fix(lex): parse let f = (): RT<T>=>null;

Test Plan:
`let f = (): RT<T>=>null;`

Fixes #1169

Author: vegerot

src/quick-lint-js/fe/lex.cpp

@@ -998,7 +998,13 @@ void Lexer::skip_less_less_as_less() {
 void Lexer::skip_as_greater() {
   switch (this->last_token_.type) {
   case Token_Type::greater_equal:
-    this->last_token_.type = Token_Type::equal;
+    if (this->input_[0] == '>') {
+      // >=>  ➤  > =>
+      this->last_token_.type = Token_Type::equal_greater;
+      this->input_ += 1;
+    } else {
+      this->last_token_.type = Token_Type::equal;
+    }
     break;
   case Token_Type::greater_greater_equal:
     this->last_token_.type = Token_Type::greater_equal;
@@ -1018,6 +1024,7 @@ void Lexer::skip_as_greater() {
   }
   this->last_token_.has_leading_newline = false;
   this->last_token_.begin += 1;
+  this->last_token_.end = this->input_;
   this->last_last_token_end_ = this->last_token_.begin;
 }
 

test/test-parse-typescript-generic.cpp

@@ -1098,6 +1098,64 @@ TEST_F(
   }
 }
 
+TEST_F(Test_Parse_TypeScript_Generic,
+       greater_equal_greater_is_split_into_two_tokens) {
+  {
+    Spy_Visitor p = test_parse_and_visit_statement(
+        u8"let f = (): RT<T>=>{};"_sv, no_diags, typescript_options);
+    EXPECT_THAT(p.visits, ElementsAreArray({
+                              "visit_enter_function_scope",       //
+                              "visit_enter_type_scope",           // :
+                              "visit_variable_type_use",          // RT
+                              "visit_variable_type_use",          // T
+                              "visit_exit_type_scope",            //
+                              "visit_enter_function_scope_body",  // {
+                              "visit_exit_function_scope",        // }
+                              "visit_variable_declaration",       // let f
+                          }));
+    EXPECT_THAT(p.variable_declarations,
+                ElementsAreArray({let_init_decl(u8"f")}));
+  }
+  {
+    Spy_Visitor p = test_parse_and_visit_statement(
+        u8"let f = (): RT<RT<T>>=> {};"_sv, no_diags, typescript_options);
+    EXPECT_THAT(p.visits, ElementsAreArray({
+                              "visit_enter_function_scope",       //
+                              "visit_enter_type_scope",           // :
+                              "visit_variable_type_use",          // RT
+                              "visit_variable_type_use",          // RT
+                              "visit_variable_type_use",          // T
+                              "visit_exit_type_scope",            //
+                              "visit_enter_function_scope_body",  // {
+                              "visit_exit_function_scope",        // }
+                              "visit_variable_declaration",       // let f
+                          }));
+    EXPECT_THAT(p.variable_declarations,
+                ElementsAreArray({let_init_decl(u8"f")}));
+  }
+  {
+    Spy_Visitor p = test_parse_and_visit_statement(
+        u8"class C<T> { method(): C<T>=> {} }"_sv,  //
+        u8"                           ^^ Diag_Functions_Or_Methods_Should_Not_Have_Arrow_Operator"_diag,
+        typescript_options);
+    EXPECT_THAT(p.visits, ElementsAreArray({
+                              "visit_enter_class_scope",  //
+                              "visit_variable_declaration",
+                              "visit_enter_class_scope_body",     //
+                              "visit_enter_function_scope",       //
+                              "visit_enter_type_scope",           // :
+                              "visit_variable_type_use",          // T
+                              "visit_variable_type_use",          // C
+                              "visit_exit_type_scope",            //
+                              "visit_enter_function_scope_body",  //
+                              "visit_exit_function_scope",        //
+                              "visit_property_declaration",       // method
+                              "visit_exit_class_scope",
+                              "visit_variable_declaration",  // C
+                          }));
+  }
+}
+
 TEST_F(Test_Parse_TypeScript_Generic,
        unambiguous_generic_arguments_are_parsed_in_javascript) {
   {