fix #19266; allow reassign discriminant field (#19567)

* add inUncheckedAssignSection

* add one more test

compiler/ccgexprs.nim

@@ -2932,7 +2932,21 @@ proc expr(p: BProc, n: PNode, d: var TLoc) =
      nkFromStmt, nkTemplateDef, nkMacroDef, nkStaticStmt:
     discard
   of nkPragma: genPragma(p, n)
-  of nkPragmaBlock: expr(p, n.lastSon, d)
+  of nkPragmaBlock:
+    var inUncheckedAssignSection = 0
+    let pragmaList = n[0]
+    for pi in pragmaList:
+      if whichPragma(pi) == wCast:
+        case whichPragma(pi[1])
+        of wUncheckedAssign:
+          inUncheckedAssignSection = 1
+        else:
+          discard
+
+    inc p.inUncheckedAssignSection, inUncheckedAssignSection
+    expr(p, n.lastSon, d)
+    dec p.inUncheckedAssignSection, inUncheckedAssignSection
+
   of nkProcDef, nkFuncDef, nkMethodDef, nkConverterDef:
     if n[genericParamsPos].kind == nkEmpty:
       var prc = n[namePos].sym

compiler/ccgstmts.nim

@@ -1566,7 +1566,7 @@ proc asgnFieldDiscriminant(p: BProc, e: PNode) =
   initLocExpr(p, e[0], a)
   getTemp(p, a.t, tmp)
   expr(p, e[1], tmp)
-  if optTinyRtti notin p.config.globalOptions:
+  if optTinyRtti notin p.config.globalOptions and p.inUncheckedAssignSection == 0:
     let field = dotExpr[1].sym
     genDiscriminantCheck(p, a, tmp, dotExpr[0].typ, field)
     message(p.config, e.info, warnCaseTransition)

compiler/cgendata.nim

@@ -99,6 +99,7 @@ type
     withinTryWithExcept*: int # required for goto based exception handling
     withinBlockLeaveActions*: int # complex to explain
     sigConflicts*: CountTable[string]
+    inUncheckedAssignSection*: int
 
   TTypeSeq* = seq[PType]
   TypeCache* = Table[SigHash, Rope]

tests/objvariant/treassign.nim

@@ -34,3 +34,25 @@ proc passToVar(x: var Token) = discard
   t.curr = TokenObject(kind: t.curr.kind, foo: "abc")
 
   t.curr.kind = Token.foo
+
+
+block:
+  type
+    TokenKind = enum
+      strLit, intLit
+    Token = object
+      case kind*: TokenKind
+      of strLit:
+        s*: string
+      of intLit:
+        i*: int64
+
+  var t = Token(kind: strLit, s: "abc")
+
+  {.cast(uncheckedAssign).}:
+
+    # inside the 'cast' section it is allowed to assign to the 't.kind' field directly:
+    t.kind = intLit
+
+  {.cast(uncheckedAssign).}:
+    t.kind = strLit