Fix #12517 Allow single branch when nimvm statements (#20577)

Allow single branch when statements

(cherry picked from commit 2102e3b02f)

compiler/semexprs.nim

@@ -2418,8 +2418,8 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
   #   ...
   var whenNimvm = false
   var typ = commonTypeBegin
-  if n.len == 2 and n[0].kind == nkElifBranch and
-      n[1].kind == nkElse:
+  if n.len in 1..2 and n[0].kind == nkElifBranch and (
+      n.len == 1 or n[1].kind == nkElse):
     let exprNode = n[0][0]
     if exprNode.kind == nkIdent:
       whenNimvm = lookUp(c, exprNode).magic == mNimvm
@@ -2457,7 +2457,10 @@ proc semWhen(c: PContext, n: PNode, semCheck = true): PNode =
     else: illFormedAst(n, c.config)
   if result == nil:
     result = newNodeI(nkEmpty, n.info)
-  if whenNimvm: result.typ = typ
+  if whenNimvm:
+    result.typ = typ
+    if n.len == 1:
+      result.add(newTree(nkElse, newNode(nkStmtList)))
 
 proc semSetConstr(c: PContext, n: PNode): PNode =
   result = newNodeI(nkCurly, n.info)

tests/whenstmt/t12517.nim

@@ -0,0 +1,21 @@
+# Test based on issue #12517
+
+discard """
+  nimout: '''
+nimvm
+both
+'''
+  output: '''
+both
+'''
+"""
+
+proc test() =
+  when nimvm:
+    echo "nimvm"
+  echo "both"
+
+static:
+  test()
+test()
+