Fix accessing members of const/partial types (#7406)

In `PerformActionHelper`, use the unqualified type for lookup.

In `PerformInstanceBinding`, propagate qualifiers to the unbound element
type's class type when doing the `ConvertToValueOrRefOfType` conversion,
and to the element type when forming the `ClassElementAccess` instr
(except for `partial`, which is only used if the member being accessed
is `base`).

In handle_operator.cpp, prevent assignment to a reference to a const
type.

toolchain/check/handle_operator.cpp

@@ -104,9 +104,13 @@ auto HandleParseNode(Context& context, Parse::InfixOperatorEqualId node_id)
 
   auto [rhs_node, rhs_id] = context.node_stack().PopExprWithNodeId();
   auto [lhs_node, lhs_id] = context.node_stack().PopExprWithNodeId();
+  auto lhs_type_id = context.insts().Get(lhs_id).type_id();
+  auto lhs_quals =
+      context.types().GetUnqualifiedTypeAndQualifiers(lhs_type_id).second;
   if (auto lhs_cat = SemIR::GetExprCategory(context.sem_ir(), lhs_id);
-      lhs_cat != SemIR::ExprCategory::DurableRef &&
-      lhs_cat != SemIR::ExprCategory::Error) {
+      (lhs_cat != SemIR::ExprCategory::DurableRef &&
+       lhs_cat != SemIR::ExprCategory::Error) ||
+      lhs_quals.HasAnyOf(SemIR::TypeQualifiers::Const)) {
     CARBON_DIAGNOSTIC(AssignmentToNonAssignable, Error,
                       "expression is not assignable");
     context.emitter().Emit(lhs_node, AssignmentToNonAssignable);

toolchain/check/member_access.cpp

@@ -406,11 +406,18 @@ static auto PerformInstanceBinding(Context& context, SemIR::LocId loc_id,
   if (auto unbound_element_type =
           context.types().TryGetAs<SemIR::UnboundElementType>(
               context.insts().Get(member_id).type_id())) {
+    auto base_type_id = context.insts().Get(base_id).type_id();
+    auto [unqualified_base_type_id, qualifiers] =
+        context.types().GetUnqualifiedTypeAndQualifiers(base_type_id);
+
     // Convert the base to the type of the element if necessary.
-    base_id = ConvertToValueOrRefOfType(
-        context, loc_id, base_id,
-        context.types().GetTypeIdForTypeInstId(
-            unbound_element_type->class_type_inst_id));
+    auto element_class_type_id =
+        GetQualifiedType(context,
+                         context.types().GetTypeIdForTypeInstId(
+                             unbound_element_type->class_type_inst_id),
+                         qualifiers);
+    base_id = ConvertToValueOrRefOfType(context, loc_id, base_id,
+                                        element_class_type_id);
 
     // Find the specified element, which could be either a field or a base
     // class, and build an element access expression.
@@ -419,12 +426,18 @@ static auto PerformInstanceBinding(Context& context, SemIR::LocId loc_id,
                  "Non-constant value {0} of unbound element type",
                  context.insts().Get(member_id));
     auto index = GetClassElementIndex(context, element_id);
+    // Only propagate the `partial` qualifier to the `base` field.
+    if (!context.insts().Is<SemIR::BaseDecl>(element_id)) {
+      qualifiers.Remove(SemIR::TypeQualifiers::Partial);
+    }
+    auto access_type_id =
+        GetQualifiedType(context,
+                         context.types().GetTypeIdForTypeInstId(
+                             unbound_element_type->element_type_inst_id),
+                         qualifiers);
     auto access_id = GetOrAddInst<SemIR::ClassElementAccess>(
         context, loc_id,
-        {.type_id = context.types().GetTypeIdForTypeInstId(
-             unbound_element_type->element_type_inst_id),
-         .base_id = base_id,
-         .index = index});
+        {.type_id = access_type_id, .base_id = base_id, .index = index});
     if (SemIR::GetExprCategory(context.sem_ir(), base_id) ==
             SemIR::ExprCategory::Value &&
         SemIR::GetExprCategory(context.sem_ir(), access_id) !=
@@ -582,7 +595,11 @@ static auto PerformActionHelper(Context& context, SemIR::LocId loc_id,
   base_id = ConvertToValueOrRefExpr(context, base_id);
   base_type_id = context.insts().Get(base_id).type_id();
 
-  auto lookup_const_id = context.types().GetConstantId(base_type_id);
+  // If the type has qualifiers, use the unqualified type for lookup.
+  auto unqualified_base_type_id =
+      context.types().GetUnqualifiedTypeAndQualifiers(base_type_id).first;
+  auto lookup_const_id =
+      context.types().GetConstantId(unqualified_base_type_id);
 
