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.

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:"

compiler/semstmts.nim

@@ -985,6 +985,7 @@ proc semConst(c: PContext, n: PNode): PNode =
     var typFlags: TTypeAllowedFlags = {}
 
     # don't evaluate here since the type compatibility check below may add a converter
+    openScope(c)
     var def = semExprWithType(c, a[^1], {efTypeAllowed}, typ)
 
     if def.kind == nkSym and def.sym.kind in {skTemplate, skMacro}:
@@ -1011,6 +1012,7 @@ proc semConst(c: PContext, n: PNode): PNode =
       if c.matchedConcept != nil:
         typFlags.incl taConcept
       typeAllowedCheck(c, a.info, typ, skConst, typFlags)
+    closeScope(c)
 
     if a.kind == nkVarTuple:
       # generate new section from tuple unpacking and embed it into this one

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

tests/vm/tconstscope1.nim

@@ -0,0 +1,5 @@
+# issue #5395
+
+const a = (var b = 3; b)
+echo b #[tt.Error
+     ^ undeclared identifier: 'b']#

tests/vm/tconstscope2.nim

@@ -0,0 +1,5 @@
+const
+  a = (var x = 3; x)
+  # should we allow this?
+  b = x #[tt.Error
+      ^ undeclared identifier: 'x']#