[Property Wrappers] Add an unsupported error message for property wrapper

parameters in subscript declarations.

Author: hborla

include/swift/AST/DiagnosticsSema.def

@@ -5482,6 +5482,9 @@ ERROR(property_wrapper_dynamic_self_type, none,
 ERROR(property_wrapper_attribute_not_on_property, none,
       "property wrapper attribute %0 can only be applied to a property",
       (Identifier))
+ERROR(property_wrapper_param_not_supported,none,
+      "property wrapper attribute on parameter not yet supported on %0",
+      (DescriptiveDeclKind))
 NOTE(property_wrapper_declared_here,none,
      "property wrapper type %0 declared here", (Identifier))
 ERROR(property_wrapper_param_no_projection,none,

lib/Sema/TypeCheckAttr.cpp

@@ -2995,6 +2995,18 @@ void AttributeChecker::visitCustomAttr(CustomAttr *attr) {
       return;
     }
 
+    if (isa<ParamDecl>(D)) {
+      // Check for unsupported declarations.
+      auto *context = D->getDeclContext()->getAsDecl();
+      if (context && isa<SubscriptDecl>(context)) {
+        diagnose(attr->getLocation(),
+                 diag::property_wrapper_param_not_supported,
+                 context->getDescriptiveKind());
+        attr->setInvalid();
+        return;
+      }
+    }
+
     return;
   }
 

test/Sema/property_wrapper_parameter_invalid.swift

@@ -68,3 +68,8 @@ func testWrapperAttributeArg(projection: Projection<Int>) {
   // expected-error@+1 {{cannot use property wrapper projection argument; pass wrapped value type 'Int' instead}}
   hasWrapperAttributeArg($value: projection)
 }
+
+struct S {
+  // expected-error@+1 {{property wrapper attribute on parameter not yet supported on subscript}}
+  subscript(@Wrapper position: Int) -> Int { 0 }
+}