[NFC] Allow getParameter{List,At} to be called on any ValueDecl

These now return nullptr for parameter-list-free types; previously those would have failed a cast.

Author: beccadax

include/swift/AST/Decl.h

@@ -7405,10 +7405,12 @@ inline EnumElementDecl *EnumDecl::getUniqueElement(bool hasValue) const {
   return result;
 }
 
-/// Retrieve the parameter list for a given declaration.
+/// Retrieve the parameter list for a given declaration, or nullputr if there
+/// is none.
 ParameterList *getParameterList(ValueDecl *source);
 
-/// Retrieve parameter declaration from the given source at given index.
+/// Retrieve parameter declaration from the given source at given index, or
+/// nullptr if the source does not have a parameter list.
 const ParamDecl *getParameterAt(const ValueDecl *source, unsigned index);
 
 /// Display Decl subclasses.

lib/AST/Decl.cpp

@@ -6623,14 +6623,19 @@ ParameterList *swift::getParameterList(ValueDecl *source) {
     return AFD->getParameters();
   } else if (auto *EED = dyn_cast<EnumElementDecl>(source)) {
     return EED->getParameterList();
-  } else {
-    return cast<SubscriptDecl>(source)->getIndices();
+  } else if (auto *SD = dyn_cast<SubscriptDecl>(source)) {
+    return SD->getIndices();
   }
+
+  return nullptr;
 }
 
 const ParamDecl *swift::getParameterAt(const ValueDecl *source,
                                        unsigned index) {
-  return getParameterList(const_cast<ValueDecl *>(source))->get(index);
+  if (auto *params = getParameterList(const_cast<ValueDecl *>(source))) {
+    return params->get(index);
+  }
+  return nullptr;
 }
 
 Type AbstractFunctionDecl::getMethodInterfaceType() const {