Source

File: wp-includes/class-wp-theme.php

public function get_post_templates() {
		// If you screw up your current theme and we invalidate your parent, most things still work. Let it slide.
		if ( $this->errors() && $this->errors()->get_error_codes() !== array( 'theme_parent_invalid' ) ) {
			return array();
		}

		$post_templates = $this->cache_get( 'post_templates' );

		if ( ! is_array( $post_templates ) ) {
			$post_templates = array();

			$files = (array) $this->get_files( 'php', 1, true );

			foreach ( $files as $file => $full_path ) {
				if ( ! preg_match( '|Template Name:(.*)$|mi', file_get_contents( $full_path ), $header ) ) {
					continue;
				}

				$types = array( 'page' );
				if ( preg_match( '|Template Post Type:(.*)$|mi', file_get_contents( $full_path ), $type ) ) {
					$types = explode( ',', _cleanup_header_comment( $type[1] ) );
				}

				foreach ( $types as $type ) {
					$type = sanitize_key( $type );
					if ( ! isset( $post_templates[ $type ] ) ) {
						$post_templates[ $type ] = array();
					}

					$post_templates[ $type ][ $file ] = _cleanup_header_comment( $header[1] );
				}
			}

			$this->cache_add( 'post_templates', $post_templates );
		}

		if ( $this->load_textdomain() ) {
			foreach ( $post_templates as &$post_type ) {
				foreach ( $post_type as &$post_template ) {
					$post_template = $this->translate_header( 'Template Name', $post_template );
				}
			}
		}

		return $post_templates;
	}