[CS] Add null check for ParameterList in findFavoredChoicesBasedOnArity

rdar://164116965

Author: hamishknight

lib/Sema/CSOptimizer.cpp

@@ -658,11 +658,20 @@ static void findFavoredChoicesBasedOnArity(
           return;
         }
 
-        if (overloadType->getNumParams() == argumentList->size() ||
-            llvm::count_if(*decl->getParameterList(), [](auto *param) {
-              return !param->isDefaultArgument();
-            }) == argumentList->size())
+        if (overloadType->getNumParams() == argumentList->size()) {
           favored.push_back(choice);
+          return;
+        }
+
+        if (auto *paramList = decl->getParameterList()) {
+          auto numNonDefaulted = llvm::count_if(*paramList, [](auto *param) {
+            return !param->isDefaultArgument();
+          });
+          if (numNonDefaulted == argumentList->size()) {
+            favored.push_back(choice);
+            return;
+          }
+        }
       });
 
   if (hasVariadicGenerics)

validation-test/compiler_crashers/findFavoredChoicesBasedOnArity-259af6.swift renamed to validation-test/compiler_crashers_fixed/findFavoredChoicesBasedOnArity-259af6.swift

@@ -1,4 +1,4 @@
 // {"kind":"typecheck","signature":"void llvm::function_ref<void (swift::constraints::Constraint*, swift::ValueDecl*, swift::FunctionType*)>::callback_fn<(anonymous namespace)::findFavoredChoicesBasedOnArity(swift::constraints::ConstraintSystem&, swift::constraints::Constraint*, swift::ArgumentList*, llvm::function_ref<void (swift::constraints::Constraint*)>)::$_0>(long, swift::constraints::Constraint*, swift::ValueDecl*, swift::FunctionType*)"}
-// RUN: not --crash %target-swift-frontend -typecheck %s
+// RUN: not %target-swift-frontend -typecheck %s
 var a: Any
 let a : () -> Void = a (b)

validation-test/compiler_crashers_fixed/findFavoredChoicesBasedOnArity-b8ad37.swift

@@ -0,0 +1,5 @@
+// {"kind":"typecheck","signature":"void llvm::function_ref<void (swift::constraints::Constraint*, swift::ValueDecl*, swift::FunctionType*)>::callback_fn<(anonymous namespace)::findFavoredChoicesBasedOnArity(swift::constraints::ConstraintSystem&, swift::constraints::Constraint*, swift::ArgumentList*, llvm::function_ref<void (swift::constraints::Constraint*)>)::$_0>(long, swift::constraints::Constraint*, swift::ValueDecl*, swift::FunctionType*)"}
+// RUN: not %target-swift-frontend -typecheck %s
+let foo: (String) -> Void = {_ in}
+func foo(_ x: String) {}
+foo()