Introduce FunctionKind::Declaration and allow accessing function signatures via contract name.

test/libsolidity/semanticTests/functionSelector/function_selector_via_contract_name.sol

@@ -0,0 +1,20 @@
+contract A {
+    function f() external {}
+    function g(uint256) external {}
+}
+contract B {
+    function f() external returns (uint256) {}
+    function g(uint256) external returns (uint256) {}
+}
+contract C {
+    function test1() external returns(bytes4, bytes4, bytes4, bytes4) {
+        return (A.f.selector, A.g.selector, B.f.selector, B.g.selector);
+    }
+    function test2() external returns(bytes4, bytes4, bytes4, bytes4) {
+        A a; B b;
+        return (a.f.selector, a.g.selector, b.f.selector, b.g.selector);
+    }
+}
+// ----
+// test1() -> left(0x26121ff0), left(0xe420264a), left(0x26121ff0), left(0xe420264a)
+// test2() -> left(0x26121ff0), left(0xe420264a), left(0x26121ff0), left(0xe420264a)

test/libsolidity/syntaxTests/types/function_types/definition/assign_function_via_contract_name_to_var.sol

@@ -0,0 +1,14 @@
+contract A {
+    function f() external {}
+    function g() external pure {}
+}
+
+contract B {
+    function h() external {
+        function() external f = A.f;
+        function() external pure g = A.g;
+    }
+}
+// ----
+// TypeError: (128-155): Type function A.f() is not implicitly convertible to expected type function () external.
+// TypeError: (165-197): Type function A.g() pure is not implicitly convertible to expected type function () pure external.

test/libsolidity/syntaxTests/types/function_types/definition/call_function_via_contract_name.sol

@@ -0,0 +1,17 @@
+contract A {
+    function f() external {}
+    function g() external pure {}
+    function h() public pure {}
+}
+
+contract B {
+    function i() external {
+        A.f();
+        A.g();
+        A.h(); // might be allowed in the future
+    }
+}
+// ----
+// TypeError: (160-165): Cannot call function via contract name.
+// TypeError: (175-180): Cannot call function via contract name.
+// TypeError: (190-195): Cannot call function via contract name.

test/libsolidity/syntaxTests/types/function_types/definition/function_selector_via_contract_name.sol

@@ -0,0 +1,9 @@
+contract A {
+    function f() external {}
+}
+
+contract B {
+    function g() external pure {
+        A.f.selector;
+    }
+}

test/libsolidity/syntaxTests/types/function_types/definition/function_selector_via_interface_name.sol

@@ -0,0 +1,9 @@
+interface I {
+    function f() external;
+}
+
+contract B {
+    function g() external pure {
+        I.f.selector;
+    }
+}

test/libsolidity/syntaxTests/types/function_types/definition/function_via_contract_name_internal.sol

@@ -0,0 +1,11 @@
+contract A {
+    function f() internal {}
+}
+
+contract B {
+    function g() external {
+        A.f;
+    }
+}
+// ----
+// TypeError: (94-97): Member "f" not found or not visible after argument-dependent lookup in type(contract A).

test/libsolidity/syntaxTests/types/function_types/definition/function_via_contract_name_overloaded.sol

@@ -0,0 +1,12 @@
+contract A {
+    function f() external {}
+    function f(uint256) external {}
+}
+
+contract B {
+    function g() external {
+        A.f;
+    }
+}
+// ----
+// TypeError: (130-133): Member "f" not unique after argument-dependent lookup in type(contract A).

test/libsolidity/syntaxTests/types/function_types/definition/function_via_contract_name_private.sol

@@ -0,0 +1,11 @@
+contract A {
+    function f() private {}
+}
+
+contract B {
+    function g() external {
+        A.f;
+    }
+}
+// ----
+// TypeError: (93-96): Member "f" not found or not visible after argument-dependent lookup in type(contract A).

test/libsolidity/syntaxTests/types/function_types/definition/function_via_contract_name_public.sol

@@ -0,0 +1,9 @@
+contract A {
+    function f() public {}
+}
+
+contract B {
+    function g() external pure {
+        A.f.selector;
+    }
+}