[Property Wrappers] Fix a diagnostic crash when a parameter has a wrapped value

mismatch.

If there's an error in property wrapper application, the backing property
wrapper type request will return a null type, which would cause the compiler
to crash because most error recovery code expects ErrorType. If the wrapper
application has an error, use the interface type of the parameter instead.

Author: hborla

lib/AST/Decl.cpp

@@ -6355,10 +6355,12 @@ Type ParamDecl::getVarargBaseTy(Type VarArgT) {
 
 AnyFunctionType::Param ParamDecl::toFunctionParam(Type type) const {
   if (!type) {
+    type = getInterfaceType();
+
     if (hasExternalPropertyWrapper()) {
-      type = getPropertyWrapperBackingPropertyType();
-    } else {
-      type = getInterfaceType();
+      if (auto wrapper = getPropertyWrapperBackingPropertyType()) {
+        type = wrapper;
+      }
     }
   }
 

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -3026,10 +3026,12 @@ void TypeChecker::checkParameterList(ParameterList *params,
       (void) param->getPropertyWrapperInitializerInfo();
 
     auto *SF = param->getDeclContext()->getParentSourceFile();
-    param->visitAuxiliaryDecls([&](VarDecl *auxiliaryDecl) {
-      if (!isa<ParamDecl>(auxiliaryDecl))
-        DeclChecker(param->getASTContext(), SF).visitBoundVariable(auxiliaryDecl);
-    });
+    if (!param->isInvalid()) {
+      param->visitAuxiliaryDecls([&](VarDecl *auxiliaryDecl) {
+        if (!isa<ParamDecl>(auxiliaryDecl))
+          DeclChecker(param->getASTContext(), SF).visitBoundVariable(auxiliaryDecl);
+      });
+    }
   }
 
   // For source compatibilty, allow duplicate internal parameter names

test/Sema/property_wrapper_parameter_invalid.swift

@@ -187,3 +187,15 @@ func testMissingWrapperType() {
     return
   }
 }
+
+@propertyWrapper
+struct OptionalWrapper<Value> { // expected-note {{'Value' declared as parameter to type 'OptionalWrapper'}}
+  var wrappedValue: Value?
+  var projectedValue: Self { self }
+  init(wrappedValue: Value?) { self.wrappedValue = wrappedValue }
+  init(projectedValue: Self) { self = projectedValue }
+}
+
+// expected-error@+2 {{generic parameter 'Value' could not be inferred}} expected-note@+2 {{}}
+// expected-error@+1 {{property type 'Int' does not match 'wrappedValue' type 'Value?'}}
+func testWrappedValueMismatch(@OptionalWrapper value: Int) {}