microsoft / microsoft/TypeScript
Explicit `never` return type in `catch` block & condition in `finally` marks the whole `finally` block as unreachable
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Description
### 🔎 Search Terms
finally, unreachable, condition, never
### 🕗 Version & Regression Information
- This builds fine in 3.2,
- produces a type error regarding the return type in 3.3 through 3.6,
- and starting 3.7, including the latest native preview, it produces the unreachable code error
### ⏯ Playground Link
https://www.typescriptlang.org/play/?noImplicitAny=false&ts=5.9.3#code/MYewdgzgLgBATgUwA4jlAXDAFAzBDMATwEoYBeAPhjAQDcE5zsFjMb7HKYBvAKBhhQAFnBAB3GAgDcvAL4zeoSLABmIEEzwRCYYNlYwACqIC2ASwgIAPGACuJgEYMqXPgKhxCPfgPgIotnBgMADMMgKyMMB4UMBCzKRuvogoaDjE4TCRKmZgeAA2+V5JAmYq2B62LN6+vkoQIPkIAHT5IADmWABEQghCXRk+ET6yclJAA
### 💻 Code
```ts
function report(e): never {
throw e;
};
const foo = async (): Promise => {
try {
return 3;
} catch (e) {
report(e);
} finally {
if (true) {
console.log("heh");
}
}
};
```
### 🙁 Actual behavior
```
test.ts:11:5 - error TS7027: Unreachable code detected.
11 if (true) {
~~~~~~~~~~~
12 console.log("heh");
~~~~~~~~~~~~~~~~~~~~~~~~~
13 }
~~~~~
test.ts:12:7 - error TS7027: Unreachable code detected.
12 console.log("heh");
~~~~~~~~~~~~~~~~~~~
```
### 🙂 Expected behavior
Builds fine.
### Additional information about the issue
This error vanishes if I do any of the following:
- Remove the explicit `never` return type of `reportError` (although that resurfaces the return type error)
- Remove the call to `reportError` entirely, or the whole `catch` block
- Remove the `if` condition from the `finally` block, but leave in the `console.log` call
I found a similar issue #61259, but it does not include a `catch` block, which is integral to the issue here - although the issues might still be related. Similarly to that issue, the `finally` block also becomes reachable when adding another `return` to the `catch` block, but it is itself unreachable.
Contributor guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
Research direction
Reproduce the diagnostic in the linked TypeScript Playground and compare the behavior across the versions described, especially 3.6 and 3.7. Read related issue #61259 and trace the control-flow handling for the catch and finally blocks; done means the example builds without TS7027 while retaining the explicit never return type.
Written by the indexing model from the issue text.
Assessment
- Tech stack
- typescript
- Domain
- compilers
- Issue type
- Bug
- Difficulty
- 4/5
- Estimated time
- 3-5 days
- Activity status
- Stale
- Clarity
- Clearly specified
- Newbie friendliness
- 48/100