[Sol->Yul] Provide selector for some internal functions.

libsolidity/codegen/ir/IRGeneratorForStatements.cpp

@@ -1689,9 +1689,15 @@ void IRGeneratorForStatements::endVisit(MemberAccess const& _memberAccess)
 				functionType.kind() == FunctionType::Kind::DelegateCall
 			)
 				define(IRVariable{_memberAccess}, IRVariable(_memberAccess.expression()).part("functionSelector"));
-			else if (functionType.kind() == FunctionType::Kind::Declaration)
+			else if (
+				functionType.kind() == FunctionType::Kind::Declaration ||
+				// In some situations, internal function types also provide the "selector" member.
+				// See Types.cpp for details.
+				functionType.kind() == FunctionType::Kind::Internal
+			)
 			{
 				solAssert(functionType.hasDeclaration(), "");
+				solAssert(functionType.declaration().isPartOfExternalInterface(), "");
 				define(IRVariable{_memberAccess}) << formatNumber(functionType.externalIdentifier() << 224) << "\n";
 			}
 			else

test/libsolidity/semanticTests/constants/function_unreferenced.sol

@@ -0,0 +1,11 @@
+contract B {
+    function g() public {}
+}
+contract C is B {
+    bytes4 constant s2 = B.g.selector;
+    function f() external pure returns (bytes4) { return s2; }
+}
+// ====
+// compileViaYul: also
+// ----
+// f() -> 0xe2179b8e00000000000000000000000000000000000000000000000000000000

test/libsolidity/semanticTests/functionTypes/selector_1.sol

@@ -0,0 +1,14 @@
+contract B {
+    function ext() external {}
+    function pub() public {}
+}
+
+contract C is B {
+    function test() public returns (bytes4, bytes4, bytes4, bytes4) {
+        return (B.ext.selector, B.pub.selector, this.ext.selector, pub.selector);
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// test() -> 0xcf9f23b500000000000000000000000000000000000000000000000000000000, 0x7defb41000000000000000000000000000000000000000000000000000000000, 0xcf9f23b500000000000000000000000000000000000000000000000000000000, 0x7defb41000000000000000000000000000000000000000000000000000000000

test/libsolidity/semanticTests/functionTypes/selector_2.sol

@@ -0,0 +1,14 @@
+contract B {
+    function ext() external {}
+    function pub() public {}
+}
+
+contract D {
+    function test() public returns (bytes4, bytes4) {
+        return (B.ext.selector, B.pub.selector);
+    }
+}
+// ====
+// compileViaYul: true
+// ----
+// test() -> 0xcf9f23b500000000000000000000000000000000000000000000000000000000, 0x7defb41000000000000000000000000000000000000000000000000000000000

test/libsolidity/syntaxTests/functionTypes/selectors.sol

@@ -0,0 +1,26 @@
+contract B {
+    function ext() external {}
+    function pub() public {}
+}
+
+contract C is B {
+    function test() public pure {
+        B.ext.selector;
+        B.pub.selector;
+        this.ext.selector;
+        pub.selector;
+    }
+}
+
+contract D {
+    function test() public pure {
+        B.ext.selector;
+        B.pub.selector;
+    }
+}
+// ----
+// Warning 6133: (136-150): Statement has no effect.
+// Warning 6133: (160-174): Statement has no effect.
+// Warning 6133: (184-201): Statement has no effect.
+// Warning 6133: (289-303): Statement has no effect.
+// Warning 6133: (313-327): Statement has no effect.