New tests for function state variables.

test/libsolidity/syntaxTests/functionTypes/external_function_type_public_variable.sol

@@ -0,0 +1,11 @@
+contract C {
+    function (uint) external public x;
+
+    function g(uint) public {
+        x = this.g;
+    }
+    function f() public view returns (function(uint) external) {
+        return this.x();
+    }
+}
+// ----

test/libsolidity/syntaxTests/functionTypes/function_type_internal_public_variable.sol

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+contract C {
+    function(bytes memory) internal public a;
+}
+// ----
+// TypeError: (17-57): Internal or recursive type is not allowed for public state variables.

test/libsolidity/syntaxTests/functionTypes/function_type_variable_external_internal.sol

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+contract test {
+    function fa(bytes memory) { }
+    function(bytes memory) external internal a = fa;
+}
+// ----
+// TypeError: (99-101): Type function (bytes memory) is not implicitly convertible to expected type function (bytes memory) external.

test/libsolidity/syntaxTests/functionTypes/function_types_internal_visibility_error.sol

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+contract C {
+    // This is an error, you should explicitly use
+    // `external public` to fix it - `internal public` does not exist.
+    function(bytes memory) public a;
+}
+// ----
+// TypeError: (139-170): Invalid visibility, can only be "external" or "internal".

test/libsolidity/syntaxTests/functionTypes/function_types_variable_visibility.sol

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+contract C {
+    function(bytes memory) a1;
+    function(bytes memory) internal b1;
+    function(bytes memory) internal internal b2;
+    function(bytes memory) external c1;
+    function(bytes memory) external internal c2;
+    function(bytes memory) external public c3;
+}
+// ----

test/libsolidity/syntaxTests/functionTypes/valid_function_type_variables.sol

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+contract test {
+    function fa(uint) {}
+    function fb(uint) internal {}
+    function fc(uint) internal {}
+    function fd(uint) external {}
+    function fe(uint) external {}
+    function ff(uint) internal {}
+    function fg(uint) internal pure {}
+    function fh(uint) pure internal {}
+
+    function(uint) a = fa;
+    function(uint) internal b = fb; // (explicit internal applies to the function type)
+    function(uint) internal internal c = fc;
+    function(uint) external d = this.fd;
+    function(uint) external internal e = this.fe;
+    function(uint) internal public f = ff;
+    function(uint) internal pure public g = fg;
+    function(uint) pure internal public h = fh;
+}
+// ----
+// TypeError: (545-582): Internal or recursive type is not allowed for public state variables.
+// TypeError: (588-630): Internal or recursive type is not allowed for public state variables.
+// TypeError: (636-678): Internal or recursive type is not allowed for public state variables.