enforce 'var T' produces a view into the first parameter; refs #7373

1 files changed, 7 insertions, 4 deletions

diff --git a/compiler/semexprs.nim b/compiler/semexprs.nim
index f6226ad77..7e7c496e3 100644
--- a/compiler/semexprs.nim
+++ b/compiler/semexprs.nim
@@ -1304,10 +1304,13 @@ proc takeImplicitAddr(c: PContext, n: PNode; isLent: bool): PNode =
   # See RFC #7373, calls returning 'var T' are assumed to
   # return a view into the first argument (if there is one):
   let root = exprRoot(n)
-  if root != nil and root.owner == c.p.owner and
-      root.kind in {skLet, skVar, skTemp} and sfGlobal notin root.flags:
-    localError(n.info, "'$1' escapes its stack frame; context: '$2'" % [
-      root.name.s, renderTree(n, {renderNoComments})])
+  if root != nil and root.owner == c.p.owner:
+    if root.kind in {skLet, skVar, skTemp} and sfGlobal notin root.flags:
+      localError(n.info, "'$1' escapes its stack frame; context: '$2'" % [
+        root.name.s, renderTree(n, {renderNoComments})])
+    elif root.kind == skParam and root.position != 0:
+      localError(n.info, "'$1' is not the first parameter; context: '$2'" % [
+        root.name.s, renderTree(n, {renderNoComments})])
   case n.kind
   of nkHiddenAddr, nkAddr: return n
   of nkHiddenDeref, nkDerefExpr: return n.sons[0]