Corsa differences in `export=` module augmentation

[RyanCavanaugh]

## Corsa changes how module augmentation interacts with export-equals and exported name visibility
augmentExportEquals2.errors.txt.diff
exportAssignmentMembersVisibleInAugmentation.errors.txt.diff

[ahejlsberg]

The issue in augmentExportEquals2.errors.txt.diff is actually that the test is broken. There are two // @filename: file3.ts in a row, which we apparently no longer handle.

[apostate0]

So from what I found guys augmentExportEquals2.errors.txt.diff isn't a real bug like ahejlsberg mentioned, there's a duplicate // @filename: file3.ts in the fixture which the harness no longer handles resulting in in the second file3 getting wiped completely so I wanted to ask why we're using duplicate directives? if it's necessary we should add the harness tolerate it or just drop it completely if not.

Another thing is exportAssignmentMembersVisibleInAugmentation.errors.txt.diff is the real semantic regression, tsgo emits TS4060 on bare T in declare module "foo" { export function f(): T; } but foo.T in the sibling test is fine, meaning the unqualified lookup inside the augmentation body can't see the merged namespace's members. Same root cause as microsoft/typescript-go#3571. Most likely the augmentation merge at internal/checker/checker.go:1409-1438 isn't propagating the upstream namespace's bindings into the augmentation body's in-scope symbol tables, so getAccessibleSymbolChain returns an empty chain and the declaration-emit tracker drops both the diagnostic and the a.d.ts.

[hesam-oxe]

I can investigate this. 𝑨𝖕𝖔𝖘𝖙𝖆𝖙𝖊† (@apostate0)'s analysis points to
checker.go:1409-1438 — the augmentation merge isn't propagating
upstream namespace bindings into the augmentation body's symbol tables.

I'll trace through getAccessibleSymbolChain and the merge logic
to fix the binding propagation.

/assign

[hesam-oxe]

Ryan Cavanaugh (@RyanCavanaugh) Anders Hejlsberg (@ahejlsberg) 𝑨𝖕𝖔𝖘𝖙𝖆𝖙𝖊† (@apostate0)

I've opened PR microsoft/typescript-go#4291 with the fix. The root cause was that
mergeModuleAugmentation wasn't propagating the main module's
bindings into the augmentation's symbol table.

Four changes:

1. Copy main module exports into augmentation before merge
2. Initialize target.Exports before mergeSymbolTable
3. Include parent exports in getResolvedMembersOrExportsOfSymbol
4. Recursive parent search in getSymbol

Based on 𝑨𝖕𝖔𝖘𝖙𝖆𝖙𝖊† (@apostate0)'s analysis — the binding propagation gap
was correctly identified. 🙏

[goldserg]

Real-world instance of this, in case it helps scope the fix вАФ augmenting
PlatformPath from @types/node silently stops applying in 7.0.2.

Repro

aug.d.ts:

type AbsolutePath = string & {__type: 'path'; __mode: 'absolute'};

declare module 'node:path' {
    interface PlatformPath {
        resolve(...paths: string[]): AbsolutePath;
    }
}

use.ts:

import {resolve} from 'node:path';

const r: AbsolutePath = resolve('a', 'b');

tsconfig.json:

{
  "compilerOptions": {
    "module": "esnext",
    "moduleResolution": "bundler",
    "strict": true,
    "skipLibCheck": true,
    "noEmit": true,
    "types": ["node"]
  },
  "include": ["aug.d.ts", "use.ts"]
}

Expected

resolve() returns AbsolutePath; the project compiles cleanly, as it does on 6.0.3.

Actual

use.ts(3,25): error TS2322: Type 'string' is not assignable to type 'AbsolutePath'.

The augmentation is silently ignored and the declared return type stays string.

Versions

Compiler	Result
5.9.3	works
6.0.3	works
7.0.2	fails

Notes

• Not resolution-dependent: reproduces identically with bundler, node16 and nodenext.
• Not specifier-dependent: the same failure occurs when augmenting 'path' instead of
  'node:path', with or without the namespace path { ... } wrapper, and whether the
  augmenting file is a global script or a module (export {}).
• In @types/node, node:path is an alias (import path = require("path"); export = path)
  and PlatformPath lives in the path namespace, so the merge has to pass through
  export =. That is what appears to have stopped working.
• Unlike fix: Corsa differences in export= module augmentation typescript-go#4446, this is not a declaration-emit visibility problem вАФ no TS4060 is
  involved and the failure happens during plain type checking with --noEmit.
• Real-world impact: a codebase using branded path types via this augmentation lost them
  everywhere at once вАФ ~105 errors, since every join/resolve result silently degraded
  to string.