fix(grainfmt): Fix formatting of default argument types (#2081)

Author: spotandjake

compiler/src/formatting/fmt.re

@@ -2538,8 +2538,7 @@ let print_parsed_type_argument = (fmt, arg) => {
   (
     switch (arg.ptyp_arg_label) {
     | Unlabeled => empty
-    | Labeled({txt: label, loc: label_loc})
-    | Default({txt: label, loc: label_loc}) =>
+    | Labeled({txt: label, loc: label_loc}) =>
       string(label)
       ++ string(":")
       ++ fmt.print_comment_range(
@@ -2550,6 +2549,18 @@ let print_parsed_type_argument = (fmt, arg) => {
            label_loc,
            arg.ptyp_arg_type.ptyp_loc,
          )
+    | Default({txt: label, loc: label_loc}) =>
+      string("?")
+      ++ string(label)
+      ++ string(":")
+      ++ fmt.print_comment_range(
+           fmt,
+           ~none=space,
+           ~lead=space,
+           ~trail=space,
+           label_loc,
+           arg.ptyp_arg_type.ptyp_loc,
+         )
     }
   )
   ++ fmt.print_type(fmt, arg.ptyp_arg_type);

compiler/test/grainfmt/function_params.expected.gr

@@ -39,3 +39,5 @@ let stringTailMatcher = (toMatch, len) =>
   }
 
 let f: Number => (Number, Number) => Number = a => (b, c) => a + b + c
+
+let namedArg: (?suffix: String) => String = (suffix="") => suffix

compiler/test/grainfmt/function_params.input.gr

@@ -34,3 +34,5 @@ let stringTailMatcher =  (toMatch, len) =>
   }
 
 let f: Number -> (Number, Number) -> Number = a => (b, c) => a + b + c
+
+let namedArg: (?suffix: String) => String = (suffix="") => suffix