Merge pull request #10333 from jwbay/better-type-as-value-error

Add clearer error message when types are used as values

src/compiler/checker.ts

@@ -877,7 +877,8 @@ namespace ts {
                 if (nameNotFoundMessage) {
                     if (!errorLocation ||
                         !checkAndReportErrorForMissingPrefix(errorLocation, name, nameArg) &&
-                        !checkAndReportErrorForExtendingInterface(errorLocation)) {
+                        !checkAndReportErrorForExtendingInterface(errorLocation) &&
+                        !checkAndReportErrorForUsingTypeAsValue(errorLocation, name, meaning)) {
                         error(errorLocation, nameNotFoundMessage, typeof nameArg === "string" ? nameArg : declarationNameToString(nameArg));
                     }
                 }
@@ -987,6 +988,16 @@ namespace ts {
             }
         }
 
+        function checkAndReportErrorForUsingTypeAsValue(errorLocation: Node, name: string, meaning: SymbolFlags): boolean {
+            if (meaning & (SymbolFlags.Value & ~SymbolFlags.NamespaceModule)) {
+                const symbol = resolveSymbol(resolveName(errorLocation, name, SymbolFlags.Type & ~SymbolFlags.Value, /*nameNotFoundMessage*/undefined, /*nameArg*/ undefined));
+                if (symbol && !(symbol.flags & SymbolFlags.NamespaceModule)) {
+                    error(errorLocation, Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_value_here, name);
+                    return true;
+                }
+            }
+            return false;
+        }
 
         function checkResolvedBlockScopedVariable(result: Symbol, errorLocation: Node): void {
             Debug.assert((result.flags & SymbolFlags.BlockScopedVariable) !== 0);

src/compiler/diagnosticMessages.json

@@ -1947,6 +1947,10 @@
         "category": "Error",
         "code": 2692
     },
+    "'{0}' only refers to a type, but is being used as a value here.": {
+        "category": "Error",
+        "code": 2693
+    },
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",
         "code": 4000