Cpp codegen: handling of imported exceptions. Fixes #3571 (#7360)

compiler/semstmts.nim

@@ -257,13 +257,19 @@ proc semCase(c: PContext, n: PNode): PNode =
 proc semTry(c: PContext, n: PNode): PNode =
 
   var check = initIntSet()
-  template semExceptBranchType(typeNode: PNode): PNode =
+  template semExceptBranchType(typeNode: PNode): bool =
+    # returns true if exception type is imported type
     let typ = semTypeNode(c, typeNode, nil).toObject()
-    if typ.kind != tyObject:
-      localError(typeNode.info, errExprCannotBeRaised)
+    var is_imported = false
+    if isImportedException(typ):
+      is_imported = true
+    elif not isException(typ):
+      localError(typeNode.info, errExprCannotBeRaised)   
+
     if containsOrIncl(check, typ.id):
       localError(typeNode.info, errExceptionAlreadyHandled)
-    newNodeIT(nkType, typeNode.info, typ)
+    typeNode = newNodeIT(nkType, typeNode.info, typ)
+    is_imported
 
   result = n
   inc c.p.inTryStmt
@@ -286,18 +292,24 @@ proc semTry(c: PContext, n: PNode): PNode =
       
       if a.len == 2 and a[0].isInfixAs():
         # support ``except Exception as ex: body``
-        a[0][1] = semExceptBranchType(a[0][1])
-
+        let is_imported = semExceptBranchType(a[0][1])
         let symbol = newSymG(skLet, a[0][2], c)
-        symbol.typ = a[0][1].typ.toRef()
+        symbol.typ = if is_imported: a[0][1].typ
+                     else: a[0][1].typ.toRef()
         addDecl(c, symbol)
         # Overwrite symbol in AST with the symbol in the symbol table.
         a[0][2] = newSymNode(symbol, a[0][2].info)
 
       else:
         # support ``except KeyError, ValueError, ... : body``
+        var is_native, is_imported: bool
         for j in 0..a.len-2:
-          a[j] = semExceptBranchType(a[j])
+          let tmp = semExceptBranchType(a[j])
+          if tmp: is_imported = true
+          else: is_native = true
+
+        if is_native and is_imported:
+          localError(a[0].info, "Mix of imported and native exception types is not allowed in one except branch")
      
     elif a.kind != nkFinally:
       illFormedAst(n)
@@ -731,16 +743,19 @@ proc semFor(c: PContext, n: PNode): PNode =
 proc semRaise(c: PContext, n: PNode): PNode =
   result = n
   checkSonsLen(n, 1)
-  if n.sons[0].kind != nkEmpty:
-    n.sons[0] = semExprWithType(c, n.sons[0])
-    var typ = n.sons[0].typ
-    if typ.kind != tyRef or typ.lastSon.kind != tyObject:
-      localError(n.info, errExprCannotBeRaised)
+  if n[0].kind != nkEmpty:
+   
+    n[0] = semExprWithType(c, n[0])
+    let typ = n[0].typ
 
-    # check if the given object inherits from Exception
-    if not typ.lastSon.isException():
+    if not isImportedException(typ):
+    
+      if typ.kind != tyRef or typ.lastSon.kind != tyObject:
+        localError(n.info, errExprCannotBeRaised)
+
+      if not isException(typ.lastSon):
         localError(n.info, "raised object of type $1 does not inherit from Exception",
-                           [typeToString(typ)])
+                          [typeToString(typ)])
 
 
 proc addGenericParamListToScope(c: PContext, n: PNode) =