[go] reflect: fix TypeAssert on nil interface values

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Joseph Tsai (Gerrit)

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Jun 27, 2025, 2:11:20 PMJun 27
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Commit message

reflect: fix TypeAssert on nil interface values

In the Go language a type assertion of a nil interface value
will always report false:

	var err error
	v, ok := err.(error) // always reports (nil, false)

Consequently, assertion on a reflect.Value.Interface()
will also report false:

	var err error
	rv := ValueOf(&err).Elem()
	v, ok := rv.Interface().(error) // reports (nil, false)

However, prior to this change, a TypeAssert would report true:

	var err error
	rv := ValueOf(&err).Elem()
	v, ok := TypeAssert[error](rv) // reports (nil, true)

when it should report false.

This fixes TypeAssert to match the Go language by
pushing the typ != v.typ check to the very end after
we have validated that neither v nor T are interface kinds.

Fixes #74404
Change-Id: Ie14d5cf18c8370c3e27ce4bdf4570c89519d8a16

Change diff

diff --git a/src/reflect/all_test.go b/src/reflect/all_test.go
index fb1a29d..cd3e306 100644
--- a/src/reflect/all_test.go
+++ b/src/reflect/all_test.go
@@ -8719,6 +8719,11 @@
 	testTypeAssert(t, any(int(1)), int(1), true)
 	testTypeAssert(t, any(int(1)), byte(0), false)
 	testTypeAssert(t, fmt.Stringer(vv), vv, true)
+
+	testTypeAssert(t, any(nil), any(nil), false)
+	testTypeAssert(t, any(nil), error(nil), false)
+	testTypeAssert(t, error(nil), any(nil), false)
+	testTypeAssert(t, error(nil), error(nil), false)
 }
 
 func testTypeAssert[T comparable, V any](t *testing.T, val V, wantVal T, wantOk bool) {
diff --git a/src/reflect/value.go b/src/reflect/value.go
index 68b97e9..effe2f2 100644
--- a/src/reflect/value.go
+++ b/src/reflect/value.go
@@ -1514,46 +1514,46 @@
 	}
 
 	typ := abi.TypeFor[T]()
+
+	// Return the element inside the interface.
+	//
+	// T is a concrete type and v is an interface. For example:
+	//
+	//	var v any = int(1)
+	//	val := ValueOf(&v).Elem()
+	//	TypeAssert[int](val) == val.Interface().(int)
+	//
+	// T is a interface and v is an interface, but the iface types are different. For example:
+	//
+	//	var v any = &someError{}
+	//	val := ValueOf(&v).Elem()
+	//	TypeAssert[error](val) == val.Interface().(error)
+	//
+	// T is a interface and v is a nil interface value. For example:
+	//
+	//	var v errror
+	//	val := ValueOf(&v).Elem()
+	//	TypeAssert[error](val) == val.Interface().(error)
+	if v.kind() == Interface {
+		v, ok := packIfaceValueIntoEmptyIface(v).(T)
+		return v, ok
+	}
+
+	// T is an interface, v is a concrete type. For example:
+	//
+	//	TypeAssert[any](ValueOf(1)) == ValueOf(1).Interface().(any)
+	//	TypeAssert[error](ValueOf(&someError{})) == ValueOf(&someError{}).Interface().(error)
+	if typ.Kind() == abi.Interface {
+		v, ok := packEface(v).(T)
+		return v, ok
+	}
+
+	// Both v and T must be concrete types.
+	// The only way for an type-assertion to match is if the types are equal.
 	if typ != v.typ() {
-		// We can't just return false here:
-		//
-		//	var zero T
-		//	return zero, false
-		//
-		// since this function should work in the same manner as v.Interface().(T) does.
-		// Thus we have to handle two cases specially.
-
-		// Return the element inside the interface.
-		//
-		// T is a concrete type and v is an interface. For example:
-		//
-		// var v any = int(1)
-		// val := ValueOf(&v).Elem()
-		// TypeAssert[int](val) == val.Interface().(int)
-		//
-		// T is a interface and v is an interface, but the iface types are different. For example:
-		//
-		// var v any = &someError{}
-		// val := ValueOf(&v).Elem()
-		// TypeAssert[error](val) == val.Interface().(error)
-		if v.kind() == Interface {
-			v, ok := packIfaceValueIntoEmptyIface(v).(T)
-			return v, ok
-		}
-
-		// T is an interface, v is a concrete type. For example:
-		//
-		// TypeAssert[any](ValueOf(1)) == ValueOf(1).Interface().(any)
-		// TypeAssert[error](ValueOf(&someError{})) == ValueOf(&someError{}).Interface().(error)
-		if typ.Kind() == abi.Interface {
-			v, ok := packEface(v).(T)
-			return v, ok
-		}
-
 		var zero T
 		return zero, false
 	}
-
 	if v.flag&flagIndir == 0 {
 		return *(*T)(unsafe.Pointer(&v.ptr)), true
 	}

Change information

Files:
  • M src/reflect/all_test.go
  • M src/reflect/value.go
Change size: M
Delta: 2 files changed, 41 insertions(+), 36 deletions(-)
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Mateusz Poliwczak added 1 comment

File src/reflect/value.go
Line 1526, Patchset 1 (Latest): // T is a interface and v is an interface, but the iface types are different. For example:
Mateusz Poliwczak . unresolved

I think that this is not true anymore.

