functions taking calldata args should be assignable to function pointers of same type

docs/types/value-types.rst

@@ -789,6 +789,18 @@ This includes private, internal and public functions of both contracts and libra
 functions.
 External function types, on the other hand, are only compatible with public and external contract
 functions.
+
+.. note::
+    External functions with ``calldata`` parameters are incompatible with external function types with ``calldata`` parameters.
+    They are compatible with the corresponding types with ``memory`` parameters instead.
+    For example, there is no function that can be pointed at by a value of type ``function (string calldata) external`` while
+    ``function (string memory) external`` can point at both ``function f(string memory) external {}`` and
+    ``function g(string calldata) external {}``.
+    This is because for both locations the arguments are passed to the function in the same way.
+    The caller cannot pass its calldata directly to an external function and always ABI-encodes the arguments into memory.
+    Marking the parameters as ``calldata`` only affects the implementation of the external function and is
+    meaningless in a function pointer on the caller's side.
+
 Libraries are excluded because they require a ``delegatecall`` and use :ref:`a different ABI
 convention for their selectors <library-selectors>`.
 Functions declared in interfaces do not have definitions so pointing at them does not make sense either.

test/libsolidity/semanticTests/functionTypes/external_functions_with_calldata_args_assigned_to_function_pointers_with_memory_type.sol

@@ -0,0 +1,10 @@
+contract C {
+    function g(string calldata) external returns (bool) { return true; }
+
+    function main() external returns (bool) {
+        function (string memory) external returns (bool) ptr = this.g;
+        return ptr("testString");
+    }
+}
+// ----
+// main() -> true

test/libsolidity/syntaxTests/functionTypes/external_functions_with_calldata_args_assigned_to_function_pointers_of_same_type.sol

@@ -0,0 +1,14 @@
+contract C {
+    function f(function (string calldata) external) external {}
+    function g(string calldata) external {}
+
+    function main() external {
+        function (string calldata) external ptr = this.g;
+        abi.encodeCall(this.f, (this.g));
+        this.f(this.g);
+    }
+}
+// ----
+// TypeError 9574: (161-209): Type function (string memory) external is not implicitly convertible to expected type function (string calldata) external.
+// TypeError 5407: (242-250): Cannot implicitly convert component at position 0 from "function (string memory) external" to "function (string calldata) external".
+// TypeError 9553: (268-274): Invalid type for argument in function call. Invalid implicit conversion from function (string memory) external to function (string calldata) external requested.

test/libsolidity/syntaxTests/functionTypes/external_functions_with_calldata_args_assigned_to_function_pointers_with_memory_type.sol

@@ -0,0 +1,9 @@
+contract C {
+    function g(string calldata) external {}
+
+    function main() view external {
+        function (string memory) external ptr = this.g;
+        ptr;
+    }
+}
+// ----

test/libsolidity/syntaxTests/functionTypes/internal_functions_with_calldata_args_assigned_to_function_pointers_of_same_type.sol

@@ -0,0 +1,9 @@
+contract C {
+    function g(bytes calldata b) pure internal {}
+
+    function main() pure external {
+        function (bytes calldata) internal ptr = g;
+        ptr;
+    }
+}
+// ----