Update assertion: symbol in union type may be a Function

src/services/symbolDisplay.ts

@@ -61,7 +61,8 @@ namespace ts.SymbolDisplay {
                     if (rootSymbolFlags & (SymbolFlags.PropertyOrAccessor | SymbolFlags.Variable)) {
                         return ScriptElementKind.memberVariableElement;
                     }
-                    Debug.assert(!!(rootSymbolFlags & SymbolFlags.Method));
+                    // May be a Function if this was from `typeof N` with `namespace N { function f();. }`.
+                    Debug.assert(!!(rootSymbolFlags & (SymbolFlags.Method | SymbolFlags.Function)));
                 });
                 if (!unionPropertyKind) {
                     // If this was union of all methods,

tests/cases/fourslash/quickInfoUnionOfNamespaces.ts

@@ -0,0 +1,15 @@
+// See GH#18461
+
+/// <reference path='fourslash.ts' />
+
+////declare const x: typeof A | typeof B;
+////x./**/f;
+////
+////namespace A {
+////    export function f() {}
+////}
+////namespace B {
+////    export function f() {}
+////}
+
+verify.quickInfoAt("", "(method) f(): void");