catchable defects

changelog.md

@@ -161,6 +161,10 @@ echo f
   errors like `IndexError` or `OverflowError` into fatal errors that **cannot**
   be caught via Nim's `try` statement. `--panics:on` can improve the
   runtime efficiency and code size of your program significantly.
+- The compiler now warns about inheriting directly from `system.Exception` as
+  this is **very bad** style. You should inherit from `ValueError`, `IOError`,
+  `OSError` or from a different specific exception type that cannot be confused
+  with a `Defect`.
 
 
 ## Bugfixes

compiler/lineinfos.nim

@@ -34,6 +34,7 @@ type
     warnTypelessParam,
     warnUseBase, warnWriteToForeignHeap, warnUnsafeCode,
     warnUnusedImportX,
+    warnInheritFromException,
     warnEachIdentIsTuple,
     warnProveInit, warnProveField, warnProveIndex, warnGcUnsafe, warnGcUnsafe2,
     warnUninit, warnGcMem, warnDestructor, warnLockLevel, warnResultShadowed,
@@ -81,6 +82,7 @@ const
     warnWriteToForeignHeap: "write to foreign heap",
     warnUnsafeCode: "unsafe code: '$1'",
     warnUnusedImportX: "imported and not used: '$1'",
+    warnInheritFromException: "inherit from a more precise exception type like ValueError, IOError or OSError",
     warnEachIdentIsTuple: "each identifier is a tuple",
     warnProveInit: "Cannot prove that '$1' is initialized. This will become a compile time error in the future.",
     warnProveField: "cannot prove that field '$1' is accessible",
@@ -138,7 +140,8 @@ const
     "LanguageXNotSupported", "FieldXNotSupported",
     "CommentXIgnored",
     "TypelessParam", "UseBase", "WriteToForeignHeap",
-    "UnsafeCode", "UnusedImport", "EachIdentIsTuple",
+    "UnsafeCode", "UnusedImport", "InheritFromException",
+    "EachIdentIsTuple",
     "ProveInit", "ProveField", "ProveIndex", "GcUnsafe", "GcUnsafe2", "Uninit",
     "GcMem", "Destructor", "LockLevel", "ResultShadowed",
     "Spacing", "CaseTransition", "CycleCreated", "User"]

compiler/semstmts.nim

@@ -1283,7 +1283,7 @@ proc typeSectionFinalPass(c: PContext, n: PNode) =
         while x.kind in {nkStmtList, nkStmtListExpr} and x.len > 0:
           x = x.lastSon
         if x.kind notin {nkObjectTy, nkDistinctTy, nkEnumTy, nkEmpty} and
-            s.typ.kind notin {tyObject, tyEnum}:
+            s.typ.skipTypes(abstractPtrs).kind notin {tyObject, tyEnum}:
           # type aliases are hard:
           var t = semTypeNode(c, x, nil)
           assert t != nil

compiler/semtypes.nim

@@ -847,6 +847,9 @@ proc semObjectNode(c: PContext, n: PNode, prev: PType; isInheritable: bool): PTy
         # specialized object, there will be second check after instantiation
         # located in semGeneric.
         if concreteBase.kind == tyObject:
+          if concreteBase.sym != nil and concreteBase.sym.magic == mException and
+              sfSystemModule notin c.module.flags:
+            message(c.config, n.info, warnInheritFromException, "")
           addInheritedFields(c, check, pos, concreteBase)
       else:
         if concreteBase.kind != tyError: