isArray type guards still causing issues in latest nightly build

Bug Report

🔎 Search Terms

• isArray

🕗 Version & Regression Information

Tested in 4.3.0-dev.20210211 which

💻 Code

In the VS Code codebase, we are still seeing the isArray type guards causing problems with the latest TS 4.3 nightly build.

Here's an example from this file

The type of DocumentSelector is string | vscode.DocumentFilter | readonly (string | vscode.DocumentFilter)[]

	export function from(selector: vscode.DocumentSelector | undefined): languageSelector.LanguageSelector | undefined {
		if (!selector) {
			return undefined;
		} else if (Array.isArray(selector)) {
			return <languageSelector.LanguageSelector>selector.map(from);
		} else if (typeof selector === 'string') {
			return selector;
		} else {
			return <languageSelector.LanguageFilter>{
				language: selector.language,
				scheme: selector.scheme,
				pattern: typeof selector.pattern === 'undefined' ? undefined : GlobPattern.from(selector.pattern),
				exclusive: selector.exclusive
			};
		}
	}

The error in the else block on the line language: selector.language:

[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/workbench/api/common/extHostTypeConverters.ts(1284,24): Property 'language' does not exist on type 'DocumentFilter | readonly (string | DocumentFilter)[]'.
[watch-client    ]   Property 'language' does not exist on type 'readonly (string | DocumentFilter)[]'.

These errors did not show up with typescript@4.2.0-dev.20201207.

Here are the other errors:

[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/workbench/services/extensions/common/abstractExtensionService.ts(545,58): Argument of type 'ILocalization[] | IAuthenticationContribution[] | IDebugger[] | { [location: string]: IView[]; } | ISnippet[] | IConfiguration | ... 12 more ... | undefined' is not assignable to parameter of type 'T'.
[watch-client    ]   'T' could be instantiated with an arbitrary type which could be unrelated to 'ILocalization[] | IAuthenticationContribution[] | IDebugger[] | { [location: string]: IView[]; } | ISnippet[] | IConfiguration | ... 12 more ... | undefined'.
[watch-client    ]     Type 'undefined' is not assignable to type 'T'.
[watch-client    ]       'T' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/workbench/services/extensions/common/abstractExtensionService.ts(700,6): Type 'ILocalization[] | IAuthenticationContribution[] | IDebugger[] | { [location: string]: IView[]; } | ISnippet[] | IConfiguration | ... 12 more ... | undefined' is not assignable to type 'T'.
[watch-client    ]   'T' could be instantiated with an arbitrary type which could be unrelated to 'ILocalization[] | IAuthenticationContribution[] | IDebugger[] | { [location: string]: IView[]; } | ISnippet[] | IConfiguration | ... 12 more ... | undefined'.
[watch-client    ]     Type 'undefined' is not assignable to type 'T'.
[watch-client    ]       'T' could be instantiated with an arbitrary type which could be unrelated to 'undefined'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/base/browser/ui/dropdown/dropdownActionViewItem.ts(175,99): Property 'getActions' does not exist on type 'readonly IAction[] | IActionProvider'.
[watch-client    ]   Property 'getActions' does not exist on type 'readonly IAction[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/workbench/api/common/extHostTypeConverters.ts(1284,24): Property 'language' does not exist on type 'DocumentFilter | readonly (string | DocumentFilter)[]'.
[watch-client    ]   Property 'language' does not exist on type 'readonly (string | DocumentFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/workbench/api/common/extHostTypeConverters.ts(1285,22): Property 'scheme' does not exist on type 'DocumentFilter | readonly (string | DocumentFilter)[]'.
[watch-client    ]   Property 'scheme' does not exist on type 'readonly (string | DocumentFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/workbench/api/common/extHostTypeConverters.ts(1286,30): Property 'pattern' does not exist on type 'DocumentFilter | readonly (string | DocumentFilter)[]'.
[watch-client    ]   Property 'pattern' does not exist on type 'readonly (string | DocumentFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/workbench/api/common/extHostTypeConverters.ts(1286,94): Property 'pattern' does not exist on type 'DocumentFilter | readonly (string | DocumentFilter)[]'.
[watch-client    ]   Property 'pattern' does not exist on type 'readonly (string | DocumentFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/workbench/api/common/extHostTypeConverters.ts(1287,25): Property 'exclusive' does not exist on type 'DocumentFilter | readonly (string | DocumentFilter)[]'.
[watch-client    ]   Property 'exclusive' does not exist on type 'readonly (string | DocumentFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/workbench/api/common/extHost.api.impl.ts(201,26): Property 'scheme' does not exist on type 'DocumentFilter | readonly (string | DocumentFilter)[]'.
[watch-client    ]   Property 'scheme' does not exist on type 'readonly (string | DocumentFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/workbench/api/common/extHost.api.impl.ts(204,58): Property 'exclusive' does not exist on type 'DocumentFilter | readonly (string | DocumentFilter)[]'.
[watch-client    ]   Property 'exclusive' does not exist on type 'readonly (string | DocumentFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/editor/common/modes/languageSelector.ts(58,11): Property 'language' does not exist on type 'LanguageFilter | readonly (string | LanguageFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/editor/common/modes/languageSelector.ts(58,21): Property 'pattern' does not exist on type 'LanguageFilter | readonly (string | LanguageFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/editor/common/modes/languageSelector.ts(58,30): Property 'scheme' does not exist on type 'LanguageFilter | readonly (string | LanguageFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/editor/common/modes/languageSelector.ts(58,38): Property 'hasAccessToAllModels' does not exist on type 'LanguageFilter | readonly (string | LanguageFilter)[]'.
[watch-client    ] [13:45:27] Error: /Users/matb/projects/vscode/src/vs/editor/common/modes/languageFeatureRegistry.ts(28,21): Property 'exclusive' does not exist on type 'LanguageFilter | readonly (string | LanguageFilter)[]'.
[watch-client    ]   Property 'exclusive' does not exist on type 'readonly (string | LanguageFilter)[]'.

@orta @RyanCavanaugh I believed that https://github.com/microsoft/TypeScript/pull/41851 was supposed to fix these issues. Can you please take a look at our errors and me know if we need to change anything on the VS Code side or if these are unexpected errors