[SE-0258] Warn about (but still use) 'delegateValue' in lieu of 'wrapperValue'.

Provide some backward compatibilty for property delegates written before
they became property wrapper, picking up 'delegateValue' (if present)
but warning about it.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -4462,6 +4462,9 @@ ERROR(property_wrapper_type_not_usable_from_inline,none,
       "%select{%select{variable|constant}0|property}1 "
       "must be '@usableFromInline' or public",
       (bool, bool))
+WARNING(property_wrapper_delegateValue,none,
+        "property wrapper's `delegateValue` property should be renamed to "
+        "'wrapperValue'; use of 'delegateValue' is deprecated", ())
 
 //------------------------------------------------------------------------------
 // MARK: function builder diagnostics

lib/Sema/TypeCheckPropertyWrapper.cpp

@@ -245,6 +245,18 @@ PropertyWrapperTypeInfoRequest::evaluate(
   result.wrapperValueVar =
     findValueProperty(ctx, nominal, ctx.Id_wrapperValue, /*allowMissing=*/true);
 
+  // If there was no wrapperValue property, but there is a delegateValue
+  // property, use that and warn.
+  if (!result.wrapperValueVar) {
+    result.wrapperValueVar =
+      findValueProperty(ctx, nominal, ctx.Id_delegateValue,
+                        /*allowMissing=*/true);
+    if (result.wrapperValueVar) {
+      result.wrapperValueVar->diagnose(diag::property_wrapper_delegateValue)
+        .fixItReplace(result.wrapperValueVar->getNameLoc(), "wrapperValue");
+    }
+  }
+
   return result;
 }
 

test/decl/var/property_wrapper_aliases.swift

@@ -5,6 +5,18 @@
 @propertyDelegate
 struct Delegate<T> {
   var value: T
+
+  var delegateValue: Wrapper<T> { // expected-warning{{property wrapper's `delegateValue` property should be renamed to 'wrapperValue'; use of 'delegateValue' is deprecated}}{{7-20=wrapperValue}}
+    return Wrapper(value: value)
+  }
+}
+
+struct TestDelegateValue {
+  @Delegate var foo: String
+
+  func test() -> Wrapper<String> {
+    return $foo
+  }
 }
 
 @propertyWrapper