Typecasting nil results in non-nil #843
Comments
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Nice find. Note, however, that this is a case of type conversion, not type casting. |
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Quick-and-possibly-dirty solution that may be a real fix, or at least may help point the way towards one: diff --git a/compiler/prelude/types.go b/compiler/prelude/types.go
index a8c29d5..7e81b22 100644
--- a/compiler/prelude/types.go
+++ b/compiler/prelude/types.go
@@ -273,6 +273,7 @@ var $newType = function(size, kind, string, named, pkg, exported, constructor) {
});
typ.ptr.nil = Object.create(constructor.prototype, properties);
typ.ptr.nil.$val = typ.ptr.nil;
+ typ.ptr.nil.isNil = true;
/* methods for embedded fields */
$addMethodSynthesizer(function() {
var synthesizeMethod = function(target, m, f) {
diff --git a/compiler/prelude/prelude.go b/compiler/prelude/prelude.go
index 1eed6c4..2c754ec 100644
--- a/compiler/prelude/prelude.go
+++ b/compiler/prelude/prelude.go
@@ -317,6 +317,9 @@ var $pointerOfStructConversion = function(obj, type) {
obj.$proxies[type.string] = proxy;
proxy.$proxies = obj.$proxies;
}
+ if (obj.isNil) {
+ return type.nil;
+ }
return proxy;
};Test case: var foo *FooType = nil
fmt.Println("foo is nil?", foo == nil)
var bar *BarType = (*BarType)(foo)
fmt.Println("bar is nil?", bar == nil)
foo = nil
fmt.Printf("foo is %v\n", foo)
bar = (*BarType)(foo)
fmt.Printf("bar is %v\n", bar)=> |
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Appreciate the correction on the terminology. |
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I discovered a case very similar to the second example @seanburke-wf posted https://play.golang.org/p/4Qn21VLSJq8 If a pointer to a struct is nil and casted to another another type declared in a type definition, the gopherjs version will panic for a nil pointer dereference. |
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If a variable has value
niland is typecast, the result will have a non-nil value which appears to be an invalid pointer. This differs from the behavior of Go.The following illustrate the difference in behaviors between Go and compiled gopherjs:
https://play.golang.org/p/_feur6S2EjC
https://gopherjs.github.io/playground/#/y785Hlp5q0
Additionally, the non-nil value appears to be an invalid reference:
https://play.golang.org/p/ugzWXe4x2HE
https://gopherjs.github.io/playground/#/HkVf0Bqs1m
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