[Concurrency] TildeSendable: Account for inherited conformances

This helps to properly detect and account for derived conformances
to `Sendable` protocol that come from parent classes.

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -7378,6 +7378,12 @@ bool swift::checkSendableConformance(
   // Sendable supression allows conditional conformances only.
   if (nominal->suppressesConformance(KnownProtocolKind::Sendable)) {
     bool hasUnconditionalConformance = false;
+
+    if (auto *inherited = dyn_cast<InheritedProtocolConformance>(conformance)) {
+      hasUnconditionalConformance =
+          inherited->getConditionalRequirements().empty();
+    }
+
     if (auto *normalConf = dyn_cast<NormalProtocolConformance>(conformance)) {
       hasUnconditionalConformance =
           normalConf->getConditionalRequirements().empty();

test/Sema/tilde_sendable.swift

@@ -119,4 +119,16 @@ do {
 
   testSendable(IsolatedC())
   // expected-warning@-1 {{type 'IsolatedC' does not conform to the 'Sendable' protocol}}
+
+  @MainActor
+  class IsolatedBase {} // derived as Sendable
+
+  class Child1: IsolatedBase, ~Sendable {}
+  // expected-error@-1 {{cannot both conform to and suppress conformance to 'Sendable'}}
+
+  class Base: Sendable {}
+  // expected-warning@-1 {{non-final class 'Base' cannot conform to the 'Sendable' protocol}}
+
+  class Child2: Base, ~Sendable {}
+  // expected-error@-1 {{cannot both conform to and suppress conformance to 'Sendable'}}
 }