rescript-lang · JonoPrest · May 27, 2026 · May 20, 2026 · May 21, 2026 · May 21, 2026
diff --git a/AGENTS.md b/AGENTS.md
@@ -168,7 +168,25 @@ tests/
 2. **Integration tests** (`tests/tests/`) - End-to-end behavior
 3. **Unit tests** (`tests/ounit_tests/`) - Compiler functions
 4. **Build tests** (`tests/build_tests/`) - Error cases and edge cases
-5. **Type tests** (`tests/build_tests/super_errors/`) - Type checking behavior
+5. **Type tests** (`tests/build_tests/super_errors/`) - Single-file type checking errors
+6. **Multi-file error tests** (`tests/build_tests/super_errors_multi/`) - Cross-module errors that need separate `.res` / `.resi` files
+
+#### Error variant catalog
+
+[`tests/ERROR_VARIANTS.md`](tests/ERROR_VARIANTS.md) is a per-module
+catalog of every error and warning variant the compiler can emit, with
+each entry mapped to a fixture (or a documented reason it's unreachable).
+
+**When adding or removing an error variant**, also update the catalog:
+
+1. Add (or remove) the row in the relevant module section.
+2. Set the status (`✓` covered / `⚠` unreachable / `☐` TODO).
+3. If covered, link the fixture path; if unreachable, note the reason.
+
+**When adding or removing a fixture**, update the corresponding row's
+`Fixture` and status columns so the catalog stays in sync with the test
+suite. The catalog is the primary tool for finding coverage gaps and
+dead-code removal candidates; stale entries make both jobs harder.
 
 ## Build Commands & Development
 

diff --git a/CHANGELOG.md b/CHANGELOG.md
@@ -52,6 +52,7 @@
 - Expand `super_errors` fixture coverage for the remaining reachable single-file error variants. https://github.com/rescript-lang/rescript/pull/8432
 - Cache OPAM env, rewatch build, and instrumented dune state in the coverage workflow. https://github.com/rescript-lang/rescript/pull/8434
 - Add a multi-file fixture harness (`super_errors_multi`) for cross-module errors and warnings. https://github.com/rescript-lang/rescript/pull/8433
+- Catalog every named compiler error variant in `tests/ERROR_VARIANTS.md` and add fixtures for the remaining reachable ones. https://github.com/rescript-lang/rescript/pull/8446
 
 
 # 13.0.0-alpha.4

diff --git a/tests/ERROR_VARIANTS.md b/tests/ERROR_VARIANTS.md
diff --git a/tests/build_tests/super_errors/expected/UntaggedAtMostOneBigint.res.expected b/tests/build_tests/super_errors/expected/UntaggedAtMostOneBigint.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedAtMostOneBigint.res[0m:[2m2:20-30[0m
+
+  1 [2m│[0m @unboxed
+  [1;31m2[0m [2m│[0m type t = A(bigint) [1;31m| B(bigint)[0m
+  3 [2m│[0m 
+
+  This untagged variant definition is invalid: At most one case can be a bigint type.
diff --git a/tests/build_tests/super_errors/expected/UntaggedAtMostOneBoolean.res.expected b/tests/build_tests/super_errors/expected/UntaggedAtMostOneBoolean.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedAtMostOneBoolean.res[0m:[2m2:18-26[0m
+
+  1 [2m│[0m @unboxed
+  [1;31m2[0m [2m│[0m type t = A(bool) [1;31m| B(bool)[0m
+  3 [2m│[0m 
+
+  This untagged variant definition is invalid: At most one case can be a boolean type.
diff --git a/tests/build_tests/super_errors/expected/UntaggedAtMostOneFunction.res.expected b/tests/build_tests/super_errors/expected/UntaggedAtMostOneFunction.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedAtMostOneFunction.res[0m:[2m2:24-44[0m
+
+  1 [2m│[0m @unboxed
+  [1;31m2[0m [2m│[0m type t = A(int => int) [1;31m| B(string => string)[0m
+  3 [2m│[0m 
+
+  This untagged variant definition is invalid: At most one case can be a function type.
