[Sema] Don't crash in matchCallArguments if autoclosure type is not a function type

During code completion, the function type may be a placeholder type. Don't crash in those cases.

Author: ahoppen

lib/Sema/CSSimplify.cpp

@@ -1815,7 +1815,6 @@ static ConstraintSystem::TypeMatchResult matchCallArguments(
       auto *argExpr = getArgumentExpr(locator.getAnchor(), argIdx);
       if (param.isAutoClosure() && !isSynthesizedArgument(argument)) {
         auto &ctx = cs.getASTContext();
-        auto *fnType = paramTy->castTo<FunctionType>();
 
         // If this is a call to a function with a closure argument and the
         // parameter is an autoclosure, let's just increment the score here
@@ -1835,7 +1834,9 @@ static ConstraintSystem::TypeMatchResult matchCallArguments(
         if (ctx.isSwiftVersionAtLeast(5) || !isAutoClosureArgument(argExpr)) {
           // In Swift >= 5 mode there is no @autoclosure forwarding,
           // so let's match result types.
-          paramTy = fnType->getResult();
+          if (auto *fnType = paramTy->getAs<FunctionType>()) {
+            paramTy = fnType->getResult();
+          }
         } else {
           // Matching @autoclosure argument to @autoclosure parameter
           // directly would mean introducting a function conversion