[Diagnostics] Tailored diagnostic for missing argument in property wrapper init

Author: xedin

lib/Sema/CSDiagnostics.cpp

@@ -3740,6 +3740,10 @@ bool MissingArgumentsFailure::diagnoseSingleMissingArgument() const {
   if (label.empty()) {
     emitDiagnostic(insertLoc, diag::missing_argument_positional, position + 1)
         .fixItInsert(insertLoc, insertText.str());
+  } else if (isPropertyWrapperInitialization()) {
+    auto *TE = cast<TypeExpr>(fnExpr);
+    emitDiagnostic(TE->getLoc(), diag::property_wrapper_missing_arg_init, label,
+                   resolveType(TE->getInstanceType())->getString());
   } else {
     emitDiagnostic(insertLoc, diag::missing_argument_named, label)
         .fixItInsert(insertLoc, insertText.str());
@@ -3880,6 +3884,23 @@ bool MissingArgumentsFailure::diagnoseInvalidTupleDestructuring() const {
   return true;
 }
 
+bool MissingArgumentsFailure::isPropertyWrapperInitialization() const {
+  auto *call = dyn_cast<CallExpr>(getRawAnchor());
+  if (!(call && call->isImplicit()))
+    return false;
+
+  auto TE = dyn_cast<TypeExpr>(call->getFn());
+  if (!TE)
+    return false;
+
+  auto instanceTy = TE->getInstanceType();
+  if (!instanceTy)
+    return false;
+
+  auto *NTD = resolveType(instanceTy)->getAnyNominal();
+  return NTD && NTD->getAttrs().hasAttribute<PropertyWrapperAttr>();
+}
+
 bool ClosureParamDestructuringFailure::diagnoseAsError() {
   auto *closure = cast<ClosureExpr>(getAnchor());
   auto params = closure->getParameters();

lib/Sema/CSDiagnostics.h

@@ -1207,6 +1207,11 @@ class MissingArgumentsFailure final : public FailureDiagnostic {
   /// Diagnose cases when instead of multiple distinct arguments
   /// call got a single tuple argument with expected arity/types.
   bool diagnoseInvalidTupleDestructuring() const;
+
+  /// Determine whether missing arguments are associated with
+  /// an implicit call to a property wrapper initializer e.g.
+  /// `@Foo(answer: 42) var question = "ultimate question"`
+  bool isPropertyWrapperInitialization() const;
 };
 
 class OutOfOrderArgumentFailure final : public FailureDiagnostic {