fix(typescript): don't assume : after '=> ()' is part of '? :'

In the following two TypeScript examples, the treatment of ':' after
'=> (somevar)' differs:

    true ? () => (somevar) : thing => { };
           () => (somevar) : thing => { };

When parsing an arrow function body, quick-lint-js assumes that a ':' is
*not* a type annotation. This works well for the first case, but causes
quick-lint-js to misunderstand the second case.

Teach arrow function body parsing to behave differently depending on
whether we're inside the truthy branch of a conditional expression.
Parsing of the truthy branch (parse-expression.cpp line 1801) sets
.colon_type_annotation = allow_type_annotations::never, so pass that
along; whereas calls which should treat the ':' as an type annotation
set .colon_type_annotation = allow_type_annotations::typescript_only
(default), so pass that along too.

Author: strager

src/quick-lint-js/fe/parse-expression.cpp

@@ -2465,7 +2465,7 @@ expression* parser::parse_arrow_function_body_no_scope(
         v, precedence{
                .commas = false,
                .in_operator = prec.in_operator,
-               .colon_type_annotation = allow_type_annotations::never,
+               .colon_type_annotation = prec.colon_type_annotation,
            });
   }
 

test/test-parse-typescript-function.cpp

@@ -364,6 +364,27 @@ TEST_F(test_parse_typescript_function,
   }
 }
 
+TEST_F(test_parse_typescript_function, arrow_with_arrow_body) {
+  {
+    // This used to confuse the quick-lint-js parser.
+    test_parser p(u8"() => (): ReturnType => myVariable"_sv,
+                  typescript_options);
+    p.parse_and_visit_expression();
+    EXPECT_THAT(p.visits, ElementsAreArray({
+                              "visit_enter_function_scope",       //
+                              "visit_enter_function_scope_body",  //
+                              "visit_enter_function_scope",       //
+                              "visit_variable_type_use",          // ReturnType
+                              "visit_enter_function_scope_body",  //
+                              "visit_variable_use",               // myVariable
+                              "visit_exit_function_scope",        //
+                              "visit_exit_function_scope",        //
+                          }));
+    EXPECT_THAT(p.variable_uses,
+                ElementsAreArray({u8"ReturnType", u8"myVariable"}));
+  }
+}
+
 TEST_F(test_parse_typescript_function, object_method_return_type_annotation) {
   {
     test_parser p(u8"({ method(param): C {} })"_sv, typescript_options);