Merge pull request swiftlang#77057 from sophiapoirier/preconcurrency_import_warning_for_suppressed_sendable_diagnostic

[Concurrency] fix erroneous "@preconcurrency import" diagnostic

Author: sophiapoirier

lib/Sema/TypeCheckProtocol.cpp

@@ -4263,13 +4263,17 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
           diagnoseSendabilityErrorBasedOn(conformance->getProtocol(), sendFrom,
                                           [&](DiagnosticBehavior limit) {
             auto &diags = DC->getASTContext().Diags;
-            diags.diagnose(getLocForDiagnosingWitness(conformance, witness),
-                           diag::witness_not_as_sendable,
-                           witness, conformance->getProtocol())
+            auto preconcurrencyBehaviorLimit =
+                sendFrom.preconcurrencyBehavior(nominal);
+            diags
+                .diagnose(getLocForDiagnosingWitness(conformance, witness),
+                          diag::witness_not_as_sendable, witness,
+                          conformance->getProtocol())
                 .limitBehaviorUntilSwiftVersion(limit, 6)
-                .limitBehaviorIf(sendFrom.preconcurrencyBehavior(nominal));
+                .limitBehaviorIf(preconcurrencyBehaviorLimit);
             diags.diagnose(requirement, diag::less_sendable_reqt_here);
-            return false;
+            return preconcurrencyBehaviorLimit &&
+                   (*preconcurrencyBehaviorLimit == DiagnosticBehavior::Ignore);
           });
         });
       }

test/Concurrency/sendable_conformance_checking.swift

@@ -204,3 +204,16 @@ public struct TestSendableWitnesses2 : NoSendableReqs {
   var prop = globalFn // Ok (no warnings)
   static let staticProp = globalFn // Ok (no warnings)
 }
+
+// @preconcurrency attributes to make it akin to an imported Obj-C API
+@preconcurrency @MainActor public protocol EscapingSendableProtocol {
+  // expected-complete-and-tns-note @+1 {{protocol requires function 'f(handler:)' with type '(@escaping @MainActor @Sendable (Int) -> Void) -> ()'}}
+  @preconcurrency func f(handler: @escaping @MainActor @Sendable (Int) -> Void)
+}
+
+// expected-complete-and-tns-error @+2 {{type 'TestEscapingOnly' does not conform to protocol 'EscapingSendableProtocol'}}
+// expected-complete-and-tns-note @+1 {{add stubs for conformance}}
+class TestEscapingOnly: EscapingSendableProtocol {
+    // expected-complete-and-tns-note @+1 {{candidate has non-matching type '(@escaping (Int) -> Void) -> ()'}}
+    func f(handler: @escaping (Int) -> Void) {}
+}