Correctly match boolean literal values (#899)

`boolean` is effectively the union type `true | false` but the current
`isMatch` implementation will treat overlapping union types as matching.

This patch introduces a more strict type equivalence check based on
microsoft/TypeScript#27024 (comment)
and uses it when checking types that extend `boolean` for equivalence to
`boolean`.

The test added in this patch fails without the corresponding code changes
here.

Fixes #754.

src/utils.ts

@@ -208,11 +208,22 @@ export type EnumSchema<T extends string | number | undefined> = {
 }
 
 /**
- * Check if a type is an exact match.
+ * Check if a type is a match for another whilst treating overlapping
+ * unions as a match.
  */
 
 export type IsMatch<T, G> = T extends G ? (G extends T ? T : never) : never
 
+/**
+ * Check if a type is an exact match.
+ */
+
+export type IsExactMatch<T, U> = (<G>() => G extends T ? 1 : 2) extends <
+  G
+>() => G extends U ? 1 : 2
+  ? T
+  : never
+
 /**
  * Check if a type is a record type.
  */
@@ -322,7 +333,7 @@ export type StructSchema<T> = [T] extends [string | undefined]
     ? EnumSchema<T>
     : T
   : [T] extends [boolean]
-  ? [T] extends [IsMatch<T, boolean>]
+  ? [T] extends [IsExactMatch<T, boolean>]
     ? null
     : T
   : T extends

test/typings/describe.ts

@@ -70,6 +70,10 @@ test<Describe<string & number>>((x) => {
   return intersection([string(), number()])
 })
 
+test<Describe<false>>((x) => {
+  return literal(false)
+})
+
 test<Describe<42>>((x) => {
   return literal(42)
 })