Fix #13093 C++ Atomics: operator= is implicitly deleted because the default definition would be ill-formed (#21169)

* add test

* fix #17982 Invalid C++ code generation when returning discardable var T

* fix #13093

* cpp atomic good example

* clearify the condition

compiler/cgen.nim

@@ -491,6 +491,10 @@ proc constructLoc(p: BProc, loc: var TLoc, isTemp = false) =
       nilLoc.r = rope("NIM_NIL")
       genRefAssign(p, loc, nilLoc)
     else:
+      if typ.kind == tyVar:
+        let tt = typ.skipTypes({tyVar})
+        if isImportedType(tt) and tfRequiresInit in tt.flags:
+          return
       linefmt(p, cpsStmts, "$1 = ($2)0;$n", [rdLoc(loc),
         getTypeDesc(p.module, typ, mapTypeChooser(loc))])
   else:

compiler/semstmts.nim

@@ -717,7 +717,11 @@ proc semVarOrLet(c: PContext, n: PNode, symkind: TSymKind): PNode =
                                           tyUserTypeClassInst})
         if actualType.kind in {tyObject, tyDistinct} and
            actualType.requiresInit:
-          defaultConstructionError(c, v.typ, v.info)
+           # imported type use requiresInit pragma prevent implicit initialization
+          if (tfRequiresInit in actualType.flags and sfImportc in actualType.sym.flags):
+            discard
+          else:
+            defaultConstructionError(c, v.typ, v.info)
         else:
           checkNilable(c, v)
         # allow let to not be initialised if imported from C: