"Cannot compile namespaces" error with --isolatedModules and no namespaces

TypeScript Version: nightly (2.3.0-dev.20170417)

Code

function f() {}

Expected behavior:

No error, or an error message about a non-module file in an --isolatedModules project.

Actual behavior:

src/a.ts(1,1): error TS1208: Cannot compile namespaces when the '--isolatedModules' flag is provided.

This occurs because we in program.ts verifyCompilerOptions to look for any source file that isn't an external module declaration, and fail on the first one, whatever it may be.
The error message should be upgraded to reflect the real reason we issue this error, and not mention namespaces.
Alternately, we could just allow files without imports and actually look for a namespace before adding this error.

[mhegazy]

The error message needs to be clearer. instead of namespaces it can say non-module. @DanielRosenwasser recommendations?

[haiyamicode]

There should not be an error message in this situation. The --isolatedModules option should just turn off the internal module system completely, if tsc still errors out this way then the option didn't do its job.

[haiyamicode]

In #15839 I intended to modify the checking behavior but later realized that this check is just pointless and the ideal solution is to remove it.
Typescript has always allowed usage of namespace inside an external module with --isolatedModules on:

export function something() {} // export found, so this module is considered 'external'
namespace ns {} // namespace is still valid in an external module

Thus the check isn't doing anything in that (seemingly invalid) case, when it's supposed to complain. So why complain when the programmer isn't even using any namespace, doing nothing wrong?

@wilonth The reason for this issue is that the error message is wrong: namespaces are perfectly fine if part of a (es6) module, but any non-module file should be an error.

[haiyamicode]

@Andy-MS I was trying to point out that the whole code to check for this error is just pointless, and we should just remove it instead of complicating the code more.
My point is, when --isolatedModules is on, every file should be regarded as an external (ES6) module, period. In that sense, usage of Namespaces should be legal because right now, it is perfectly legal to use Namespaces inside ES6 modules (with --isolatedModules on).

[mhegazy]

My point is, when --isolatedModules is on, every file should be regarded as an external (ES6) module, period.

The semantics here is that a module is a file with at least one top-level import or export. it does not matter how you invoke the compiler.

[haiyamicode]

@mhegazy Yes and that rule is exactly the root of the problem we have here. The rule unfairly treats a file without import or export as a part of the legacy internal module system, so --isolatedModules rejects the file.
Now we should not change the rule because that will probably break a lot of things, but we can solve the current issue very easily just by removing this error message.
Let's imagine this "cannot compile namespaces" error message being removed, what could go wrong? What about a codebase that uses internal module, like the Typescript compiler itself? With --isolatedModules on, would it compile to garbage code and confuse the user? No it won't, there would be a lot of Typescript error messages about undefined symbols, because --isolatedModules broke the links between the files, pretty obvious to understand.
Can you find ANY downsides with removing this error message? I couldn't. Why wouldn't we choose the easiest solution, that involves zero work (just remove the lines) and has no downsides?

[RyanCavanaugh]

With --isolatedModules on, would it compile to garbage code and confuse the user? No it won't

Yes, it will.

The point of the --isolatedModules flag is to validate that a program can be successfully compiled through single-file transpilation.

The TypeScript codebase can't be successfully compiled through single-file transpilation. There is code like this everywhere:

file1

namespace ts {
  export var x = 10;
}

file2

namespace ts {
  var y = x; // transpiles to ts.x during whole-program compilation
}

If we single-file transpiled file2, it would break.

[haiyamicode]

My bad, I screwed up my testing. I hereby retract my proposal. Thank you @RyanCavanaugh for reminding.