Explorer: basic `abstract` class support (#2441)

Relates to #1881 

Features:
* Add basic support for `abstract` class
    * Allow extending an abstract class
    * Prevent direct instantiation of an abstract class
Changes:
* Check that class extensibility for `VariableDefinition` is not `Abstract`
* Add corresponding set of lit tests

explorer/interpreter/interpreter.cpp

@@ -1777,6 +1777,15 @@ auto Interpreter::StepStmt() -> ErrorOr<Success> {
     }
     case StatementKind::VariableDefinition: {
       const auto& definition = cast<VariableDefinition>(stmt);
+      const auto* dest_type = &definition.pattern().static_type();
+      if (const auto* dest_class = dyn_cast<NominalClassType>(dest_type)) {
+        if (dest_class->declaration().extensibility() ==
+            ClassExtensibility::Abstract) {
+          return ProgramError(stmt.source_loc())
+                 << "Cannot instantiate abstract class "
+                 << dest_class->declaration().name();
+        }
+      }
       if (act.pos() == 0 && definition.has_init()) {
         //    { {(var x = e) :: C, E, F} :: S, H}
         // -> { {e :: (var x = []) :: C, E, F} :: S, H}
@@ -1790,8 +1799,7 @@ auto Interpreter::StepStmt() -> ErrorOr<Success> {
         Nonnull<const Value*> v;
         if (definition.has_init()) {
           CARBON_ASSIGN_OR_RETURN(
-              v, Convert(act.results()[0], &definition.pattern().static_type(),
-                         stmt.source_loc()));
+              v, Convert(act.results()[0], dest_type, stmt.source_loc()));
         } else {
           v = arena_->New<UninitializedValue>(p);
         }

explorer/interpreter/type_checker.cpp

@@ -4504,11 +4504,6 @@ auto TypeChecker::DeclareClassDeclaration(Nonnull<ClassDeclaration*> class_decl,
   ImplScope class_scope;
   class_scope.AddParent(scope_info.innermost_scope);
 
-  if (class_decl->extensibility() == ClassExtensibility::Abstract) {
-    return ProgramError(class_decl->source_loc())
-           << "Class prefix `abstract` is not supported yet";
-  }
-
   std::optional<Nonnull<const NominalClassType*>> base_class;
   if (class_decl->base_expr().has_value()) {
     Nonnull<Expression*> base_class_expr = *class_decl->base_expr();

explorer/testdata/class/abstract_class.carbon

@@ -0,0 +1,27 @@
+// 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
+//
+// AUTOUPDATE
+// RUN: %{explorer-run}
+// RUN: %{explorer-run-trace}
+// CHECK:STDOUT: d.a=1
+// CHECK:STDOUT: d.b=2
+// CHECK:STDOUT: result: 0
+
+package ExplorerTest api;
+
+abstract class C {
+  var a: i32;
+}
+
+class D extends C {
+  var b: i32;
+}
+
+fn Main() -> i32 {
+  var d: D = { .base = {.a = 1}, .b = 2 };
+  Print("d.a={0}", d.a);
+  Print("d.b={0}", d.b);
+  return 0;
+}

explorer/testdata/class/fail_abstract_class_subtyping.carbon

@@ -0,0 +1,25 @@
+// 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
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package ExplorerTest api;
+
+abstract class C {
+  var a: i32;
+}
+
+class D extends C {
+  var b: i32;
+}
+
+fn Main() -> i32 {
+  var d: D = { .base = {.a = 1}, .b = 2 };
+  // Not supported yet.
+  // CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_abstract_class_subtyping.carbon:[[@LINE+1]]: type error in name binding: 'class D*' is not implicitly convertible to 'class C*'
+  var c: C* = &d;
+  return 0;
+}

explorer/testdata/class/fail_instantiate_abstract_class_constructor.carbon

@@ -0,0 +1,23 @@
+// 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
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package ExplorerTest api;
+
+abstract class C {
+  // TODO: Returning `Self` for an abstract class should error: `partial Self` should be used instead. This should be updated when `partial` is implemented.
+  fn Create() -> Self {
+    return { .a = 1 };
+  }
+  var a: i32;
+}
+
+fn Main() -> i32 {
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/class/fail_instantiate_abstract_class_constructor.carbon:[[@LINE+1]]: Cannot instantiate abstract class C
+  var c: C = C.Create();
+  return 0;
+}

explorer/testdata/class/fail_abstract_class.carbon → explorer/testdata/class/fail_instantiate_abstract_class_empty.carbon

@@ -9,10 +9,11 @@
 package ExplorerTest api;
 
 abstract class C {
-  fn F() {}
-// CHECK:STDERR: COMPILATION ERROR: {{.*}}/explorer/testdata/class/fail_abstract_class.carbon:[[@LINE+1]]: Class prefix `abstract` is not supported yet
+  var a: i32;
 }
 
 fn Main() -> i32 {
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/class/fail_instantiate_abstract_class_empty.carbon:[[@LINE+1]]: Cannot instantiate abstract class C
+  var c: C;
   return 0;
 }

explorer/testdata/class/fail_instantiate_abstract_class_struct.carbon

@@ -0,0 +1,19 @@
+// 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
+//
+// AUTOUPDATE
+// RUN: %{not} %{explorer-run}
+// RUN: %{not} %{explorer-run-trace}
+
+package ExplorerTest api;
+
+abstract class C {
+  var a: i32;
+}
+
+fn Main() -> i32 {
+  // CHECK:STDERR: RUNTIME ERROR: {{.*}}/explorer/testdata/class/fail_instantiate_abstract_class_struct.carbon:[[@LINE+1]]: Cannot instantiate abstract class C
+  var c: C = { .a = 1 };
+  return 0;
+}