diff --git a/docs/modules/index.ts.md b/docs/modules/index.ts.md
index d1dbde41f..cc26efb20 100644
--- a/docs/modules/index.ts.md
+++ b/docs/modules/index.ts.md
@@ -666,31 +666,33 @@ Added in v1.5.3
 **Signature**
 
 ```ts
-export interface TupleC<CS extends [Mixed, ...Array<Mixed>]>
+export interface TupleC<CS extends readonly [Mixed, ...Array<Mixed>]>
   extends TupleType<
     CS,
-    CS extends { length: 1 }
+    CS extends { readonly length: 1 }
       ? [TypeOf<CS[0]>]
-      : CS extends { length: 2 }
+      : CS extends { readonly length: 2 }
       ? [TypeOf<CS[0]>, TypeOf<CS[1]>]
-      : CS extends { length: 3 }
+      : CS extends { readonly length: 3 }
       ? [TypeOf<CS[0]>, TypeOf<CS[1]>, TypeOf<CS[2]>]
-      : CS extends { length: 4 }
+      : CS extends { readonly length: 4 }
       ? [TypeOf<CS[0]>, TypeOf<CS[1]>, TypeOf<CS[2]>, TypeOf<CS[3]>]
-      : CS extends { length: 5 }
+      : CS extends { readonly length: 5 }
       ? [TypeOf<CS[0]>, TypeOf<CS[1]>, TypeOf<CS[2]>, TypeOf<CS[3]>, TypeOf<CS[4]>]
       : unknown,
-    CS extends { length: 1 }
+    // : { [K in keyof CS]: CS[K] extends Mixed ? TypeOf<CS[K]> : unknown },
+    CS extends { readonly length: 1 }
       ? [OutputOf<CS[0]>]
-      : CS extends { length: 2 }
+      : CS extends { readonly length: 2 }
       ? [OutputOf<CS[0]>, OutputOf<CS[1]>]
-      : CS extends { length: 3 }
+      : CS extends { readonly length: 3 }
       ? [OutputOf<CS[0]>, OutputOf<CS[1]>, OutputOf<CS[2]>]
-      : CS extends { length: 4 }
+      : CS extends { readonly length: 4 }
       ? [OutputOf<CS[0]>, OutputOf<CS[1]>, OutputOf<CS[2]>, OutputOf<CS[3]>]
-      : CS extends { length: 5 }
+      : CS extends { readonly length: 5 }
       ? [OutputOf<CS[0]>, OutputOf<CS[1]>, OutputOf<CS[2]>, OutputOf<CS[3]>, OutputOf<CS[4]>]
       : unknown,
+    // : { [K in keyof CS]: CS[K] extends Mixed ? OutputOf<CS[K]> : unknown },
     unknown
   > {}
 ```
@@ -2259,20 +2261,10 @@ Added in v1.0.0
 **Signature**
 
 ```ts
-export declare function tuple<A extends Mixed, B extends Mixed, C extends Mixed, D extends Mixed, E extends Mixed>(
-  codecs: [A, B, C, D, E],
-  name?: string
-): TupleC<[A, B, C, D, E]>
-export declare function tuple<A extends Mixed, B extends Mixed, C extends Mixed, D extends Mixed>(
-  codecs: [A, B, C, D],
-  name?: string
-): TupleC<[A, B, C, D]>
-export declare function tuple<A extends Mixed, B extends Mixed, C extends Mixed>(
-  codecs: [A, B, C],
-  name?: string
-): TupleC<[A, B, C]>
-export declare function tuple<A extends Mixed, B extends Mixed>(codecs: [A, B], name?: string): TupleC<[A, B]>
-export declare function tuple<A extends Mixed>(codecs: [A], name?: string): TupleC<[A]>
+export declare function tuple<CS extends readonly [Mixed, ...Array<Mixed>]>(
+  codecs: CS,
+  name: string = `[${codecs.map((type) => type.name).join(', ')}]`
+): TupleC<CS>
 ```
 
