[SE-0470] Downgrade some isolated conformance-related errors to warnings in Swift 5

(cherry picked from commit e8b3065)

Author: DougGregor

include/swift/AST/ASTContextGlobalCache.h

@@ -55,6 +55,7 @@ struct WitnessIsolationError {
 /// Describes an isolation error involving an associated conformance.
 struct AssociatedConformanceIsolationError {
   ProtocolConformance *isolatedConformance;
+  DiagnosticBehavior behavior = DiagnosticBehavior::Unspecified;
 
   /// Diagnose this associated conformance isolation error.
   void diagnose(const NormalProtocolConformance *conformance) const;

lib/Sema/TypeCheckConcurrency.cpp

@@ -8009,7 +8009,8 @@ namespace {
           firstConformance->getIsolation(),
           firstConformance->getType(),
           firstConformance->getProtocol()->getName(),
-          getContextIsolation());
+          getContextIsolation())
+        .warnUntilSwiftVersion(6);
       return true;
     }
   };

lib/Sema/TypeCheckProtocol.cpp

@@ -4947,6 +4947,9 @@ static void diagnoseConformanceIsolationErrors(
       hasIsolatedConformances = true;
     }
 
+    // Take the least-restrictive behavior.
+    behavior = behavior.merge(assocConformanceError.behavior);
+
     anyNonDistributedIssues = true;
   }
 
@@ -5405,9 +5408,19 @@ static void ensureRequirementsAreSatisfied(ASTContext &ctx,
             // If the isolation doesn't match, record an error.
             if (!outerIsolation.isGlobalActor() ||
                 outerIsolation != innerIsolation) {
+              DiagnosticBehavior behavior = DiagnosticBehavior::Unspecified;
+              // If we're working with requirements imported from Clang, or with
+              // global actor isolation in general, use the default diagnostic
+              // behavior based on the conformance context.
+              if (proto->hasClangNode() ||
+                  outerIsolation.isGlobalActor() ||
+                  innerIsolation.isGlobalActor())
+                behavior = SendableCheckContext(dc).defaultDiagnosticBehavior();
+
               ctx.getGlobalCache().conformanceIsolationErrors[conformance]
                 .push_back(
-                  AssociatedConformanceIsolationError{isolatedConformance});
+                  AssociatedConformanceIsolationError{
+                    isolatedConformance, behavior});
               return true;
             }
 

test/Concurrency/isolated_conformance.swift

@@ -1,4 +1,4 @@
-// RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.1-abi-triple -swift-version 6 -enable-experimental-feature IsolatedConformances %s
+// RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.1-abi-triple -swift-version 5 -strict-concurrency=complete -enable-experimental-feature IsolatedConformances %s
 
 // REQUIRES: swift_feature_IsolatedConformances
 // REQUIRES: concurrency
@@ -11,7 +11,7 @@ protocol P {
 // Definition of isolated conformances
 // ----------------------------------------------------------------------------
 
-// expected-error@+4{{conformance of 'CWithNonIsolated' to protocol 'P' crosses into main actor-isolated code and can cause data races}}
+// expected-warning@+4{{conformance of 'CWithNonIsolated' to protocol 'P' crosses into main actor-isolated code and can cause data races}}
 // expected-note@+3{{mark all declarations used in the conformance 'nonisolated'}}
 // expected-note@+2{{isolate this conformance to the main actor with '@MainActor'}}{{25-25=@MainActor }}
 @MainActor
@@ -54,7 +54,7 @@ protocol Q {
   associatedtype A: P
 }
 
-// expected-error@+2{{conformance of 'SMissingIsolation' to protocol 'Q' crosses into main actor-isolated code and can cause data races}}
+// expected-warning@+2{{conformance of 'SMissingIsolation' to protocol 'Q' crosses into main actor-isolated code and can cause data races}}
 @MainActor
 struct SMissingIsolation: Q {
   // expected-note@-1{{conformance depends on main actor-isolated conformance of 'C' to protocol 'P'}}
@@ -66,7 +66,7 @@ struct PWrapper<T: P>: P {
   func f() { }
 }
 
-// expected-error@+2{{conformance of 'SMissingIsolationViaWrapper' to protocol 'Q' crosses into main actor-isolated code and can cause data races}}
+// expected-warning@+2{{conformance of 'SMissingIsolationViaWrapper' to protocol 'Q' crosses into main actor-isolated code and can cause data races}}
 @MainActor
 struct SMissingIsolationViaWrapper: Q {
   // expected-note@-1{{conformance depends on main actor-isolated conformance of 'C' to protocol 'P'}}
@@ -84,7 +84,7 @@ struct S: @MainActor Q {
   typealias A = C
 }
 
-// expected-error@+3{{conformance of 'SMismatchedActors' to protocol 'Q' crosses into global actor 'SomeGlobalActor'-isolated code and can cause data races}}
+// expected-warning@+3{{conformance of 'SMismatchedActors' to protocol 'Q' crosses into global actor 'SomeGlobalActor'-isolated code and can cause data races}}
 // expected-note@+2{{conformance depends on global actor 'SomeGlobalActor'-isolated conformance of 'C2' to protocol 'P'}}
 @MainActor
 struct SMismatchedActors: @MainActor Q {
@@ -149,9 +149,9 @@ func testIsolatedConformancesOfOtherGlobalActor(c: CMismatchedIsolation) {
 }
 
 func testIsolationConformancesFromOutside(c: C) {
-  acceptP(c) // expected-error{{main actor-isolated conformance of 'C' to 'P' cannot be used in nonisolated context}}
-  let _: any P = c // expected-error{{main actor-isolated conformance of 'C' to 'P' cannot be used in nonisolated context}}
-  let _ = PWrapper<C>() // expected-error{{main actor-isolated conformance of 'C' to 'P' cannot be used in nonisolated context}}
+  acceptP(c) // expected-warning{{main actor-isolated conformance of 'C' to 'P' cannot be used in nonisolated context}}
+  let _: any P = c // expected-warning{{main actor-isolated conformance of 'C' to 'P' cannot be used in nonisolated context}}
+  let _ = PWrapper<C>() // expected-warning{{main actor-isolated conformance of 'C' to 'P' cannot be used in nonisolated context}}
 }
 
 protocol HasAssociatedType {