Fix literal_only_boolean_expressions is False-Positive

Prior to this commit - a conditonal expression which used `is` to test against a type
parameter was treated as literal expression causing the lint to flag such conditions.

The actual type of a type parameter is not known for a program until a literal type is
substituted hence the flagging was a false-positive. This commit prevents such flagging.

Change-Id: I7b33510e83354fb652b159df452ed5063f51e875
Bug: #61903
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/461680
Commit-Queue: Samuel Rawlins <[email protected]>
Auto-Submit: Rohit Saily <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Reviewed-by: Samuel Rawlins <[email protected]>

Author: RohitSaily

pkg/linter/lib/src/rules/literal_only_boolean_expressions.dart

@@ -8,6 +8,7 @@ import 'package:analyzer/analysis_rule/rule_visitor_registry.dart';
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
 import 'package:analyzer/dart/ast/visitor.dart';
+import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/error/error.dart';
 
 import '../analyzer.dart';
@@ -31,6 +32,10 @@ bool _onlyLiterals(Expression? rawExpression) {
     return _onlyLiterals(expression.leftOperand) &&
         _onlyLiterals(expression.rightOperand);
   }
+  if (expression is IsExpression) {
+    if (expression.type.type?.element is TypeParameterElement) return false;
+    return _onlyLiterals(expression.expression);
+  }
   return false;
 }
 

pkg/linter/test/rules/literal_only_boolean_expressions_test.dart

@@ -218,6 +218,33 @@ void f() {
 ''');
   }
 
+  test_is_expression_both_operands_known() async {
+    await assertDiagnostics(
+      r'''
+void f() {
+  if (20 is int) {}
+}
+''',
+      [lint(13, 17), error(diag.unnecessaryTypeCheckTrue, 17, 9)],
+    );
+  }
+
+  test_is_expression_only_left_operand_unknown() async {
+    await assertNoDiagnostics(r'''
+void f(a) {
+  if (a is int) {}
+}
+''');
+  }
+
+  test_is_expression_only_right_operand_unknown() async {
+    await assertNoDiagnostics(r'''
+void f<T>(a) {
+  if (20 is T) {}
+}
+''');
+  }
+
   test_nullAware() async {
     await assertDiagnostics(
       r'''