Encode the conditional presence of 'error' in the type system.

Author: DanielRosenwasser

src/compiler/commandLineParser.ts

@@ -327,11 +327,8 @@ namespace ts {
                                 if (hasProperty(map, key)) {
                                     options[opt.name] = map[key];
                                 }
-                                else if (opt.error) {
-                                    errors.push(createCompilerDiagnostic(opt.error));
-                                }
                                 else {
-                                    Debug.fail(`Command line option for '${opt.name}' doesn't account for invalid options.`);
+                                    errors.push(createCompilerDiagnostic((<CommandLineOptionOfCustomType>opt).error));
                                 }
                         }
                     }
@@ -444,7 +441,7 @@ namespace ts {
                                     value = optType[key];
                                 }
                                 else {
-                                    errors.push(createCompilerDiagnostic(opt.error));
+                                    errors.push(createCompilerDiagnostic((<CommandLineOptionOfCustomType>opt).error));
                                     value = 0;
                                 }
                             }

src/compiler/types.ts

@@ -2112,17 +2112,30 @@ namespace ts {
     }
 
     /* @internal */
-    export interface CommandLineOption {
+    interface CommandLineOptionBase {
         name: string;
         type: string | Map<number>;         // "string", "number", "boolean", or an object literal mapping named values to actual values
         isFilePath?: boolean;               // True if option value is a path or fileName
         shortName?: string;                 // A short mnemonic for convenience - for instance, 'h' can be used in place of 'help'
         description?: DiagnosticMessage;    // The message describing what the command line switch does
         paramType?: DiagnosticMessage;      // The name to be used for a non-boolean option's parameter
-        error?: DiagnosticMessage;          // The error given when the argument does not fit a customized 'type'
         experimental?: boolean;
     }
 
+    /* @internal */
+    export interface CommandLineOptionOfPrimitiveType extends CommandLineOptionBase {
+        type: string;                   // "string" | "number" | "boolean"
+    }
+
+    /* @internal */
+    export interface CommandLineOptionOfCustomType extends CommandLineOptionBase {
+        type: Map<number>;             // an object literal mapping named values to actual values
+        error: DiagnosticMessage;      // The error given when the argument does not fit a customized 'type'
+    }
+
+    /* @internal */
+    export type CommandLineOption = CommandLineOptionOfCustomType | CommandLineOptionOfPrimitiveType;
+
     /* @internal */
     export const enum CharacterCodes {
         nullCharacter = 0,