Fix control-flow handling of modifiers without body.

libsolidity/analysis/ControlFlowBuilder.cpp

@@ -308,7 +308,7 @@ bool ControlFlowBuilder::visit(ModifierInvocation const& _modifierInvocation)
 		_modifierInvocation.name().annotation().referencedDeclaration
 	);
 	if (!modifierDefinition) return false;
-	solAssert(!!modifierDefinition, "");
+	if (!modifierDefinition->isImplemented()) return false;
 	solAssert(!!m_returnNode, "");
 
 	m_placeholderEntry = newLabel();

libsolidity/analysis/ControlFlowBuilder.h

@@ -29,9 +29,8 @@
 namespace solidity::frontend
 {
 
-/** Helper class that builds the control flow of a function or modifier.
- * Modifiers are not yet applied to the functions. This is done in a second
- * step in the CFG class.
+/**
+ * Helper class that builds the control flow of a function or modifier.
  */
 class ControlFlowBuilder: private ASTConstVisitor, private yul::ASTWalker
 {

test/libsolidity/smtCheckerTests/modifiers/modifier_overriding_2.sol

@@ -23,5 +23,4 @@ contract C is B {
 // ====
 // SMTEngine: all
 // ----
-// Warning 5740: (62-111): Unreachable code.
 // Warning 6328: (66-75): CHC: Assertion violation happens here.\nCounterexample:\ns = false\n\nTransaction trace:\nB.constructor()\nState: s = false\nA.f()

test/libsolidity/smtCheckerTests/modifiers/modifier_overriding_3.sol

@@ -18,5 +18,4 @@ contract B is A {
 // ====
 // SMTEngine: all
 // ----
-// Warning 5740: (62-123): Unreachable code.
 // Warning 6328: (94-104): CHC: Assertion violation happens here.\nCounterexample:\ns = true\nx = true\n\nTransaction trace:\nB.constructor()\nState: s = false\nA.f()

test/libsolidity/syntaxTests/controlFlow/modifiers/implemented_without_placeholder.sol

@@ -0,0 +1,8 @@
+abstract contract A {
+    function f() public view mod {
+        require(block.timestamp > 10);
+    }
+    modifier mod() virtual { }
+}
+// ----
+// SyntaxError 2883: (129-132): Modifier body does not contain '_'.

test/libsolidity/syntaxTests/controlFlow/modifiers/non_implemented_modifer.sol

@@ -0,0 +1,6 @@
+abstract contract A {
+    function f() public view mod {
+        require(block.timestamp > 10);
+    }
+    modifier mod() virtual;
+}