fix(45221): suggest unknown symbols in type positions (#45323)

Author: a-tarasyuk

src/services/completions.ts

@@ -3309,10 +3309,9 @@ namespace ts.Completions {
     /** True if symbol is a type or a module containing at least one type. */
     function symbolCanBeReferencedAtTypeLocation(symbol: Symbol, checker: TypeChecker, seenModules = new Map<SymbolId, true>()): boolean {
         const sym = skipAlias(symbol.exportSymbol || symbol, checker);
-        return !!(sym.flags & SymbolFlags.Type) ||
-            !!(sym.flags & SymbolFlags.Module) &&
-            addToSeen(seenModules, getSymbolId(sym)) &&
-            checker.getExportsOfModule(sym).some(e => symbolCanBeReferencedAtTypeLocation(e, checker, seenModules));
+        return !!(sym.flags & SymbolFlags.Type) || checker.isUnknownSymbol(sym) ||
+            !!(sym.flags & SymbolFlags.Module) && addToSeen(seenModules, getSymbolId(sym)) &&
+                checker.getExportsOfModule(sym).some(e => symbolCanBeReferencedAtTypeLocation(e, checker, seenModules));
     }
 
     function isDeprecated(symbol: Symbol, checker: TypeChecker) {

tests/cases/fourslash/completionsNonExistentImport.ts

@@ -0,0 +1,9 @@
+/// <reference path="fourslash.ts" />
+
+////import { NonExistentType } from "non-existent-module";
+////let foo: /**/
+
+verify.completions({
+    marker: "",
+    includes: ["NonExistentType"]
+});

tests/cases/fourslash/nonExistingImport.ts

@@ -5,4 +5,4 @@
 ////    var n: num/*1*/
 ////}
 
-verify.completions({ marker: "1", exact: completion.globalTypes });
+verify.completions({ marker: "1", exact: ["foo", ...completion.globalTypes] });