fix: ensure correct type when providing a dtype option

---
type: pre_commit_static_analysis_report
description: Results of running static analysis checks when committing changes.
report:
  - task: lint_filenames
    status: passed
  - task: lint_editorconfig
    status: passed
  - task: lint_markdown
    status: na
  - task: lint_package_json
    status: na
  - task: lint_repl_help
    status: na
  - task: lint_javascript_src
    status: na
  - task: lint_javascript_cli
    status: na
  - task: lint_javascript_examples
    status: na
  - task: lint_javascript_tests
    status: na
  - task: lint_javascript_benchmarks
    status: na
  - task: lint_python
    status: na
  - task: lint_r
    status: na
  - task: lint_c_src
    status: na
  - task: lint_c_examples
    status: na
  - task: lint_c_benchmarks
    status: na
  - task: lint_c_tests_fixtures
    status: na
  - task: lint_shell
    status: na
  - task: lint_typescript_declarations
    status: passed
  - task: lint_typescript_tests
    status: passed
  - task: lint_license_headers
    status: passed
---

Author: kgryte

lib/node_modules/@stdlib/ndarray/flatten/docs/types/index.d.ts

@@ -20,12 +20,12 @@
 
 /// <reference types="@stdlib/types"/>
 
-import { ndarray, Order, DataType } from '@stdlib/types/ndarray';
+import { ndarray, typedndarray, Order, DataTypeMap } from '@stdlib/types/ndarray';
 
 /**
-* Interface defining function options.
+* Interface defining "base" function options.
 */
-interface Options {
+interface BaseOptions {
 	/**
 	* Maximum number of dimensions to flatten.
 	*
@@ -50,16 +50,47 @@ interface Options {
 	* -   Default: 'row-major'.
 	*/
 	order?: Order | 'same' | 'any';
+}
 
+/**
+* Function options.
+*/
+type Options<U> = BaseOptions & {
 	/**
 	* Output ndarray data type.
-	*
-	* ## Notes
-	*
-	* -   By default, the function returns an ndarray having the same data type as a provided input ndarray.
 	*/
-	dtype?: DataType;
-}
+	dtype: U;
+};
+
+/**
+* Returns a flattened copy of an input ndarray.
+*
+* ## Notes
+*
+* -   The function **always** returns a copy of input ndarray data, even when an input ndarray already has the desired number of dimensions.
+* -   By default, the function returns an ndarray having the same data type as a provided input ndarray.
+*
+* @param x - input ndarray
+* @param options - function options
+* @param options.depth - maximum number of dimensions to flatten
+* @param options.order - order in which input ndarray elements should be flattened
+* @param options.dtype - output ndarray data type
+* @returns output ndarray
+*
+* @example
+* var array = require( '@stdlib/ndarray/array' );
+* var ndarray2array = require( '@stdlib/ndarray/to-array' );
+*
+* var x = array( [ [ [ 1.0, 2.0 ] ], [ [ 3.0, 4.0 ] ], [ [ 5.0, 6.0 ] ] ] );
+* // return <ndarray>
+*
+* var y = flatten( x );
+* // returns <ndarray>
+*
+* var arr = ndarray2array( y );
+* // returns [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ]
+*/
+declare function flatten<T extends ndarray>( x: T, options?: BaseOptions ): T;
 
 /**
 * Returns a flattened copy of an input ndarray.
@@ -72,6 +103,7 @@ interface Options {
 * @param options - function options
 * @param options.depth - maximum number of dimensions to flatten
 * @param options.order - order in which input ndarray elements should be flattened
+* @param options.dtype - output ndarray data type
 * @returns output ndarray
 *
 * @example
@@ -87,7 +119,7 @@ interface Options {
 * var arr = ndarray2array( y );
 * // returns [ 1.0, 2.0, 3.0, 4.0, 5.0, 6.0 ]
 */
-declare function flatten<T extends ndarray>( x: T, options?: Options ): T;
+declare function flatten<T = unknown, U extends keyof DataTypeMap<T> = 'generic'>( x: typedndarray<T>, options: Options<U> ): DataTypeMap<T>[U];
 
 
 // EXPORTS //

lib/node_modules/@stdlib/ndarray/flatten/docs/types/test.ts

@@ -31,6 +31,10 @@ import flatten = require( './index' );
 	flatten( zeros( 'float64', [ 2, 2, 2 ], 'row-major' ), {} ); // $ExpectType float64ndarray
 	flatten( zeros( 'complex128', [ 2, 2, 2 ], 'row-major' ), {} ); // $ExpectType complex128ndarray
 	flatten( zeros( 'generic', [ 2, 2, 2 ], 'row-major' ), {} ); // $ExpectType genericndarray<number>
+
+	flatten( zeros( 'float64', [ 2, 2, 2 ], 'row-major' ), { 'dtype': 'float32' } ); // $ExpectType float32ndarray
+	flatten( zeros( 'int32', [ 2, 2, 2 ], 'row-major' ), { 'dtype': 'float64' } ); // $ExpectType float64ndarray
+	flatten( zeros( 'int32', [ 2, 2, 2 ], 'row-major' ), { 'dtype': 'generic' } ); // $ExpectType genericndarray<number>
 }
 
 // The compiler throws an error if the function is provided a first argument which is not an ndarray-like object...