Fix panic in getClassElementPropertyKeyType for private identifiers (#3819)

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: RyanCavanaugh <6685088+RyanCavanaugh@users.noreply.github.com>
Co-authored-by: Gabriela Araujo Britto <gabrielaa@microsoft.com>
Co-authored-by: Gabriela Araujo Britto <garaujobritto@gmail.com>

Author: 3 people

internal/checker/checker.go

@@ -30220,7 +30220,7 @@ func (c *Checker) getClassElementPropertyKeyType(element *ast.Node) *Type {
 		}
 		return c.stringType
 	}
-	panic("Unhandled case in getClassElementPropertyKeyType")
+	return c.errorType
 }
 
 func (c *Checker) getTypeOfPropertyOfContextualType(t *Type, name string) *Type {

internal/fourslash/tests/decoratorOnPrivateMethod_test.go

@@ -0,0 +1,36 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	. "github.com/microsoft/typescript-go/internal/fourslash/tests/util"
+	"github.com/microsoft/typescript-go/internal/testutil"
+)
+
+func TestDecoratorCompletionOnPrivateMethod(t *testing.T) {
+	t.Parallel()
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+	const content = `
+// @experimentalDecorators: true
+declare function dec(target: any, key: string): void;
+class C {
+    @dec/**/
+    #method() {}
+}`
+	f, done := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	defer done()
+	// Verify completions don't panic on decorator applied to private method
+	f.VerifyCompletions(t, "", &fourslash.CompletionsExpectedList{
+		IsIncomplete: false,
+		ItemDefaults: &fourslash.CompletionsExpectedItemDefaults{
+			CommitCharacters: &DefaultCommitCharacters,
+			EditRange:        Ignored,
+		},
+		Items: &fourslash.CompletionsExpectedItems{
+			Includes: []fourslash.CompletionsExpectedItem{
+				"dec",
+			},
+		},
+	})
+}