Add nil assertion for thisType.symbol

internal/checker/checker.go

@@ -28999,7 +28999,7 @@ func (c *Checker) getContextualTypeForAssignmentExpression(binary *ast.BinaryExp
 			thisType := c.getTypeOfExpression(expr)
 			if ast.IsPropertyAccessExpression(left) {
 				name := left.Name()
-				if ast.IsPrivateIdentifier(name) {
+				if ast.IsPrivateIdentifier(name) && thisType.symbol != nil {
 					symbol = c.getPropertyOfType(thisType, binder.GetSymbolNameForPrivateIdentifier(thisType.symbol, name.Text()))
 				} else {
 					symbol = c.getPropertyOfType(thisType, name.Text())

internal/fourslash/tests/privatePropertyOfUndefinedThis_test.go

@@ -0,0 +1,38 @@
+package fourslash_test
+
+import (
+	"testing"
+
+	"github.com/microsoft/typescript-go/internal/fourslash"
+	"github.com/microsoft/typescript-go/internal/lsp/lsproto"
+	"github.com/microsoft/typescript-go/internal/testutil"
+)
+
+func TestPrivatePropertyOfUndefinedThis(t *testing.T) {
+	t.Parallel()
+	defer testutil.RecoverAndFail(t, "Panic on fourslash test")
+	const content = `
+this.#a = {};
+export {};
+`
+	f, done := fourslash.NewFourslash(t, nil /*capabilities*/, content)
+	defer done()
+	f.VerifyNonSuggestionDiagnostics(t, []*lsproto.Diagnostic{
+		{
+			Code:    &lsproto.IntegerOrString{Integer: new(int32(2532))},
+			Message: "Object is possibly 'undefined'.",
+			Range:   lsproto.Range{
+				Start: lsproto.Position{Line: 0, Character: 0},
+				End:   lsproto.Position{Line: 0, Character: 4},
+			},
+		},
+		{
+			Code:    &lsproto.IntegerOrString{Integer: new(int32(18016))},
+			Message: "Private identifiers are not allowed outside class bodies.",
+			Range:   lsproto.Range{
+				Start: lsproto.Position{Line: 0, Character: 5},
+				End:   lsproto.Position{Line: 0, Character: 7},
+			},
+		},
+	})
+}