 Added in v1.0.0
diff --git a/dtslint/after3.8/index.d.ts b/dtslint/after3.8/index.d.ts
new file mode 100644
index 000000000..45362d6a6
--- /dev/null
+++ b/dtslint/after3.8/index.d.ts
@@ -0,0 +1 @@
+// TypeScript Version: 3.8
diff --git a/dtslint/ts3.5/index.d.ts b/dtslint/after3.8/ts3.8/index.d.ts
similarity index 100%
rename from dtslint/ts3.5/index.d.ts
rename to dtslint/after3.8/ts3.8/index.d.ts
diff --git a/dtslint/after3.8/ts3.8/index.ts b/dtslint/after3.8/ts3.8/index.ts
new file mode 100644
index 000000000..243c74455
--- /dev/null
+++ b/dtslint/after3.8/ts3.8/index.ts
@@ -0,0 +1,9 @@
+import * as t from '../../../src'
+
+const Tuple8 = t.tuple([t.string, t.string, t.string, t.string, t.string, t.string])
+type Tuple8TypeTest = t.TypeOf<typeof Tuple8> // $ExpectType [string, string, string, string, string, string]
+type Tuple8OutputTest = t.OutputOf<typeof Tuple8> // $ExpectType [string, string, string, string, string, string]
+
+const Tuple9 = t.tuple([t.string, t.string, t.boolean, t.string, t.string, t.string, t.string])
+type Tuple9TypeTest = t.TypeOf<typeof Tuple9> // $ExpectType [string, string, boolean, string, string, string, string]
+type Tuple9OutputTest = t.OutputOf<typeof Tuple9> // $ExpectType [string, string, boolean, string, string, string, string]
diff --git a/dtslint/ts3.5/tsconfig.json b/dtslint/after3.8/ts3.8/tsconfig.json
similarity index 100%
rename from dtslint/ts3.5/tsconfig.json
rename to dtslint/after3.8/ts3.8/tsconfig.json
diff --git a/dtslint/ts3.5/tslint.json b/dtslint/after3.8/ts3.8/tslint.json
similarity index 100%
rename from dtslint/ts3.5/tslint.json
rename to dtslint/after3.8/ts3.8/tslint.json
diff --git a/dtslint/index.d.ts b/dtslint/base/index.d.ts
similarity index 100%
rename from dtslint/index.d.ts
rename to dtslint/base/index.d.ts
diff --git a/dtslint/base/ts3.5/index.d.ts b/dtslint/base/ts3.5/index.d.ts
new file mode 100644
index 000000000..e69de29bb
diff --git a/dtslint/ts3.5/index.ts b/dtslint/base/ts3.5/index.ts
similarity index 99%
rename from dtslint/ts3.5/index.ts
rename to dtslint/base/ts3.5/index.ts
index 69c1cbc19..3f15af6a7 100644
--- a/dtslint/ts3.5/index.ts
+++ b/dtslint/base/ts3.5/index.ts
@@ -1,4 +1,4 @@
-import * as t from '../../src'
+import * as t from '../../../src'
 import { either } from 'fp-ts/lib/Either'
 
 //
@@ -239,9 +239,6 @@ const Tuple7 = t.tuple([t.string, t.number, t.boolean, t.null, t.undefined]) //
 type Tuple7TypeTest = t.TypeOf<typeof Tuple7> // $ExpectType [string, number, boolean, null, undefined]
 type Tuple7OutputTest = t.OutputOf<typeof Tuple7> // $ExpectType [string, number, boolean, null, undefined]
 
-// $ExpectError
-const Tuple8 = t.tuple([t.string, t.string, t.string, t.string, t.string, t.string])
-
 //
 // partial
 //
diff --git a/dtslint/base/ts3.5/tsconfig.json b/dtslint/base/ts3.5/tsconfig.json
new file mode 100644
index 000000000..a83515bca
--- /dev/null
+++ b/dtslint/base/ts3.5/tsconfig.json
@@ -0,0 +1,16 @@
+{
+  "compilerOptions": {
+    "noEmit": true,
+    "strict": true,
+    "noImplicitAny": true,
+    "noImplicitThis": true,
+    "strictNullChecks": true,
+    "strictFunctionTypes": true,
+    "noImplicitReturns": false,
+    "noUnusedLocals": false,
+    "noUnusedParameters": true,
+    "noFallthroughCasesInSwitch": true,
+    "target": "es5",
+    "lib": ["es2015"]
+  }
+}
diff --git a/dtslint/base/ts3.5/tslint.json b/dtslint/base/ts3.5/tslint.json
new file mode 100644
index 000000000..9ce37507e
--- /dev/null
+++ b/dtslint/base/ts3.5/tslint.json
@@ -0,0 +1,19 @@
+{
+  "extends": "dtslint/dtslint.json",
+  "rules": {
+    "semicolon": false,
+    "array-type": false,
+    "no-unnecessary-generics": false,
+    "member-access": false,
+    "no-empty-interface": false,
+    "no-arg": false,
+    "no-object-literal-type-assertion": false,
+    "no-unnecessary-class": false,
+    "radix": false,
+    "no-angle-bracket-type-assertion": false,
+    "object-literal-shorthand": false,
+    "prefer-object-spread": false,
+    "whitespace": false,
+    "use-default-type-parameter": false
+  }
+}
diff --git a/package.json b/package.json
index 36c5d24cb..12c7a2ade 100644
--- a/package.json
+++ b/package.json
@@ -20,7 +20,7 @@
     "build": "npm run clean && tsc && tsc -p tsconfig.es6.json && npm run import-path-rewrite",
     "prepublish": "npm run build",
     "perf": "ts-node perf/index",
-    "dtslint": "dtslint dtslint",
+    "dtslint": "dtslint dtslint/base && dtslint dtslint/after3.8",
     "declaration": "tsc -p declaration/tsconfig.json",
     "mocha": "TS_NODE_CACHE=false mocha -r ts-node/register test/*.ts",
     "doctoc": "doctoc README.md",
diff --git a/src/index.ts b/src/index.ts
index 0af62149d..ea6bcbbb5 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -812,7 +812,7 @@ const getNameFromProps = (props: Props): string =>
     .map((k) => `${k}: ${props[k].name}`)
     .join(', ')
 
