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, 9 insertions, 0 deletions

diff --git a/doc/manual.md b/doc/manual.md
index 693fb7330..5c36a0a7b 100644
--- a/doc/manual.md
+++ b/doc/manual.md
@@ -3220,6 +3220,15 @@ A const section declares constants whose values are constant expressions:
 
 Once declared, a constant's symbol can be used as a constant expression.
 
+The value part of a constant declaration opens a new scope for each constant,
+so no symbols declared in the constant value are accessible outside of it.
+
+  ```nim
+  const foo = (var a = 1; a)
+  const bar = a # error
+  let baz = a # error
+  ```
+
 See [Constants and Constant Expressions] for details.
 
 Static statement/expression