECK:STDOUT:     %p.param_patt: %pattern_type = value_param_pattern %p.patt, call_param0 [concrete]
+// CHECK:STDOUT:   } {
+// CHECK:STDOUT:     %p.param: %.a34 = value_param call_param0
+// CHECK:STDOUT:     %.loc6_9.1: type = splice_block %.loc6_9.2 [concrete = constants.%.a34] {
+// CHECK:STDOUT:       %C.ref: type = name_ref C, file.%C.decl [concrete = constants.%C]
+// CHECK:STDOUT:       %.loc6_18: type = partial_type %C.ref [concrete = constants.%.43f]
+// CHECK:STDOUT:       %.loc6_9.2: type = partial_type %.loc6_18 [concrete = constants.%.a34]
+// CHECK:STDOUT:     }
+// CHECK:STDOUT:     %p: %.a34 = bind_name p, %p.param
+// CHECK:STDOUT:   }
+// CHECK:STDOUT: }
+// CHECK:STDOUT:
+// CHECK:STDOUT: fn @F(%p.param: %.a34);
+// CHECK:STDOUT:

toolchain/check/type_completion.cpp

@@ -140,6 +140,10 @@ class TypeCompleter {
   auto BuildInfoForInst(SemIR::TypeId /*type_id*/, SemIR::ConstType inst) const
       -> SemIR::CompleteTypeInfo;
 
+  auto BuildInfoForInst(SemIR::TypeId /*type_id*/,
+                        SemIR::PartialType inst) const
+      -> SemIR::CompleteTypeInfo;
+
   auto BuildInfoForInst(SemIR::TypeId /*type_id*/,
                         SemIR::ImplWitnessAssociatedConstant inst) const
       -> SemIR::CompleteTypeInfo;
@@ -290,6 +294,10 @@ auto TypeCompleter::AddNestedIncompleteTypes(SemIR::Inst type_inst) -> bool {
       Push(context_->types().GetTypeIdForTypeInstId(inst.inner_id));
       break;
     }
+    case CARBON_KIND(SemIR::PartialType inst): {
+      Push(context_->types().GetTypeIdForTypeInstId(inst.inner_id));
+      break;
+    }
     case CARBON_KIND(SemIR::FacetType inst): {
       auto identified_id = RequireIdentifiedFacetType(*context_, inst);
       const auto& identified =
@@ -512,6 +520,14 @@ auto TypeCompleter::BuildInfoForInst(SemIR::TypeId /*type_id*/,
   return GetNestedInfo(context_->types().GetTypeIdForTypeInstId(inst.inner_id));
 }
 
+auto TypeCompleter::BuildInfoForInst(SemIR::TypeId /*type_id*/,
+                                     SemIR::PartialType inst) const
+    -> SemIR::CompleteTypeInfo {
+  // The value representation of `partial T` is the same as that of `T`.
+  // Objects are not modifiable through their value representations.
+  return GetNestedInfo(context_->types().GetTypeIdForTypeInstId(inst.inner_id));
+}
+
 auto TypeCompleter::BuildInfoForInst(
     SemIR::TypeId /*type_id*/, SemIR::ImplWitnessAssociatedConstant inst) const
     -> SemIR::CompleteTypeInfo {

toolchain/lower/file_context.cpp

@@ -760,6 +760,12 @@ static auto BuildTypeForInst(FileContext& context, SemIR::ConstType inst)
       context.sem_ir().types().GetTypeIdForTypeInstId(inst.inner_id));
 }
 
+static auto BuildTypeForInst(FileContext& context, SemIR::PartialType inst)
+    -> llvm::Type* {
+  return context.GetType(
+      context.sem_ir().types().GetTypeIdForTypeInstId(inst.inner_id));
+}
+
 static auto BuildTypeForInst(FileContext& context,
                              SemIR::ImplWitnessAssociatedConstant inst)
     -> llvm::Type* {

toolchain/sem_ir/expr_info.cpp

@@ -134,6 +134,7 @@ auto GetExprCategory(const File& file, InstId inst_id) -> ExprCategory {
       case InterfaceDecl::Kind:
       case LegacyFloatType::Kind:
       case NamespaceType::Kind:
+      case PartialType::Kind:
       case PatternType::Kind:
       case PointerType::Kind:
       case RefineTypeAction::Kind:

toolchain/sem_ir/inst_kind.def

@@ -94,6 +94,7 @@ CARBON_SEM_IR_INST_KIND(Namespace)
 CARBON_SEM_IR_INST_KIND(NamespaceType)
 CARBON_SEM_IR_INST_KIND(OutParam)
 CARBON_SEM_IR_INST_KIND(OutParamPattern)
+CARBON_SEM_IR_INST_KIND(PartialType)
 CARBON_SEM_IR_INST_KIND(PatternType)
 CARBON_SEM_IR_INST_KIND(PointerType)
 CARBON_SEM_IR_INST_KIND(RefParam)

toolchain/sem_ir/stringify.cpp

@@ -514,6 +514,12 @@ class Stringifier {
                                    name_scope.name_id());
   }
 
+  auto StringifyInst(InstId /*inst_id*/, PartialType inst) -> void {
+    *out_ << "partial ";
+
+    step_stack_->PushInstId(inst.inner_id);
+  }
+
   auto StringifyInst(InstId /*inst_id*/, PatternType inst) -> void {
     *out_ << "<pattern for ";
     step_stack_->Push(inst.scrutinee_type_inst_id, ">");

toolchain/sem_ir/type.cpp

@@ -77,6 +77,7 @@ auto TypeStore::GetUnqualifiedType(TypeId type_id) const -> TypeId {
   if (auto const_type = TryGetAs<ConstType>(type_id)) {
     return file_->types().GetTypeIdForTypeInstId(const_type->inner_id);
   }
+  // TODO: Look through PartialType when this is reachable/testable
   return type_id;
 }
 

toolchain/sem_ir/type_iterator.cpp

@@ -127,6 +127,12 @@ auto TypeIterator::Next() -> Step {
         PushInstId(const_type.inner_id);
         break;
       }
+      case CARBON_KIND(SemIR::PartialType partial_type): {
+        // We don't stop at `partial` since it is a modifier; just move to the
+        // inner type.
+        PushInstId(partial_type.inner_id);
+        break;
+      }
       case CARBON_KIND(SemIR::ImplWitnessAssociatedConstant assoc): {
         Push(assoc.type_id);
         break;

toolchain/sem_ir/typed_insts.h

@@ -512,6 +512,18 @@ struct ConstType {
   TypeInstId inner_id;
 };
 
+struct PartialType {
+  static constexpr auto Kind =
+      InstKind::PartialType.Define<Parse::PrefixOperatorPartialId>(
+          {.ir_name = "partial_type",
+           .is_type = InstIsType::Always,
+           .constant_kind = InstConstantKind::Conditional,
+           .deduce_through = true});
+
+  TypeId type_id;
+  TypeInstId inner_id;
+};
+
 // Records that a type conversion `original as new_type` was done, producing the
 // result.
 struct Converted {