diff --git a/tests/build_tests/super_errors/expected/UntaggedAtMostOneInstance.res.expected b/tests/build_tests/super_errors/expected/UntaggedAtMostOneInstance.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedAtMostOneInstance.res[0m:[2m2:20-30[0m
+
+  1 [2m│[0m @unboxed
+  [1;31m2[0m [2m│[0m type t = A(Date.t) [1;31m| B(Date.t)[0m
+  3 [2m│[0m 
+
+  This untagged variant definition is invalid: At most one case can be a Date type.
diff --git a/tests/build_tests/super_errors/expected/UntaggedAtMostOneNumber.res.expected b/tests/build_tests/super_errors/expected/UntaggedAtMostOneNumber.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedAtMostOneNumber.res[0m:[2m2:17-26[0m
+
+  1 [2m│[0m @unboxed
+  [1;31m2[0m [2m│[0m type t = A(int) [1;31m| B(float)[0m
+  3 [2m│[0m 
+
+  This untagged variant definition is invalid: At most one case can be a number type (int or float).
diff --git a/tests/build_tests/super_errors/expected/UntaggedAtMostOneObject.res.expected b/tests/build_tests/super_errors/expected/UntaggedAtMostOneObject.res.expected
@@ -0,0 +1,10 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedAtMostOneObject.res[0m:[2m4:15-20[0m
+
+  2 [2m│[0m type b = {y: int}
+  3 [2m│[0m @unboxed
+  [1;31m4[0m [2m│[0m type t = A(a) [1;31m| B(b)[0m
+  5 [2m│[0m 
+
+  This untagged variant definition is invalid: At most one case can be an object type.
diff --git a/tests/build_tests/super_errors/expected/UntaggedConstructorMoreThanOneArg.res.expected b/tests/build_tests/super_errors/expected/UntaggedConstructorMoreThanOneArg.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedConstructorMoreThanOneArg.res[0m:[2m2:1-32[0m
+
+  1 [2m│[0m @unboxed
+  [1;31m2[0m [2m│[0m [1;31mtype t = A(int, int) | B(string)[0m
+  3 [2m│[0m 
+
+  This untagged variant definition is invalid: Constructor A has more than one argument.
diff --git a/tests/build_tests/super_errors/expected/UntaggedDuplicateLiteral.res.expected b/tests/build_tests/super_errors/expected/UntaggedDuplicateLiteral.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedDuplicateLiteral.res[0m:[2m2:23-34[0m
+
+  1 [2m│[0m @unboxed
+  [1;31m2[0m [2m│[0m type t = | @as("x") A [1;31m| @as("x") B[0m
+  3 [2m│[0m 
+
+  This untagged variant definition is invalid: Duplicate literal x.
diff --git a/tests/build_tests/super_errors/expected/UntaggedDuplicatedBsAs.res.expected b/tests/build_tests/super_errors/expected/UntaggedDuplicatedBsAs.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedDuplicatedBsAs.res[0m:[2m2:21-23[0m
+
+  1 [2m│[0m @unboxed
+  [1;31m2[0m [2m│[0m type t = | @as("x") [1;31m@as[0m("y") A | B
+  3 [2m│[0m 
+
+  duplicate @as
diff --git a/tests/build_tests/super_errors/expected/UntaggedInvalidVariantAsAnnotation.res.expected b/tests/build_tests/super_errors/expected/UntaggedInvalidVariantAsAnnotation.res.expected
@@ -0,0 +1,8 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedInvalidVariantAsAnnotation.res[0m:[2m1:12-14[0m
+
+  [1;31m1[0m [2m│[0m type t = | [1;31m@as[0m(foo) A | B
+  2 [2m│[0m 
+
+  A variant case annotation @as(...) must be a string or integer, boolean, null, undefined
diff --git a/tests/build_tests/super_errors/expected/UntaggedInvalidVariantTagAnnotation.res.expected b/tests/build_tests/super_errors/expected/UntaggedInvalidVariantTagAnnotation.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedInvalidVariantTagAnnotation.res[0m:[2m1:1-4[0m
+
+  [1;31m1[0m [2m│[0m [1;31m@tag[0m(123)
+  2 [2m│[0m type t = A({foo: int}) | B({bar: string})
+  3 [2m│[0m 
+
+  A variant tag annotation @tag(...) must be a string
diff --git a/tests/build_tests/super_errors/expected/UntaggedOnlyOneUnknown.res.expected b/tests/build_tests/super_errors/expected/UntaggedOnlyOneUnknown.res.expected
@@ -0,0 +1,10 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedOnlyOneUnknown.res[0m:[2m4:10-18[0m
+
+  2 [2m│[0m 
+  3 [2m│[0m @unboxed
+  [1;31m4[0m [2m│[0m type t = [1;31mA(opaque)[0m | B(int)
+  5 [2m│[0m 
+
+  This untagged variant definition is invalid: Case A has a payload that is not of one of the recognized shapes (object, array, etc). Then it must be the only case with payloads.
