Support optional abstract prefix on classes for TypeScript compatibility

Author: popham

src/parser/object_parser.ml

@@ -784,6 +784,7 @@ module Object
     let env = env |> with_strict true in
     let start_loc = Peek.loc env in
     let decorators = decorators @ (decorator_list env) in
+    ignore (Expect.maybe env T_ABSTRACT);
     Expect.token env T_CLASS;
     let tmp_env = env |> with_no_let true in
     let id = (

src/parser/parser_env.ml

@@ -681,6 +681,7 @@ module Peek = struct
   let is_class env =
     match token env with
     | T_CLASS
+    | T_ABSTRACT
     | T_AT -> true
     | _ -> false
 end

src/parser/parser_flow.ml

@@ -195,7 +195,6 @@ module rec Parse : PARSER = struct
     | T_CASE
     | T_DEFAULT
     | T_EXTENDS
-    | T_ABSTRACT (*TJP: I may retract this if I end up adding support for the `abstract` prefix on `class`.*)
     | T_STATIC
     | T_EXPORT (* TODO *)
     | T_ELLIPSIS ->

src/parser/statement_parser.ml

@@ -669,6 +669,7 @@ module Statement
 
   and declare_class_statement env = with_loc (fun env ->
     Expect.token env T_DECLARE;
+    ignore (Expect.maybe env T_ABSTRACT);
     let fn = declare_class env in
     Statement.DeclareClass fn
   ) env
@@ -837,6 +838,7 @@ module Statement
     then error env Error.UnexpectedTypeDeclaration;
     (* eventually, just emit a wrapper AST node *)
     (match Peek.ith_token ~i:1 env with
+      | T_ABSTRACT
       | T_CLASS ->
           declare_class_statement env
       | T_INTERFACE ->

src/parser/test/flow/abstract_class/class_prefix.js

@@ -0,0 +1,2 @@
+abstract class PrefixedClass {}
+declare abstract class PrefixedAbstractClass {}

src/parser/test/flow/abstract_class/class_prefix.tree.json

@@ -0,0 +1,56 @@
+{
+  "type":"Program",
+  "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":2,"column":47}},
+  "range":[0,79],
+  "body":[
+    {
+      "type":"ClassDeclaration",
+      "loc":{"source":null,"start":{"line":1,"column":0},"end":{"line":1,"column":31}},
+      "range":[0,31],
+      "id":{
+        "type":"Identifier",
+        "loc":{"source":null,"start":{"line":1,"column":15},"end":{"line":1,"column":28}},
+        "range":[15,28],
+        "name":"PrefixedClass",
+        "typeAnnotation":null,
+        "optional":false
+      },
+      "body":{
+        "type":"ClassBody",
+        "loc":{"source":null,"start":{"line":1,"column":29},"end":{"line":1,"column":31}},
+        "range":[29,31],
+        "body":[]
+      },
+      "superClass":null,
+      "typeParameters":null,
+      "superTypeParameters":null,
+      "implements":[],
+      "decorators":[]
+    },
+    {
+      "type":"DeclareClass",
+      "loc":{"source":null,"start":{"line":2,"column":0},"end":{"line":2,"column":47}},
+      "range":[32,79],
+      "id":{
+        "type":"Identifier",
+        "loc":{"source":null,"start":{"line":2,"column":23},"end":{"line":2,"column":44}},
+        "range":[55,76],
+        "name":"PrefixedAbstractClass",
+        "typeAnnotation":null,
+        "optional":false
+      },
+      "typeParameters":null,
+      "body":{
+        "type":"ObjectTypeAnnotation",
+        "loc":{"source":null,"start":{"line":2,"column":45},"end":{"line":2,"column":47}},
+        "range":[77,79],
+        "exact":false,
+        "properties":[],
+        "indexers":[],
+        "callProperties":[]
+      },
+      "extends":[]
+    }
+  ],
+  "comments":[]
+}