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

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]

tests/varres/twrong_parameter.nim

@@ -0,0 +1,12 @@
+discard """
+  line: 6
+  errormsg: "'x' is not the first parameter; context: 'x'"
+"""
+
+proc forward(abc: int; x: var int): var int = result = x
+
+proc foo(): var int =
+  var y = 9
+  result = forward(45, y)
+
+echo foo()