Always check the deduced type validity for result

Fixes #8259

compiler/semexprs.nim

@@ -1560,6 +1560,8 @@ proc semAsgn(c: PContext, n: PNode; mode=asgnNormal): PNode =
           rhsTyp = rhsTyp.lastSon
         if cmpTypes(c, lhs.typ, rhsTyp) in {isGeneric, isEqual}:
           internalAssert c.config, c.p.resultSym != nil
+          # Make sure the type is valid for the result variable
+          typeAllowedCheck(c.config, n.info, rhsTyp, skResult)
           lhs.typ = rhsTyp
           c.p.resultSym.typ = rhsTyp
           c.p.owner.typ.sons[0] = rhsTyp

tests/openarray/t8259.nim

@@ -0,0 +1,7 @@
+discard """
+  line: 6
+  errormsg: "invalid type: 'openarray[int]' for result"
+"""
+
+proc foo(a: openArray[int]):auto = a
+echo foo(toOpenArray([1, 2], 0, 2))