Tests and Changelog for basefee.

test/libsolidity/syntaxTests/types/magic_block_basefee_error.sol

@@ -0,0 +1,15 @@
+contract C {
+    function f() public view returns (uint) {
+        return block.basefee;
+    }
+    function g() public view returns (uint ret) {
+        assembly {
+            ret := basefee()
+        }
+    }
+}
+// ====
+// EVMVersion: =berlin
+// ----
+// TypeError 5921: (74-87): "basefee" is not supported by the VM version.
+// TypeError 5430: (183-190): The "basefee" instruction is only available for London-compatible VMs (you are currently compiling for "berlin").

test/libsolidity/syntaxTests/types/magic_block_london.sol

@@ -0,0 +1,13 @@
+contract C {
+    function f() public view returns (uint) {
+        return block.basefee;
+    }
+    function g() public view returns (uint ret) {
+        assembly {
+            ret := basefee()
+        }
+    }
+}
+// ====
+// EVMVersion: >=london
+// ----

test/libsolidity/syntaxTests/viewPureChecker/basefee_not_pure.sol

@@ -0,0 +1,13 @@
+contract C {
+    function f() public pure {
+        assembly { pop(basefee()) }
+    }
+    function g() public pure returns (uint) {
+        return block.basefee;
+    }
+}
+// ====
+// EVMVersion: >=london
+// ----
+// TypeError 2527: (67-76): Function declared as pure, but this expression (potentially) reads from the environment or state and thus requires "view".
+// TypeError 2527: (147-160): Function declared as pure, but this expression (potentially) reads from the environment or state and thus requires "view".