Add a comment in getTypeOfAlias

src/compiler/checker.ts

@@ -2415,6 +2415,12 @@ module ts {
             let links = getSymbolLinks(symbol);
             if (!links.type) {
                 let targetSymbol = resolveAlias(symbol);
+
+                // It only makes sense to get the type of a value symbol. If the result of resolving
+                // the alias is not a value, then it has no type. To get the type associated with a
+                // type symbol, call getDeclaredTypeOfSymbol.
+                // This check is important because without it, a call to getTypeOfSymbol could end
+                // up recursively calling getTypeOfAlias, causing a stack overflow.
                 links.type = targetSymbol.flags & SymbolFlags.Value
                     ? getTypeOfSymbol(targetSymbol)
                     : unknownType;