[Concurrency] Disallow 'async' and non-async overloading.

Author: DougGregor

include/swift/AST/Decl.h

@@ -242,14 +242,10 @@ struct OverloadSignature {
   /// Whether this declaration has an opaque return type.
   unsigned HasOpaqueReturnType : 1;
 
-  /// Whether this declaration is 'async'
-  unsigned HasAsync : 1;
-
   OverloadSignature()
       : UnaryOperator(UnaryOperatorKind::None), IsInstanceMember(false),
         IsVariable(false), IsFunction(false), InProtocolExtension(false),
-        InExtensionOfGenericType(false), HasOpaqueReturnType(false),
-        HasAsync(false) {}
+        InExtensionOfGenericType(false), HasOpaqueReturnType(false) { }
 };
 
 /// Determine whether two overload signatures conflict.

lib/AST/Decl.cpp

@@ -2403,10 +2403,6 @@ bool swift::conflicting(const OverloadSignature& sig1,
              (sig2.IsVariable && !sig1.Name.getArgumentNames().empty()));
   }
 
-  // If one is asynchronous and the other is not, they can't conflict.
-  if (sig1.HasAsync != sig2.HasAsync)
-    return false;
-
   // Note that we intentionally ignore the HasOpaqueReturnType bit here.
   // For declarations that can't be overloaded by type, we want them to be
   // considered conflicting independent of their type.
@@ -2630,8 +2626,6 @@ OverloadSignature ValueDecl::getOverloadSignature() const {
   signature.IsTypeAlias = isa<TypeAliasDecl>(this);
   signature.HasOpaqueReturnType =
                        !signature.IsVariable && (bool)getOpaqueResultTypeDecl();
-  signature.HasAsync = isa<AbstractFunctionDecl>(this) &&
-    cast<AbstractFunctionDecl>(this)->hasAsync();
 
   // Unary operators also include prefix/postfix.
   if (auto func = dyn_cast<FuncDecl>(this)) {

test/Constraints/async.swift

@@ -12,7 +12,7 @@ func testNonConversions() async {
 
 // Overloading
 @available(swift, deprecated: 4.0, message: "synchronous is no fun")
-func overloadedSame() -> String { "synchronous" }
+func overloadedSame(_: Int = 0) -> String { "synchronous" }
 
 func overloadedSame() async -> String { "asynchronous" }
 

test/decl/func/async.swift

@@ -6,6 +6,9 @@
 func redecl1() async { } // expected-note{{previously declared here}}
 func redecl1() async throws { } // expected-error{{invalid redeclaration of 'redecl1()'}}
 
+func redecl2() -> String { "" } // expected-note{{previously declared here}}
+func redecl2() async -> String { "" } // expected-error{{invalid redeclaration of 'redecl2()'}}
+
 // Override checking
 
 class Super {