Interface and TypeAlias should be forbidden in Statement position, currently causing invalid emit or crash

[ajafff]

TypeScript Version: 2.8.0-dev.20180318

Search Terms:

TypeAlias

Code

if (Boolean())
    type T = number;
else
    type U = string;

Expected behavior:

Error: type alias declaration is not allowed in this position.

Actual behavior:

TypeError: Cannot read property 'kind' of undefined
    at Object.isBlock (/Users/klaus/code/wotan/node_modules/typescript/lib/tsc.js:9482:21)
    at emitEmbeddedStatement (/Users/klaus/code/wotan/node_modules/typescript/lib/tsc.js:58158:20)
    at emitIfStatement (/Users/klaus/code/wotan/node_modules/typescript/lib/tsc.js:57400:13)
    at pipelineEmitUnspecified (/Users/klaus/code/wotan/node_modules/typescript/lib/tsc.js:56654:28)
    at pipelineEmitWithHint (/Users/klaus/code/wotan/node_modules/typescript/lib/tsc.js:56525:32)
    at emitNodeWithSourceMap (/Users/klaus/code/wotan/node_modules/typescript/lib/tsc.js:54061:21)
    at pipelineEmitWithSourceMap (/Users/klaus/code/wotan/node_modules/typescript/lib/tsc.js:56513:17)
    at emitNodeWithComments (/Users/klaus/code/wotan/node_modules/typescript/lib/tsc.js:54192:17)
    at pipelineEmitWithComments (/Users/klaus/code/wotan/node_modules/typescript/lib/tsc.js:56505:17)
    at emitNodeWithNotification (/Users/klaus/code/wotan/node_modules/typescript/lib/tsc.js:53768:21)

Playground Link: http://www.typescriptlang.org/play/#src=if%20(Boolean())%0A%20%20%20%20type%20T%20%3D%20number%3B%0Aelse%0A%20%20%20%20type%20U%20%3D%20string%3B%0A

Interface

Code

if (Boolean())
    interface I {
        a: number
    }
else
    interface I {
        b: string;
    }

Expected behavior:

Error: interface declaration is not allowed in this position.

Actual behavior:

Invalid emit:

if (Boolean())
else

Playground Link: http://www.typescriptlang.org/play/#src=if%20(Boolean())%0A%20%20%20%20interface%20I%20%7B%0A%20%20%20%20%20%20%20%20a%3A%20number%0A%20%20%20%20%7D%0Aelse%0A%20%20%20%20interface%20I%20%7B%0A%20%20%20%20%20%20%20%20b%3A%20string%3B%0A%20%20%20%20%7D%0A

Related Issues:
#22682

IMO TypeAliasDeclaration and InterfaceDeclaration should be forbidden in that position to align with ClassDeclaration (once #22682 is fixed).
Otherwise the types are visible outside of the containing statement. That's pretty unintuitive, because users might think they can conditionally declare interfaces (see my interface code snippet), but in fact they are merged.

[RyanCavanaugh]

This no longer crashes; local type aliases are now allowed.