diff --git a/tests/build_tests/super_errors/expected/UntaggedTagFieldNameConflict.res.expected b/tests/build_tests/super_errors/expected/UntaggedTagFieldNameConflict.res.expected
@@ -0,0 +1,11 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/UntaggedTagFieldNameConflict.res[0m:[2m3:3-18[0m
+
+  1 [2m│[0m @tag("kind")
+  2 [2m│[0m type t =
+  [1;31m3[0m [2m│[0m   [1;31m| A({kind: int})[0m
+  4 [2m│[0m   | B({other: string})
+  5 [2m│[0m 
+
+  Constructor "A": the @tag name "kind" conflicts with the runtime value of inline record field "kind". Use a different @tag name or rename the field.
diff --git a/tests/build_tests/super_errors/expected/bs_expect_int_or_string_or_json_literal.res.expected b/tests/build_tests/super_errors/expected/bs_expect_int_or_string_or_json_literal.res.expected
@@ -0,0 +1,8 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/bs_expect_int_or_string_or_json_literal.res[0m:[2m1:21-23[0m
+
+  [1;31m1[0m [2m│[0m @val external foo: ([1;31m@as[0m(true) _, int) => unit = "foo"
+  2 [2m│[0m 
+
+  expect int, string literal or json literal {json|text here|json}
diff --git a/tests/build_tests/super_errors/expected/bs_invalid_underscore_type_in_external.res.expected b/tests/build_tests/super_errors/expected/bs_invalid_underscore_type_in_external.res.expected
@@ -0,0 +1,8 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/bs_invalid_underscore_type_in_external.res[0m:[2m1:26[0m
+
+  [1;31m1[0m [2m│[0m @obj external make: (~x: [1;31m_[0m) => _ = ""
+  2 [2m│[0m 
+
+  _ is not allowed in combination with external optional type
diff --git a/tests/build_tests/super_errors/expected/bs_optional_in_uncurried_bs_attribute.res.expected b/tests/build_tests/super_errors/expected/bs_optional_in_uncurried_bs_attribute.res.expected
@@ -0,0 +1,8 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/bs_optional_in_uncurried_bs_attribute.res[0m:[2m1:15-55[0m
+
+  [1;31m1[0m [2m│[0m let f = @this [1;31m(self, ~x=?) => self + x->Option.getOr(0)[0m
+  2 [2m│[0m 
+
+  Uncurried function doesn't support optional arguments yet
diff --git a/tests/build_tests/super_errors/expected/bs_this_simple_pattern.res.expected b/tests/build_tests/super_errors/expected/bs_this_simple_pattern.res.expected
@@ -0,0 +1,8 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/bs_this_simple_pattern.res[0m:[2m1:16-21[0m
+
+  [1;31m1[0m [2m│[0m let f = @this ([1;31m(a, b)[0m, x) => a + b + x
+  2 [2m│[0m 
+
+  %@this expect its pattern variable to be simple form
diff --git a/tests/build_tests/super_errors/expected/bs_unsupported_predicates.res.expected b/tests/build_tests/super_errors/expected/bs_unsupported_predicates.res.expected
@@ -0,0 +1,8 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/bs_unsupported_predicates.res[0m:[2m1:19-23[0m
+
+  [1;31m1[0m [2m│[0m type t = {..@get({[1;31mweird[0m: true}) "x": int}
+  2 [2m│[0m 
+
+  unsupported predicates
diff --git a/tests/build_tests/super_errors/expected/cannot_quantify.res.expected b/tests/build_tests/super_errors/expected/cannot_quantify.res.expected
@@ -0,0 +1,8 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/cannot_quantify.res[0m:[2m1:14-35[0m
+
+  [1;31m1[0m [2m│[0m type t = {f: [1;31m'a. (int as 'a) => int[0m}
+  2 [2m│[0m 
+
+  The universal type variable 'a cannot be generalized: it is not a variable.
