Sema: Ban implied 'Sendable' conformance when strict concurrency enabled

You can't do this anymore:

    struct G<T> {}
    protocol P: Sendable {}
    extension G: P where T: P {}

Fixes rdar://95695543.

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -2414,8 +2414,16 @@ checkIndividualConformance(NormalProtocolConformance *conformance) {
       ComplainLoc, diag::unchecked_conformance_not_special, ProtoType);
   }
 
+  bool allowImpliedConditionalConformance = false;
+  if (Proto->isSpecificProtocol(KnownProtocolKind::Sendable)) {
+    if (Context.LangOpts.StrictConcurrencyLevel != StrictConcurrency::Complete)
+      allowImpliedConditionalConformance = true;
+  } else if (Proto->isMarkerProtocol()) {
+    allowImpliedConditionalConformance = true;
+  }
+
   if (conformance->getSourceKind() == ConformanceEntryKind::Implied &&
-      !Proto->isMarkerProtocol()) {
+      !allowImpliedConditionalConformance) {
     // We've got something like:
     //
     //   protocol Foo : Proto {}

test/Concurrency/implied_sendable_conformance_swift6.swift

@@ -0,0 +1,27 @@
+// RUN: %target-typecheck-verify-swift -swift-version 6
+
+protocol P: Sendable {}
+protocol Q: Sendable {}
+
+struct One<T> {  // expected-note {{consider making generic parameter 'T' conform to the 'Sendable' protocol}}
+  var t: T  // expected-error {{stored property 't' of 'Sendable'-conforming generic struct 'One' has non-sendable type 'T'}}
+}
+
+extension One: P where T: P {}
+// expected-error@-1 {{conditional conformance of type 'One<T>' to protocol 'P' does not imply conformance to inherited protocol 'Sendable'}}
+// expected-note@-2 {{did you mean to explicitly state the conformance like 'extension One: Sendable where ...'}}
+
+struct Both<T> { // expected-note {{consider making generic parameter 'T' conform to the 'Sendable' protocol}}
+  var t: T  // expected-error {{stored property 't' of 'Sendable'-conforming generic struct 'Both' has non-sendable type 'T'}}
+}
+
+extension Both: P where T: P {}
+// expected-error@-1 {{conditional conformance of type 'Both<T>' to protocol 'P' does not imply conformance to inherited protocol 'Sendable'}}
+// expected-note@-2 {{did you mean to explicitly state the conformance like 'extension Both: Sendable where ...'}}
+
+extension Both: Q where T: Q {}
+
+func takesSendable<T: Sendable>(_: T) {}
+
+takesSendable(One<Int>(t: 3))
+takesSendable(Both<Int>(t: 3))