borrowchecks: fix parameter borrow overlap check

Only mutable borrows overlapping with other borrows is a problem.

compiler/sem/borrowchecks.nim

@@ -1081,7 +1081,8 @@ proc verifyParamBorrow(config: ConfigRef, c: ControlFlowGraph, start: int) =
     of borrow, mborrow:
       if instr.borrower.kind in nkCallKinds and instr.borrower != target:
         discard "ignore other call's borrows; the mut/use instruction are what's relevant"
-      elif overlaps(path, instr.n):
+      elif overlaps(path, instr.n) and mborrow in {c[start].kind, instr.kind}:
+        # only mutable borrows overlapping with other borrows is a problem
         config.localReport(instr.n):
           SemReport(kind: rsemOverlappingParamBorrows, usage: path.info,
                     isProblemMutation: c[start].kind == mborrow)

tests/lang_experimental/views/tborrow_checking_3.nim

@@ -7,6 +7,16 @@ proc error() =
   var s = @[1]
   a(s, s) # error; mutable borrows of the same location
 
+proc no_error() =
+  type Container = ref object
+    children: seq[Container]
+
+  proc p(a, b: Container) = discard
+
+  var c = Container(children: @[Container()])
+  p(c, c.children[0])
+  # not an error, even though this is a dangerous borrow of c.children[0]
+
 proc error_2() =
   type Container = object
     children: seq[Container]