adds legacy:typedescFieldAccess

changelog.md

@@ -12,6 +12,10 @@ rounding guarantees (via the
   avoid conflicts with `system.default`, so named argument usage for this
   parameter like `getOrDefault(..., default = ...)` will have to be changed.
 
+- Typedesc field access on object/tuple types (e.g. `Foo[int].val`) is now
+  restricted to `typeof` context. Use `--legacy:typedescFieldAccess` to restore
+  the previous behavior of allowing it outside `typeof`.
+
 - With `-d:nimPreviewCheckedClose`, the `close` function in the `std/syncio` module now raises an IO exception in case of an error.
 
 - Unknown warnings and hints now gives warnings `warnUnknownNotes` instead of

compiler/options.nim

@@ -262,6 +262,9 @@ type
     procParamTypeBackendAliases
       ## Keep the old proc type compatibility rules that ignore backend
       ## c type aliases.
+    typedescFieldAccess
+      ## Allow typedesc field access on object/tuple types outside of
+      ## typeof context.
 
   SymbolFilesOption* = enum
     disabledSf, writeOnlySf, readOnlySf, v2Sf, stressTest

compiler/semexprs.nim

@@ -1500,7 +1500,7 @@ proc tryReadingTypeField(c: PContext, n: PNode, i: PIdent, ty: PType): PNode =
       markUsed(c, n.info, f)
       onUse(n.info, f)
   of tyObject, tyTuple:
-    if c.inTypeofContext > 0 and
+    if (c.inTypeofContext > 0 or typedescFieldAccess in c.config.legacyFeatures) and
         ty.n != nil and ty.n.kind == nkRecList:
       let field = lookupInRecord(ty.n, i)
       if field != nil:

tests/metatype/ttypedesc_field_legacy.nim

@@ -0,0 +1,11 @@
+discard """
+  matrix: "--legacy:typedescFieldAccess"
+  output: "4"
+"""
+
+type
+  Foo[T] = object
+    val: T
+
+var x: Foo[int].val = 4
+echo x