   // TODO: If the type is a facet, we look through it into the facet's type (a
   // FacetType) for names. According to the design, we shouldn't need to do

toolchain/check/testdata/class/partial/access.carbon

@@ -0,0 +1,47 @@
+// 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/int.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/access.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/class/partial/access.carbon
+
+// --- common.carbon
+package Common;
+
+base class A {
+    var n: i32;
+    virtual fn F(self);
+}
+
+base class B {
+    extend base: A;
+}
+
+// --- partial_member_access.carbon
+library "[[@TEST_NAME]]";
+import Common;
+
+fn F(ref pb: partial Common.B) {
+    let ref _: i32 = pb.n;
+    let ref _: partial Common.A = pb.base;
+}
+
+// --- fail_invalid_partial_member_access.carbon
+library "[[@TEST_NAME]]";
+import Common;
+
+fn F(ref pb: partial Common.B) {
+    // CHECK:STDERR: fail_invalid_partial_member_access.carbon:[[@LINE+7]]:27: error: cannot implicitly convert expression of type `partial Common.A` to `Common.A` [ConversionFailure]
+    // CHECK:STDERR:     let ref _: Common.A = pb.base;
+    // CHECK:STDERR:                           ^~~~~~~
+    // CHECK:STDERR: fail_invalid_partial_member_access.carbon:[[@LINE+4]]:27: note: type `partial Common.A` does not implement interface `Core.ImplicitAs(Common.A)` [MissingImplInMemberAccessInContext]
+    // CHECK:STDERR:     let ref _: Common.A = pb.base;
+    // CHECK:STDERR:                           ^~~~~~~
+    // CHECK:STDERR:
+    let ref _: Common.A = pb.base;
+}

toolchain/check/testdata/const/access.carbon

@@ -0,0 +1,52 @@
+// 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/int.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/const/access.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/const/access.carbon
+
+// --- const_member_access.carbon
+library "[[@TEST_NAME]]";
+
+class C {
+  var x: ();
+}
+
+fn F(c: const C) {
+  c.x;
+}
+
+// --- fail_const_member_assign.carbon
+library "[[@TEST_NAME]]";
+
+class C {
+  var x: ();
+}
+
+fn F(c: const C) {
+  // CHECK:STDERR: fail_const_member_assign.carbon:[[@LINE+4]]:3: error: expression is not assignable [AssignmentToNonAssignable]
+  // CHECK:STDERR:   c.x = ();
+  // CHECK:STDERR:   ^~~
+  // CHECK:STDERR:
+  c.x = ();
+}
+
+// --- fail_const_ref_member_assign.carbon
+library "[[@TEST_NAME]]";
+
+class C {
+  var x: ();
+}
+
+fn F(ref c: const C) {
+  // CHECK:STDERR: fail_const_ref_member_assign.carbon:[[@LINE+4]]:3: error: expression is not assignable [AssignmentToNonAssignable]
+  // CHECK:STDERR:   c.x = ();
+  // CHECK:STDERR:   ^~~
+  // CHECK:STDERR:
+  c.x = ();
+}

toolchain/check/testdata/interop/cpp/const/access.carbon

@@ -0,0 +1,71 @@
+// 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/int.carbon
+//
+// AUTOUPDATE
+// TIP: To test this file alone, run:
+// TIP:   bazel test //toolchain/testing:file_test --test_arg=--file_tests=toolchain/check/testdata/interop/cpp/const/access.carbon
+// TIP: To dump output, run:
+// TIP:   bazel run //toolchain/testing:file_test -- --dump_output --file_tests=toolchain/check/testdata/interop/cpp/const/access.carbon
+
+// --- const_member_access.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+struct S {
+  int x;
+};
+
+const S& Get();
+''';
+
+fn F() {
+  Cpp.Get().x;
+}
+
+// --- fail_const_member_assign.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+struct S {
+  int x;
+};
+
+const S& Get();
+''';
+
+fn F() {
+  // CHECK:STDERR: fail_const_member_assign.carbon:[[@LINE+11]]:3: error: expression is not assignable [AssignmentToNonAssignable]
+  // CHECK:STDERR:   Cpp.Get().x = 123;
+  // CHECK:STDERR:   ^~~~~~~~~~~
+  // CHECK:STDERR:
+  // CHECK:STDERR: fail_const_member_assign.carbon:[[@LINE+7]]:3: error: cannot implicitly convert expression of type `Core.IntLiteral` to `const i32` [ConversionFailure]
+  // CHECK:STDERR:   Cpp.Get().x = 123;
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~
+  // CHECK:STDERR: fail_const_member_assign.carbon:[[@LINE+4]]:3: note: type `Core.IntLiteral` does not implement interface `Core.ImplicitAs(const i32)` [MissingImplInMemberAccessInContext]
+  // CHECK:STDERR:   Cpp.Get().x = 123;
+  // CHECK:STDERR:   ^~~~~~~~~~~~~~~~~
+  // CHECK:STDERR:
+  Cpp.Get().x = 123;
+}
+
+// --- fail_const_member_assign_cast.carbon
+library "[[@TEST_NAME]]";
+
+import Cpp inline '''
+struct S {
+  int x;
+};
+
+const S& Get();
+''';
+
+fn F() {
+  // CHECK:STDERR: fail_const_member_assign_cast.carbon:[[@LINE+4]]:3: error: expression is not assignable [AssignmentToNonAssignable]
+  // CHECK:STDERR:   Cpp.Get().x = 123 as i32;
+  // CHECK:STDERR:   ^~~~~~~~~~~
+  // CHECK:STDERR:
+  Cpp.Get().x = 123 as i32;
+}