fix(compiler): Correct impossible bug on nested char pattern (#2310)

Co-authored-by: Oscar Spencer <oscar@grain-lang.org>

Author: spotandjake

compiler/src/typed/parmatch.re

@@ -2099,7 +2099,8 @@ let untype_constant =
     Parsetree.PConstBytes(Location.mknoloc(Bytes.to_string(b)))
   | Const_string(s) =>
     Parsetree.PConstString(Location.mknoloc(Printf.sprintf("\"%s\"", s)))
-  | Const_char(c) => Parsetree.PConstChar(Location.mknoloc(c))
+  | Const_char(c) =>
+    Parsetree.PConstChar(Location.mknoloc(Printf.sprintf("\'%s\'", c)))
   | Const_bool(b) => Parsetree.PConstBool(b)
   | Const_void => Parsetree.PConstVoid;
 

compiler/src/typed/printpat.re

@@ -34,7 +34,7 @@ let pretty_const = c =>
   | Const_number(Const_number_bigint({bigint_rep})) => bigint_rep
   | Const_bytes(b) => Printf.sprintf("%S", Bytes.to_string(b))
   | Const_string(s) => Printf.sprintf("%S", s)
-  | Const_char(c) => Printf.sprintf("%S", c)
+  | Const_char(c) => Printf.sprintf("\'%s\'", c)
   | Const_float64(f)
   | Const_float32(f) => Printf.sprintf("%f", f)
   | Const_int8(i) => Printf.sprintf("%lds", i)

compiler/test/suites/pattern_matching.re

@@ -236,6 +236,13 @@ describe("pattern matching", ({test, testSkip}) => {
     "constant_match_4",
     "match ((\"foo\", 5)) { (\"foo\", n) when n == 7 => false, (\"foo\", 9) when true => false, (\"foo\", n) when n == 5 => true, _ => false }",
   );
+  assertWarning(
+    "nested_constant_match_1",
+    "match ([> 'a']) {
+       [> 'l', 'e'] => void,
+     }",
+    Warnings.PartialMatch("([> 'l', _]|[> _, _]|[> ])"),
+  );
   // Constant low level wasm type patterns
   assertSnapshot(
     "low_level_constant_match_1",