ensure the Nim compiler works with --experimental:strictFuncs --exper…

…imental:views [backport:1.4] (#15737)

(cherry picked from commit cf01945)

compiler/varpartitions.nim

@@ -545,6 +545,9 @@ proc borrowingCall(c: var Partitions; destType: PType; n: PNode; i: int) =
     localError(c.config, n[i].info, "cannot determine the target of the borrow")
 
 proc borrowingAsgn(c: var Partitions; dest, src: PNode) =
+  proc mutableParameter(n: PNode): bool {.inline.} =
+    result = n.kind == nkSym and n.sym.kind == skParam and n.sym.typ.kind == tyVar
+
   if dest.kind == nkSym:
     if directViewType(dest.typ) != noView:
       borrowFrom(c, dest.sym, src)
@@ -559,7 +562,11 @@ proc borrowingAsgn(c: var Partitions; dest, src: PNode) =
         if vid >= 0:
           c.s[vid].flags.incl viewDoesMutate
     of immutableView:
-      localError(c.config, dest.info, "attempt to mutate a borrowed location from an immutable view")
+      if dest.kind == nkBracketExpr and dest[0].kind == nkHiddenDeref and
+          mutableParameter(dest[0][0]):
+        discard "remains a mutable location anyhow"
+      else:
+        localError(c.config, dest.info, "attempt to mutate a borrowed location from an immutable view")
     of noView: discard "nothing to do"
 
 proc containsPointer(t: PType): bool =

tests/views/tcan_compile_nim.nim

@@ -0,0 +1,4 @@
+discard """
+  cmd: "nim check --hints:on --experimental:strictFuncs --experimental:views compiler/nim.nim"
+  action: "compile"
+"""

tests/views/tdont_mutate.nim

@@ -46,3 +46,14 @@ proc main() =
     echo i, ": ", x
 
 main()
+
+# This has to continue to work:
+
+type
+  PNode = ref object
+  TSrcGen = object
+    comStack: seq[PNode]
+
+proc pushCom(g: var TSrcGen, n: PNode) =
+  setLen(g.comStack, g.comStack.len + 1)
+  g.comStack[^1] = n