FlowChecker: Fix segfault with error reporting parameter name

Summary:
It's possible that the parameter doesn't have a name when reporting an
error, so check before dereferencing it.

Reviewed By: tmikov

Differential Revision: D66665056

fbshipit-source-id: 7e89b8aecf2c47ab7cc24d72e074336acddc20a7

lib/Sema/FlowChecker-expr.cpp

@@ -1894,10 +1894,17 @@ class FlowChecker::ExprVisitor {
       auto [argTypeNarrow, cf] = tryNarrowType(argType, expectedType);
 
       if (!cf.canFlow) {
-        outer_.sm_.error(
-            arg->getSourceRange(),
-            "ft: " + calleeName + " parameter '" + param.first.str() +
-                "' type mismatch");
+        if (param.first.isValid()) {
+          outer_.sm_.error(
+              arg->getSourceRange(),
+              "ft: " + calleeName + " parameter '" + param.first.str() +
+                  "' type mismatch");
+        } else {
+          outer_.sm_.error(
+              arg->getSourceRange(),
+              "ft: " + calleeName + " parameter #" + llvh::Twine(argIndex + 1) +
+                  " type mismatch");
+        }
         return false;
       }
       // If a cast is needed, replace the argument with the cast.

test/Sema/flow/regress-function-param-no-name-error.js

@@ -0,0 +1,19 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+// RUN: (! %shermes -Werror -ferror-limit=0 -typed -dump-sema %s 2>&1 ) | %FileCheckOrRegen --match-full-lines %s
+
+function f(cb: number => string) {
+  cb('a');
+}
+
+// Auto-generated content below. Please do not modify manually.
+
+// CHECK:{{.*}}regress-function-param-no-name-error.js:11:6: error: ft: function parameter #1 type mismatch
+// CHECK-NEXT:  cb('a');
+// CHECK-NEXT:     ^~~
+// CHECK-NEXT:Emitted 1 errors. exiting.