strict inference should not complain about top-level setter return types

Fixes #39250

Change-Id: I9f9ddc7ce484cc4ee0e9bbf2a7b2c673f6ad4e7e
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/124520
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>

Author: srawlins

pkg/analyzer/lib/src/generated/resolver.dart

@@ -365,7 +365,7 @@ class BestPracticesVerifier extends RecursiveAstVisitor<void> {
           node.returnType, node.functionExpression.body, element, node);
 
       // Return types are inferred only on non-recursive local functions.
-      if (node.parent is CompilationUnit) {
+      if (node.parent is CompilationUnit && !node.isSetter) {
         _checkStrictInferenceReturnType(node.returnType, node, node.name.name);
       }
       _checkStrictInferenceInParameters(node.functionExpression.parameters);
@@ -499,7 +499,7 @@ class BestPracticesVerifier extends RecursiveAstVisitor<void> {
       _checkForMissingReturn(node.returnType, node.body, element, node);
       _checkForUnnecessaryNoSuchMethod(node);
 
-      if (_strictInference && !node.isSetter && !elementIsOverride()) {
+      if (!node.isSetter && !elementIsOverride()) {
         _checkStrictInferenceReturnType(node.returnType, node, node.name.name);
       }
       _checkStrictInferenceInParameters(node.parameters);

pkg/analyzer/test/src/diagnostics/inference_failure_on_function_return_type_test.dart

@@ -190,6 +190,12 @@ mixin C {
 ''', [error(HintCode.INFERENCE_FAILURE_ON_FUNCTION_RETURN_TYPE, 12, 9)]);
   }
 
+  test_setter_topLevel() async {
+    await assertNoErrorsInCode(r'''
+set f(int x) => print(x);
+''');
+  }
+
   test_topLevelArrowFunction() async {
     await assertErrorsInCode(r'''
 f() => 7;