runtime: add missing runtime error prefix to PanicNilError

src/runtime/panic.go

@@ -700,7 +700,7 @@ type PanicNilError struct {
 	_ [0]*PanicNilError
 }
 
-func (*PanicNilError) Error() string { return "panic called with nil argument" }
+func (*PanicNilError) Error() string { return "runtime error: panic called with nil argument" }
 func (*PanicNilError) RuntimeError() {}
 
 var panicnil = &godebugInc{name: "panicnil"}

src/runtime/panic_test.go

@@ -5,6 +5,7 @@
 package runtime_test
 
 import (
+	"runtime"
 	"strings"
 	"testing"
 )
@@ -46,3 +47,56 @@ func TestPanicWithDirectlyPrintableCustomTypes(t *testing.T) {
 		})
 	}
 }
+
+func TestPanicNilErrorPrefix(t *testing.T) {
+	tests := []struct {
+		name      string
+		wantPanic string
+		fn        func()
+	}{
+		{
+			name: "panic(nil)",
+			fn: func() {
+				panic(nil)
+			},
+		},
+		{
+			name: "panic((any)(nil))",
+			fn: func() {
+				var foo any = nil
+				panic(foo)
+			},
+		},
+		{
+			name: "panic((error)(nil))",
+			fn: func() {
+				var err error
+				panic(err)
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		tt := tt
+		t.Run(tt.name, func(t *testing.T) {
+			defer func() {
+				r := recover()
+				if r == nil {
+					t.Fatal("expected a panic")
+				}
+
+				re, ok := r.(runtime.Error)
+				if !ok {
+					t.Fatalf("wrong panic type: got %T, want runtime.Error", r)
+				}
+				got := re.Error()
+				want := "runtime error: panic called with nil argument"
+				if !strings.Contains(got, want) {
+					t.Fatalf("got: %s, want: %s", got, want)
+				}
+			}()
+
+			tt.fn()
+		})
+	}
+}

test/fixedbugs/issue19658.go

@@ -47,7 +47,7 @@ func main() {
 		Input  string
 		Expect string
 	}{
-		{"", "nil", "panic: panic called with nil argument"},
+		{"", "nil", "panic: runtime error: panic called with nil argument"},
 		{"errors.New", `"test"`, "panic: test"},
 		{"S", "S{}", "panic: s-stringer"},
 		{"byte", "8", "panic: 8"},