fix: broken ChecksumConfiguration and ConditionalRecursiveTransformEx…

…act interface in TS < 4.4 (#926)

* fix: broken ChecksumConfiguration interface in TS < 4.4

Before TS 4.4, TS does not allow the index signature as union type. Consuming library with this interface will cause error:
"TS1337: An index signature parameter type cannot be a union type. Consider using a mapped object type instead.
[other: string | number]: any;". This issue was fixed in TS 4.4: https://www.typescriptlang.org/docs/handbook/release-notes/typescript-4-4.html#symbol-and-template-string-pattern-index-signatures

Fixing this issue so it won't break users with TSC < 4.4

* fix(types): conditional generic types in TS<4.1

The referred type `ConditionalRecursiveTransformExact<T, FromType, ToType>` uses
conditional generic type that is only supported in TS>=4.1. However the typesVersions
directs consumers' TSC 4.0 to the types with this definition, hence consumers will see
following error:

error TS2315: Type 'ConditionalRecursiveTransformExact' is not generic.

Changing the typesVersions directive resolves the issue.

.changeset/eleven-kids-decide.md

@@ -0,0 +1,5 @@
+---
+"@smithy/types": patch
+---
+
+fix: broken ChecksumConfiguration interface in TS < 4.4 and conditional generic types in TS<4.1

packages/types/package.json

@@ -30,7 +30,7 @@
     "node": ">=14.0.0"
   },
   "typesVersions": {
-    "<4.0": {
+    "<=4.0": {
       "dist-types/*": [
         "dist-types/ts3.4/*"
       ]

packages/types/src/extensions/checksum.ts

@@ -21,16 +21,21 @@ export interface ChecksumAlgorithm {
 }
 
 /**
- * @internal
+ * @deprecated unused.
  */
-export interface ChecksumConfiguration {
-  addChecksumAlgorithm(algo: ChecksumAlgorithm): void;
-  checksumAlgorithms(): ChecksumAlgorithm[];
-
+type ChecksumConfigurationLegacy = {
   /**
    * @deprecated unused.
    */
-  [other: string | number]: any;
+  [other in string | number]: any;
+};
+
+/**
+ * @internal
+ */
+export interface ChecksumConfiguration extends ChecksumConfigurationLegacy {
+  addChecksumAlgorithm(algo: ChecksumAlgorithm): void;
+  checksumAlgorithms(): ChecksumAlgorithm[];
 }
 
 /**