[RequirementMachine] Skip Sendable conformance requirements from preconcurrency

declarations in requirement inference.

Author: hborla

lib/AST/RequirementMachine/RequirementLowering.cpp

@@ -482,11 +482,25 @@ struct InferRequirementsWalker : public TypeWalker {
   }
 
   Action walkToTypePost(Type ty) override {
+    // Skip `Sendable` conformance requirements that are inferred from
+    // `@preconcurrency` declarations.
+    auto skipRequirement = [](Requirement req, Decl *fromDecl) {
+      if (!fromDecl->preconcurrency())
+        return false;
+
+      return (req.getKind() == RequirementKind::Conformance &&
+          req.getSecondType()->castTo<ProtocolType>()->getDecl()
+            ->isSpecificProtocol(KnownProtocolKind::Sendable));
+    };
+
     // Infer from generic typealiases.
     if (auto typeAlias = dyn_cast<TypeAliasType>(ty.getPointer())) {
       auto decl = typeAlias->getDecl();
       auto subMap = typeAlias->getSubstitutionMap();
       for (const auto &rawReq : decl->getGenericSignature().getRequirements()) {
+        if (skipRequirement(rawReq, decl))
+          continue;
+
         desugarRequirement(rawReq.subst(subMap), SourceLoc(), reqs, errors);
       }
 
@@ -567,6 +581,9 @@ struct InferRequirementsWalker : public TypeWalker {
     // Handle the requirements.
     // FIXME: Inaccurate TypeReprs.
     for (const auto &rawReq : genericSig.getRequirements()) {
+      if (skipRequirement(rawReq, decl))
+        continue;
+
       auto req = rawReq.subst(subMap);
       desugarRequirement(req, SourceLoc(), reqs, errors);
     }

test/Generics/requirement_inference_preconcurrency.swift

@@ -0,0 +1,16 @@
+// RUN: %target-swift-frontend -typecheck %s -debug-generic-signatures 2>&1 | %FileCheck %s
+
+// CHECK-LABEL: ExistingType
+// CHECK: Canonical generic signature: <τ_0_0 where τ_0_0 : Sendable>
+@preconcurrency
+public struct ExistingType<T: Sendable> : Sendable {}
+
+// CHECK-LABEL: existingClient(arg:)
+// CHECK: Canonical generic signature: <τ_0_0>
+public func existingClient<T>(arg: T.Type) -> ExistingType<T>? { nil }
+
+public typealias Alias = ExistingType
+
+// CHECK-LABEL: existingClient2(arg:)
+// CHECK: Canonical generic signature: <τ_0_0>
+public func existingClient2<T>(arg: T.Type) -> Alias<T>? { nil }