Adding Stack Height Checker and modifying the number of POP instructions to appropriately provide the pointer address

Changelog.md

@@ -17,6 +17,7 @@ Bugfixes:
  * IR Generator: Fix IR syntax error when copying storage arrays of structs containing functions.
  * Natspec: Fix ICE when overriding a struct getter with a Natspec-documented return value and the name in the struct is different.
  * TypeChecker: Fix ICE when a constant variable declaration forward references a struct.
+ * Code Generator: Fix ICE when accessing the members of external functions occupying more than two stack slots.
 
 
 Solc-Js:

libsolidity/codegen/ExpressionCompiler.cpp

@@ -1760,6 +1760,9 @@ bool ExpressionCompiler::visit(MemberAccess const& _memberAccess)
 	case Type::Category::Function:
 		if (member == "selector")
 		{
+			auto const& functionType = dynamic_cast<FunctionType const&>(*_memberAccess.expression().annotation().type);
+			if (functionType.kind() == FunctionType::Kind::External)
+				CompilerUtils(m_context).popStackSlots(functionType.sizeOnStack() - 2);
 			m_context << Instruction::SWAP1 << Instruction::POP;
 			/// need to store it as bytes4
 			utils().leftShiftNumberOnStack(224);
@@ -1768,8 +1771,7 @@ bool ExpressionCompiler::visit(MemberAccess const& _memberAccess)
 		{
 			auto const& functionType = dynamic_cast<FunctionType const&>(*_memberAccess.expression().annotation().type);
 			solAssert(functionType.kind() == FunctionType::Kind::External, "");
-			// stack: <address> <function_id>
+			CompilerUtils(m_context).popStackSlots(functionType.sizeOnStack() - 1);
-			m_context << Instruction::POP;
 		}
 		else
 			solAssert(

test/libsolidity/semanticTests/functionTypes/stack_height_check_on_adding_gas_variable_to_function.sol

@@ -0,0 +1,25 @@
+contract C {
+    function g() external {}
+    function h() external payable {}
+    function test_function() external returns (bool){
+        assert (
+            this.g.address == this.g.address &&
+            this.g{gas: 42}.address == this.g.address &&
+            this.g{gas: 42}.selector == this.g.selector
+        );
+        assert (
+            this.h.address == this.h.address &&
+            this.h{gas: 42}.address == this.h.address &&
+            this.h{gas: 42}.selector == this.h.selector
+        );
+        assert (
+            this.h{gas: 42, value: 5}.address == this.h.address &&
+            this.h{gas: 42, value: 5}.selector == this.h.selector
+        );
+        return true;
+    }
+}
+// ====
+// compileViaYul: also
+// ----
+// test_function() -> true

test/libsolidity/syntaxTests/functionTypes/external_functions_with_variable_number_of_stack_slots.sol

@@ -0,0 +1,8 @@
+contract C {
+    function f (address) external returns (bool) {
+        this.f{gas: 42}.address;
+    }
+}
+// ----
+// Warning 6321: (56-60): Unnamed return variable can remain unassigned. Add an explicit return with value to all non-reverting code paths or name the variable.
+// Warning 2018: (17-102): Function state mutability can be restricted to view