diff --git a/tests/build_tests/super_errors/expected/empty_record_literal.res.expected b/tests/build_tests/super_errors/expected/empty_record_literal.res.expected
@@ -0,0 +1,8 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/empty_record_literal.res[0m:[2m1:11-12[0m
+
+  [1;31m1[0m [2m│[0m let bad = [1;31m{}[0m
+  2 [2m│[0m 
+
+  Empty record literal {} should be type annotated or used in a record context.
diff --git a/tests/build_tests/super_errors/expected/extension_arity_mismatch.res.expected b/tests/build_tests/super_errors/expected/extension_arity_mismatch.res.expected
@@ -0,0 +1,11 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/extension_arity_mismatch.res[0m:[2m3:1-16[0m
+
+  1 [2m│[0m type t<'a> = ..
+  2 [2m│[0m 
+  [1;31m3[0m [2m│[0m [1;31mtype t += A(int)[0m
+  4 [2m│[0m 
+
+  This extension does not match the definition of type t
+  They have different arities.
diff --git a/tests/build_tests/super_errors/expected/extension_rebind_mismatch.res.expected b/tests/build_tests/super_errors/expected/extension_rebind_mismatch.res.expected
@@ -0,0 +1,10 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/extension_rebind_mismatch.res[0m:[2m5:15[0m
+
+  3 [2m│[0m 
+  4 [2m│[0m type a += A(int)
+  [1;31m5[0m [2m│[0m type b += B = [1;31mA[0m
+  6 [2m│[0m 
+
+  The constructor A has type a but was expected to be of type b
diff --git a/tests/build_tests/super_errors/expected/extension_rebind_private.res.expected b/tests/build_tests/super_errors/expected/extension_rebind_private.res.expected
@@ -0,0 +1,10 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/extension_rebind_private.res[0m:[2m5:15[0m
+
+  3 [2m│[0m type t += private A(int)
+  4 [2m│[0m 
+  [1;31m5[0m [2m│[0m type t += B = [1;31mA[0m
+  6 [2m│[0m 
+
+  The constructor A is private
diff --git a/tests/build_tests/super_errors/expected/if_return_type_mismatch.res.expected b/tests/build_tests/super_errors/expected/if_return_type_mismatch.res.expected
@@ -0,0 +1,15 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/if_return_type_mismatch.res[0m:[2m2:3[0m
+
+  1 [2m│[0m let x: string = if true {
+  [1;31m2[0m [2m│[0m   [1;31m1[0m
+  3 [2m│[0m } else {
+  4 [2m│[0m   2
+
+  This has type: [1;31mint[0m
+  But this [1;33mif[0m statement is expected to return: [1;33mstring[0m
+
+  [1;33mif[0m expressions must return the same type in all branches ([1;33mif[0m, [1;33melse if[0m, [1;33melse[0m).
