Add C4770 per cpp-docs 4253 (#4649)

* Add C4770 per cpp-docs 4253

* Acrolinx issues

* Tweaks to title

Author: colin-home

docs/error-messages/compiler-warnings/c4770.md

@@ -0,0 +1,53 @@
+---
+description: "Learn more about: Compiler Warning (level 4) C4770"
+title: "Compiler Warning (level 4) C4770"
+ms.date: 11/02/2022
+f1_keywords: ["C4770"]
+helpviewer_keywords: ["C4770"]
+---
+# Compiler Warning (level 4) C4770
+
+> partially validated enum '*symbol*' used as index
+
+The compiler warns if an enum value is cast or aliased to an integer type, but the result isn't checked for non-negative or excessive values.
+
+## Remarks
+
+This warning is new in Visual Studio 2013. It's not enabled by default. To enable it as a level 1 warning, use `/w14770`. For information on how to disable warnings by compiler version, see [Compiler warnings by compiler version](compiler-warnings-by-compiler-version.md).
+
+## Example
+
+The following code produces warning C4770:
+
+```cpp
+// c4770.cpp
+// compile by using: cl /GL /w14770 c4770.cpp
+
+enum E { a  0, b, c, E_MAX };
+
+int main(int argc, char *argv[])
+{
+    const E e1 = E(argc); // value unknown at compile time
+
+    if ((int)(e1) >= E_MAX)
+        return 0;
+
+    const int n = e1 + e1;  // C4770 partially validated enum used as index
+
+    return argv[n][n];
+}
+```
+
+To fix the warning, you could cast the value in the check to `unsigned int`, which implicitly forces a non-negative value:
+
+```cpp
+    if ((unsigned int)(e1) >= E_MAX) 
+        return 0;
+```
+
+Or, explicitly check for a non-negative value:
+
+```cpp
+    if ((int)(e1) >= E_MAX || (int)(e1) < 0)
+        return 0;
+```

docs/error-messages/compiler-warnings/compiler-warnings-c4600-through-c4799.md

@@ -2,8 +2,8 @@
 description: "Learn more about: Compiler warnings C4600 Through C4799"
 title: "Compiler warnings C4600 Through C4799"
 ms.date: 05/03/2021
-f1_keywords: ["C4609", "C4658", "C4671", "C4676", "C4689", "C4695", "C4696", "C4719", "C4720", "C4721", "C4728", "C4732", "C4751", "C4752", "C4755", "C4757", "C4767", "C4770"]
-helpviewer_keywords: ["C4609", "C4658", "C4671", "C4676", "C4689", "C4695", "C4696", "C4719", "C4720", "C4721", "C4728", "C4732", "C4751", "C4752", "C4755", "C4757", "C4767", "C4770"]
+f1_keywords: ["C4609", "C4658", "C4671", "C4676", "C4689", "C4695", "C4696", "C4719", "C4720", "C4721", "C4728", "C4732", "C4751", "C4752", "C4755", "C4757", "C4767"]
+helpviewer_keywords: ["C4609", "C4658", "C4671", "C4676", "C4689", "C4695", "C4696", "C4719", "C4720", "C4721", "C4728", "C4732", "C4751", "C4752", "C4755", "C4757", "C4767"]
 ms.assetid: 22bd4392-f3be-445c-9f23-6126aebac901
 ---
 # Compiler warnings C4600 Through C4799
@@ -147,7 +147,7 @@ The articles in this section of the documentation explain a subset of the warnin
 |[Compiler warning (level 4) C4764](compiler-warning-level-4-c4764.md)|Can not align catch objects to greater than 16 bytes|
 |Compiler warning (level 4) C4767|section name '%s' is longer than 8 characters and will be truncated by the linker|
 |[Compiler warning (level 3) C4768](c4768.md)|__declspec attributes before linkage specification are ignored|
-|Compiler warning C4770|partially validated enum '*name*' used as index|
+| [Compiler warning (level 4) C4770](./c4770.md) | partially validated enum '*name*' used as index |
 |Compiler warning C4771|Bounds must be created using a simple pointer; MPX intrinsic function ignored|
 |[Compiler warning (level 1, Error) C4772](../../error-messages/compiler-warnings/compiler-warning-level-1-c4772.md)|#import referenced a type from a missing type library; 'missing_type' used as a placeholder|
 |Compiler warning (level 4) C4774|'*string*' : format string expected in argument *number* is not a string literal|

docs/error-messages/toc.yml

@@ -4156,6 +4156,8 @@ items:
           href: compiler-warnings/compiler-warning-level-4-c4764.md
         - name: Compiler warning (level 4) C4768
           href: compiler-warnings/c4768.md
+        - name: Compiler warning (level 4) C4770
+          href: compiler-warnings/c4770.md
         - name: Compiler warning (level 1) C4772
           href: compiler-warnings/compiler-warning-level-1-c4772.md
         - name: Compiler warning (Level 1) C4788