Make exportSymbol public

[ghost]

#16742 changed the behavior of getSymbolsInScope to return local symbols and not exported symbols.

This means that in:

namespace N {
    export type T = number;
    export const x: N.T;
}

The getSymbolAtLocation at T in N.T will return the exported symbol for T.
However, getSymbolsInScope at the same location returns the local symbol for T.

Tslint used to compare getSymbolAtLocation with getSymbolsInScope to determine when a namespace qualifier was unnecessary; this was broken when getSymbolAtLocation started returning local symbols. This was "fixed" by palantir/tslint#3052 which checks every declaration for equality, but a better solution would be to test for .exportSymbol of the local symbol.

We could instead add a checker.getExportedSymbolOfLocalSymbol method if you think that would make for a better API.

[rbuckton]

I don't think we want to make exportSymbol public as we generally pass it through getMergedSymbol before using it ourselves (see getExportSymbolOfValueSymbolIfExported). I'd prefer we added a checker.getExportSymbolOfSymbol that is just a pointer to getExportSymbolOfValueSymbolIfExported.

[rbuckton]

My apologies. getExportSymbolOfValueSymbolIfExported will only get the export symbol if the symbol is a Value symbol, not a Type symbol.

Perhaps this might be more reliable:

function getExportSymbolOfSymbol(symbol: Symbol) {
  return getMergedSymbol(symbol.exportSymbol || symbol);
}

Otherwise, the example in your comment in types.ts isn't strictly valid.

[rbuckton]

It would be nice to have unit tests for this, though we don't currently have much in the way of unit tests for checker's public API surface.