open scope for each when nimvm branch

fixes #23687, fixes #23688, refs #13450

compiler/semexprs.nim

@@ -2727,7 +2727,9 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
       checkSonsLen(it, 2, c.config)
       if whenNimvm:
         if semCheck:
+          openScope(c)
           it[1] = semExpr(c, it[1], flags)
+          closeScope(c)
           typ = commonType(c, typ, it[1].typ)
         result = n # when nimvm is not elimited until codegen
       elif c.inGenericContext > 0:
@@ -2758,10 +2760,12 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
         discard
       elif result == nil or whenNimvm:
         if semCheck:
+          if whenNimvm: openScope(c)
           it[0] = semExpr(c, it[0], flags)
           typ = commonType(c, typ, it[0].typ)
           if typ != nil and typ.kind != tyUntyped:
             it[0] = fitNode(c, typ, it[0], it[0].info)
+          if whenNimvm: closeScope(c)
         if result == nil:
           result = it[0]
     else: illFormedAst(n, c.config)

doc/manual.md

@@ -3466,7 +3466,7 @@ A `when nimvm` statement must meet the following requirements:
 * It must contain an `else` branch.
 * Code in branches must not affect semantics of the code that follows the
   `when nimvm` statement. E.g. it must not define symbols that are used in
-  the following code.
+  the following code. A new scope is opened for each branch to prevent this.
 
 Return statement
 ----------------

tests/vm/twhennimvmscope1.nim

@@ -0,0 +1,13 @@
+# issue #23687
+
+when nimvm:
+  proc mytest(a: int) =
+    echo a
+else:
+  template mytest(a: int) =
+    echo a + 42
+
+
+proc xxx() =
+  mytest(100) #[tt.Error
+  ^ undeclared identifier: 'mytest']#

tests/vm/twhennimvmscope2.nim

@@ -0,0 +1,14 @@
+# issue #23688
+
+when nimvm:
+  proc mytest(a: int) =
+    echo a
+else:
+  template mytest(a: untyped) =
+    echo a + 42
+
+
+proc xxx() =
+  mytest(100) #[tt.Error
+  ^ undeclared identifier: 'mytest']#
+xxx()

tests/vm/twhennimvmscope3.nim

@@ -0,0 +1,10 @@
+# issue #13450 example 3
+
+proc bar() =
+  when nimvm:
+    let y = 1
+  else:
+    let y = 2
+  discard y #[tt.Error
+          ^ undeclared identifier: 'y']#
+bar()