Contract level checker: Disallow free function redefinition and alias

shadowing another free function

Co-authored-by: chriseth <chris@ethereum.org>

test/libsolidity/syntaxTests/freeFunctions/free_different_integer_types.sol

@@ -0,0 +1,4 @@
+function f(uint24) {}
+function f(uint16) {}
+function f(int24) {}
+function f(bool) {}

test/libsolidity/syntaxTests/freeFunctions/free_function_namesake_different_parameter_types.sol

@@ -0,0 +1,11 @@
+function g() pure returns (uint) { return 1; }
+function g() pure returns (string memory) { return "1"; }
+contract C {
+  function foo() public pure returns (uint) {
+    string memory s = g();
+    return 100/g();
+  }
+}
+// ----
+// DeclarationError 1686: (0-46): Function with same name and parameter types defined twice.
+// TypeError 9574: (168-189): Type uint256 is not implicitly convertible to expected type string memory.

test/libsolidity/syntaxTests/freeFunctions/free_identical.sol

@@ -0,0 +1,4 @@
+function f() pure returns (uint) { return 1337; }
+function f() pure returns (uint) { return 42; }
+// ----
+// DeclarationError 1686: (0-49): Function with same name and parameter types defined twice.

test/libsolidity/syntaxTests/freeFunctions/free_identical_multiple.sol

@@ -0,0 +1,5 @@
+function f() pure returns (uint) { return 1337; }
+function f() pure returns (uint) { return 42; }
+function f() pure returns (uint) { return 1; }
+// ----
+// DeclarationError 1686: (0-49): Function with same name and parameter types defined twice.

test/libsolidity/syntaxTests/freeFunctions/free_namesake_contract_function.sol

@@ -0,0 +1,8 @@
+function f() pure returns (uint) { return 1337; }
+contract C {
+  function f() public pure returns (uint) {
+    return f();
+  }
+}
+// ----
+// Warning 2519: (65-126): This declaration shadows an existing declaration.

test/libsolidity/syntaxTests/freeFunctions/free_redefinition.sol

@@ -0,0 +1,9 @@
+function f() pure returns (uint) { return 1337; }
+function f() view returns (uint) { return 42; }
+contract C {
+  function g() public pure virtual returns (uint) {
+    return f();
+  }
+}
+// ----
+// DeclarationError 1686: (0-49): Function with same name and parameter types defined twice.