open new scope for const values (#24084)

fixes #5395

Previously values of `const` statements used the same scope as the
`const` statement itself, meaning variables could be declared inside
them and referred to in other statements in the same block. Now each
`const` value opens its own scope, so any variable declared in the value
of a constant can only be accessed for that constant.

We could change this to open a new scope for the `const` *section*
rather than each constant, so the variables can be used in other
constants, but I'm not sure if this is sound.

1 files changed, 10 insertions, 0 deletions

diff --git a/changelog.md b/changelog.md
index 305409fa5..67b6c74f8 100644
--- a/changelog.md
+++ b/changelog.md
@@ -41,6 +41,16 @@
   bar[int]() # before: (1.0, "abc"), now: type mismatch, missing generic parameter
   ```
 
+- `const` values now open a new scope for each constant, meaning symbols
+  declared in them can no longer be used outside or in the value of
+  other constants.
+
+  ```nim
+  const foo = (var a = 1; a)
+  const bar = a # error
+  let baz = a # error
+  ```
+
 ## Standard library additions and changes
 
 [//]: # "Changes:"