fixes #16331; aliasing of tuple construction within a single assignme… (

#22113)

* fixes #16331; aliasing of tuple construction within a single assignment, great coding style

* added test case

compiler/ccgexprs.nim

@@ -2679,15 +2679,30 @@ proc genTupleConstr(p: BProc, n: PNode, d: var TLoc) =
  if not handleConstExpr(p, n, d):
  let t = n.typ
  discard getTypeDesc(p.module, t) # so that any fields are initialized
- if d.k == locNone: getTemp(p, t, d)
+
+ var tmp: TLoc
+ # bug #16331
+ let doesAlias = lhsDoesAlias(d.lode, n)
+ let dest = if doesAlias: addr(tmp) else: addr(d)
+ if doesAlias:
+ getTemp(p, n.typ, tmp)
+ elif d.k == locNone:
+ getTemp(p, n.typ, d)
+
  for i in 0..<n.len:
  var it = n[i]
  if it.kind == nkExprColonExpr: it = it[1]
- initLoc(rec, locExpr, it, d.storage)
- rec.r = "$1.Field$2" % [rdLoc(d), rope(i)]
+ initLoc(rec, locExpr, it, dest[].storage)
+ rec.r = "$1.Field$2" % [rdLoc(dest[]), rope(i)]
  rec.flags.incl(lfEnforceDeref)
  expr(p, it, rec)
 
+ if doesAlias:
+ if d.k == locNone:
+ d = tmp
+ else:
+ genAssignment(p, d, tmp, {})
+
 proc isConstClosure(n: PNode): bool {.inline.} =
  result = n[0].kind == nkSym and isRoutine(n[0].sym) and
  n[1].kind == nkNilLit

tests/ccgbugs/twrong_tupleconv.nim

@@ -1,3 +1,8 @@
+discard """
+ targets: "c cpp"
+ matrix: "--gc:refc; --gc:arc"
+"""
+
 # bug #1833
 iterator myitems*[T](a: var seq[T]): var T {.inline.} =
  ## iterates over each item of `a` so that you can modify the yielded value.
@@ -18,3 +23,13 @@ var ys = @[(1,"a"),(2,"b"),(3,"c")]
 for y in myitems(ys):
  inc y[0]
 
+# bug #16331
+type T1 = tuple[a, b: int]
+
+proc p(b: bool): string =
+ var x: T1 = (10, 20)
+ x = if b: (x.b, x.a) else: (-x.b, -x.a)
+ $x
+
+assert p(false) == "(a: -20, b: -10)"
+assert p(true) == "(a: 20, b: 10)"