From a76f36b4e511f2131a9b04cbcb00f8f35f4bac1b Mon Sep 17 00:00:00 2001
From: Can Lehmann <85876381+can-lehmann@users.noreply.github.com>
Date: Mon, 17 Oct 2022 08:01:53 +0200
Subject: [PATCH] Fix #12517 Allow single branch when nimvm statements (#20577)

Allow single branch when statements

(cherry picked from commit 2102e3b02f88e006494d66fbe474161bc151a1dc)
---
 compiler/semexprs.nim     |  9 ++++++---
 tests/whenstmt/t12517.nim | 21 +++++++++++++++++++++
 2 files changed, 27 insertions(+), 3 deletions(-)
 create mode 100644 tests/whenstmt/t12517.nim

diff --git a/compiler/semexprs.nim b/compiler/semexprs.nim
index 85327af201..2c1d40ff25 100644
--- a/compiler/semexprs.nim
+++ b/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)
diff --git a/tests/whenstmt/t12517.nim b/tests/whenstmt/t12517.nim
new file mode 100644
index 0000000000..8be0171e1b
--- /dev/null
+++ b/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()
+