+
+  You can convert [1;33mint[0m to [1;33mstring[0m with [1;33mInt.toString[0m.
diff --git a/tests/build_tests/super_errors/expected/illegal_recursive_module_reference.res.expected b/tests/build_tests/super_errors/expected/illegal_recursive_module_reference.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/illegal_recursive_module_reference.res[0m:[2m1:15-17[0m
+
+  [1;31m1[0m [2m│[0m module rec A: [1;31mB.S[0m = {
+  2 [2m│[0m   let x = 1
+  3 [2m│[0m }
+
+  Illegal recursive module reference
diff --git a/tests/build_tests/super_errors/expected/labeled_fn_argument_type_clash.res.expected b/tests/build_tests/super_errors/expected/labeled_fn_argument_type_clash.res.expected
@@ -0,0 +1,13 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/labeled_fn_argument_type_clash.res[0m:[2m3:14-17[0m
+
+  1 [2m│[0m let f = (~x: int) => x
+  2 [2m│[0m 
+  [1;31m3[0m [2m│[0m let _ = f(~x=[1;31m"hi"[0m)
+  4 [2m│[0m 
+
+  This has type: [1;31mstring[0m
+  But this function argument [1;33m~x[0m is expecting: [1;33mint[0m
+
+  You can convert [1;33mstring[0m to [1;33mint[0m with [1;33mInt.fromString[0m.
diff --git a/tests/build_tests/super_errors/expected/maybe_unwrap_option.res.expected b/tests/build_tests/super_errors/expected/maybe_unwrap_option.res.expected
@@ -0,0 +1,9 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/maybe_unwrap_option.res[0m:[2m1:14-20[0m
+
+  [1;31m1[0m [2m│[0m let x: int = [1;31mSome(5)[0m
+  2 [2m│[0m 
+
+  This has type: [1;31moption<'a>[0m
+  But it's expected to have type: [1;33mint[0m
diff --git a/tests/build_tests/super_errors/expected/no_value_clauses.res.expected b/tests/build_tests/super_errors/expected/no_value_clauses.res.expected
@@ -0,0 +1,11 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/no_value_clauses.res[0m:[2m2:3-4:3[0m
+
+  1 [2m│[0m let f = x =>
+  [1;31m2[0m [2m│[0m   [1;31mswitch x {[0m
+  [1;31m3[0m [2m│[0m [1;31m  | exception Not_found => 1[0m
+  [1;31m4[0m [2m│[0m [1;31m  }[0m
+  5 [2m│[0m 
+
+  None of the patterns in this 'match' expression match values.
diff --git a/tests/build_tests/super_errors/expected/non_generalizable_module.res.expected b/tests/build_tests/super_errors/expected/non_generalizable_module.res.expected
@@ -0,0 +1,21 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/non_generalizable_module.res[0m:[2m1:8-5:1[0m
+
+  [1;31m1[0m [2m│[0m module [1;31mM = {[0m
+  [1;31m2[0m [2m│[0m [1;31m  module Inner = {[0m
+  [1;31m3[0m [2m│[0m [1;31m    let r = ref(None)[0m
+  [1;31m4[0m [2m│[0m [1;31m  }[0m
+  [1;31m5[0m [2m│[0m [1;31m}[0m
+  6 [2m│[0m 
+
+  The type of this module contains type variables that cannot be generalized:
+  [1;31m{
+  module Inner: {
+    let r: ref<option<'_weak1>>
+  }
+}[0m
+
+  This happens when the type system senses there's a mutation/side-effect,
+  in combination with a polymorphic value.
+  [1;33mUsing or annotating that value usually solves it.[0m
diff --git a/tests/build_tests/super_errors/expected/pattern_type_clash_polyvariant.res.expected b/tests/build_tests/super_errors/expected/pattern_type_clash_polyvariant.res.expected
@@ -0,0 +1,12 @@
+
+  [1;31mWe've found a bug for you![0m
+  [36m/.../fixtures/pattern_type_clash_polyvariant.res[0m:[2m3:5-6[0m
+
+  1 [2m│[0m let f = (x: int) =>
+  2 [2m│[0m   switch x {
+  [1;31m3[0m [2m│[0m   | [1;31m#A[0m => 1
+  4 [2m│[0m   | _ => 0
+  5 [2m│[0m   }
+
+  This pattern matches values of type [1;31m[? #A][0m
+  but a pattern was expected which matches values of type [1;33mint[0m