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)"