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    File src/reflect/value.go
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    Mateusz Poliwczak . unresolved
    ```suggestion
    	//	var v error
    ```
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    Joseph Tsai (Gerrit)

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    File src/reflect/value.go
    Line 1526, Patchset 1: // T is a interface and v is an interface, but the iface types are different. For example:
    Mateusz Poliwczak . resolved

    I think that this is not true anymore.

    Joseph Tsai

    Done

    Line 1534, Patchset 1: // var v errror
    Mateusz Poliwczak . resolved
    ```suggestion
    	//	var v error
    ```
    Joseph Tsai

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      Mateusz Poliwczak . resolved

      Thanks for the fix.

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      Cherry Mui . resolved

      Thanks.

      File src/reflect/value.go
      Line 1518, Patchset 2 (Latest): // Return the element inside the interface.
      Cherry Mui . unresolved

      After the rearrange, this line perhaps read better with

      // If v is an interface, return the element inside the interface.

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      File src/reflect/value.go
      Line 1518, Patchset 2: // Return the element inside the interface.
      Cherry Mui . resolved

      After the rearrange, this line perhaps read better with

      // If v is an interface, return the element inside the interface.

      Joseph Tsai

      Done

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        Jun 27, 2025, 7:34:53 PMJun 27
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        Gopher Robot submitted the change with unreviewed changes

        Unreviewed changes

        2 is the latest approved patch-set.
        The change was submitted with unreviewed changes in the following files:

        ```
        The name of the file: src/reflect/value.go
        Insertions: 4, Deletions: 4.

        @@ -1515,7 +1515,7 @@

         
         	typ := abi.TypeFor[T]()
         
        -	// Return the element inside the interface.
        +	// If v is an interface, return the element inside the interface.
         	//

         	// T is a concrete type and v is an interface. For example:
         	//
        @@ -1523,7 +1523,7 @@
         	//	val := ValueOf(&v).Elem()

         	//	TypeAssert[int](val) == val.Interface().(int)
         	//
        -	// T is a interface and v is an interface. For example:
        +	// T is a interface and v is a non-nil interface value. For example:
         	//

         	//	var v any = &someError{}
         	//	val := ValueOf(&v).Elem()
        @@ -1531,7 +1531,7 @@
         	//

         	// T is a interface and v is a nil interface value. For example:
         	//
        -	//	var v error
        +	//	var v error = nil
         	//	val := ValueOf(&v).Elem()

         	//	TypeAssert[error](val) == val.Interface().(error)
         	if v.kind() == Interface {
        @@ -1539,7 +1539,7 @@
         		return v, ok

         	}
         
        -	// T is an interface, v is a concrete type. For example:
        +	// If T is an interface and v is a concrete type. For example:
         	//

         	//	TypeAssert[any](ValueOf(1)) == ValueOf(1).Interface().(any)
         	//	TypeAssert[error](ValueOf(&someError{})) == ValueOf(&someError{}).Interface().(error)
        ```

        Change information

        Commit message:
        reflect: fix TypeAssert on nil interface values

        In the Go language a type assertion of a nil interface value
        will always report false:

        	var err error
        	v, ok := err.(error) // always reports (nil, false)

        Consequently, assertion on a reflect.Value.Interface()
        will also report false:

        	var err error
        	rv := ValueOf(&err).Elem()
        	v, ok := rv.Interface().(error) // reports (nil, false)

        However, prior to this change, a TypeAssert would report true:

        	var err error
        	rv := ValueOf(&err).Elem()
        	v, ok := TypeAssert[error](rv) // reports (nil, true)

        when it should report false.

        This fixes TypeAssert to match the Go language by
        pushing the typ != v.typ check to the very end after
        we have validated that neither v nor T are interface kinds.

        Fixes #74404
        Change-Id: Ie14d5cf18c8370c3e27ce4bdf4570c89519d8a16
        Reviewed-by: Cherry Mui <cher...@google.com>
        Auto-Submit: Joseph Tsai <joe...@digital-static.net>
        Reviewed-by: Mateusz Poliwczak <mpoliw...@gmail.com>
        Reviewed-by: Dmitri Shuralyov <dmit...@google.com>
        Files:
        • M src/reflect/all_test.go
        • M src/reflect/value.go
        Change size: M
        Delta: 2 files changed, 41 insertions(+), 36 deletions(-)
        Branch: refs/heads/master
        Submit Requirements:
        • requirement satisfiedCode-Review: +2 by Cherry Mui, +1 by Mateusz Poliwczak, +1 by Dmitri Shuralyov
        • requirement satisfiedTryBots-Pass: LUCI-TryBot-Result+1 by Go LUCI
        Open in Gerrit
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        Gerrit-MessageType: merged
        Gerrit-Project: go
        Gerrit-Branch: master
        Gerrit-Change-Id: Ie14d5cf18c8370c3e27ce4bdf4570c89519d8a16
        Gerrit-Change-Number: 684675
        Gerrit-PatchSet: 4
        Gerrit-Owner: Joseph Tsai <joe...@digital-static.net>
        Gerrit-Reviewer: Cherry Mui <cher...@google.com>
        Gerrit-Reviewer: Dmitri Shuralyov <dmit...@google.com>
        Gerrit-Reviewer: Gopher Robot <go...@golang.org>
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