Update hashProposal visibility to view

.changeset/popular-horses-tell.md

@@ -0,0 +1,5 @@
+---
+'openzeppelin-solidity': minor
+---
+
+`Governor`: Change `hashProposal()` visibility to view.

contracts/governance/Governor.sol

@@ -118,16 +118,20 @@ abstract contract Governor is Context, ERC165, EIP712, Nonces, IGovernor, IERC72
      * advance, before the proposal is submitted.
      *
      * Note that the chainId and the governor address are not part of the proposal id computation. Consequently, the
-     * same proposal (with same operation and same description) will have the same id if submitted on multiple governors
-     * across multiple networks. This also means that in order to execute the same operation twice (on the same
-     * governor) the proposer will have to change the description in order to avoid proposal id conflicts.
+     * same proposal will share its id with another governor in a different network where the same proposal was
+     * submitted. We recommend using the description field as a discriminator to avoid proposal id conflicts.
+     * Similarly, developers can add both `chainId` and `address(this)` to the hash computation by overriding
+     * this function.
+     *
+     * IMPORTANT: Overriding this function may have unexpected effects after an upgrade. Consider to verify
+     * that the new implementation doesn't allow a proposer to forge an id.
      */
     function hashProposal(
         address[] memory targets,
         uint256[] memory values,
         bytes[] memory calldatas,
         bytes32 descriptionHash
-    ) public pure virtual returns (uint256) {
+    ) public view virtual returns (uint256) {
         return uint256(keccak256(abi.encode(targets, values, calldatas, descriptionHash)));
     }
 

contracts/governance/IGovernor.sol

@@ -207,7 +207,7 @@ interface IGovernor is IERC165, IERC6372 {
         uint256[] memory values,
         bytes[] memory calldatas,
         bytes32 descriptionHash
-    ) external pure returns (uint256);
+    ) external view returns (uint256);
 
     /**
      * @notice module:core