-const useIdentity = (codecs: Array<Any>): boolean => {
+const useIdentity = (codecs: readonly Any[]): boolean => {
   for (let i = 0; i < codecs.length; i++) {
     if (codecs[i].encode !== identity) {
       return false
@@ -1431,7 +1431,7 @@ export function intersection<CS extends [Mixed, Mixed, ...Array<Mixed>]>(
 /**
  * @since 1.0.0
  */
-export class TupleType<CS extends Array<Any>, A = any, O = A, I = unknown> extends Type<A, O, I> {
+export class TupleType<CS extends readonly Any[], A = any, O = A, I = unknown> extends Type<A, O, I> {
   /**
    * @since 1.0.0
    */
@@ -1450,52 +1450,38 @@ export class TupleType<CS extends Array<Any>, A = any, O = A, I = unknown> exten
 /**
  * @since 1.5.3
  */
-export interface TupleC<CS extends [Mixed, ...Array<Mixed>]>
+export interface TupleC<CS extends readonly [Mixed, ...Array<Mixed>]>
   extends TupleType<
     CS,
-    CS extends { length: 1 }
+    CS extends { readonly length: 1 }
       ? [TypeOf<CS[0]>]
-      : CS extends { length: 2 }
+      : CS extends { readonly length: 2 }
       ? [TypeOf<CS[0]>, TypeOf<CS[1]>]
-      : CS extends { length: 3 }
+      : CS extends { readonly length: 3 }
       ? [TypeOf<CS[0]>, TypeOf<CS[1]>, TypeOf<CS[2]>]
-      : CS extends { length: 4 }
+      : CS extends { readonly length: 4 }
       ? [TypeOf<CS[0]>, TypeOf<CS[1]>, TypeOf<CS[2]>, TypeOf<CS[3]>]
-      : CS extends { length: 5 }
+      : CS extends { readonly length: 5 }
       ? [TypeOf<CS[0]>, TypeOf<CS[1]>, TypeOf<CS[2]>, TypeOf<CS[3]>, TypeOf<CS[4]>]
-      : unknown,
-    CS extends { length: 1 }
+      : { [K in keyof CS]: CS[K] extends Mixed ? TypeOf<CS[K]> : unknown },
+    CS extends { readonly length: 1 }
       ? [OutputOf<CS[0]>]
-      : CS extends { length: 2 }
+      : CS extends { readonly length: 2 }
       ? [OutputOf<CS[0]>, OutputOf<CS[1]>]
-      : CS extends { length: 3 }
+      : CS extends { readonly length: 3 }
       ? [OutputOf<CS[0]>, OutputOf<CS[1]>, OutputOf<CS[2]>]
-      : CS extends { length: 4 }
+      : CS extends { readonly length: 4 }
       ? [OutputOf<CS[0]>, OutputOf<CS[1]>, OutputOf<CS[2]>, OutputOf<CS[3]>]
-      : CS extends { length: 5 }
+      : CS extends { readonly length: 5 }
       ? [OutputOf<CS[0]>, OutputOf<CS[1]>, OutputOf<CS[2]>, OutputOf<CS[3]>, OutputOf<CS[4]>]
-      : unknown,
+      : { [K in keyof CS]: CS[K] extends Mixed ? OutputOf<CS[K]> : unknown },
     unknown
   > {}
 
 /**
  * @since 1.0.0
  */
-export function tuple<A extends Mixed, B extends Mixed, C extends Mixed, D extends Mixed, E extends Mixed>(
-  codecs: [A, B, C, D, E],
-  name?: string
-): TupleC<[A, B, C, D, E]>
-export function tuple<A extends Mixed, B extends Mixed, C extends Mixed, D extends Mixed>(
-  codecs: [A, B, C, D],
-  name?: string
-): TupleC<[A, B, C, D]>
-export function tuple<A extends Mixed, B extends Mixed, C extends Mixed>(
-  codecs: [A, B, C],
-  name?: string
-): TupleC<[A, B, C]>
-export function tuple<A extends Mixed, B extends Mixed>(codecs: [A, B], name?: string): TupleC<[A, B]>
-export function tuple<A extends Mixed>(codecs: [A], name?: string): TupleC<[A]>
-export function tuple<CS extends [Mixed, ...Array<Mixed>]>(
+export function tuple<CS extends readonly [Mixed, ...Array<Mixed>]>(
   codecs: CS,
   name: string = `[${codecs.map((type) => type.name).join(', ')}]`
 ): TupleC<CS> {