Propagate global actor to a closure from its contextual type.

Author: DougGregor

lib/Sema/CSApply.cpp

@@ -6842,6 +6842,22 @@ Expr *ExprRewriter::coerceToType(Expr *expr, Type toType,
       }
     }
 
+    // If we have a ClosureExpr, then we can safely propagate a global actor
+    // to the closure without invalidating prior analysis.
+    fromEI = fromFunc->getExtInfo();
+    if (toEI.getGlobalActor() && !fromEI.getGlobalActor()) {
+      auto newFromFuncType = fromFunc->withExtInfo(
+          fromEI.withGlobalActor(toEI.getGlobalActor()));
+      if (applyTypeToClosureExpr(cs, expr, newFromFuncType)) {
+        fromFunc = newFromFuncType->castTo<FunctionType>();
+
+        // Propagating the global actor bit might have satisfied the entire
+        // conversion. If so, we're done, otherwise keep converting.
+        if (fromFunc->isEqual(toType))
+          return expr;
+      }
+    }
+
     // If we have a ClosureExpr, then we can safely propagate the 'no escape'
     // bit to the closure without invalidating prior analysis.
     fromEI = fromFunc->getExtInfo();

test/Concurrency/global_actor_function_types.swift

@@ -46,4 +46,13 @@ func testClosures() {
   acceptOnSomeGlobalActor { @SomeGlobalActor in
     onSomeGlobalActor()
   }
+
+  acceptOnSomeGlobalActor {
+    onSomeGlobalActor()
+  }
+
+  acceptOnSomeGlobalActor { () -> Int in
+    let i = onSomeGlobalActor()
+    return i
+  }
 }