Moved a check related to constants to TypeChecker

And added a proper error message when constant types containing (nested) mapping types are used.

libsolidity/analysis/DeclarationTypeChecker.cpp

@@ -437,14 +437,16 @@ void DeclarationTypeChecker::endVisit(VariableDeclaration const& _variable)
 		type = TypeProvider::withLocation(ref, typeLoc, isPointer);
 	}
 
-	if (_variable.isConstant() && !type->isValueType())
-	{
-		bool allowed = false;
-		if (auto arrayType = dynamic_cast<ArrayType const*>(type))
-			allowed = arrayType->isByteArray();
-		if (!allowed)
-			m_errorReporter.fatalDeclarationError(9259_error, _variable.location(), "Constants of non-value type not yet implemented.");
-	}
+	if (
+		_variable.isConstant() &&
+		!dynamic_cast<UserDefinedValueType const*>(type) &&
+		type->containsNestedMapping()
+	)
+		m_errorReporter.fatalDeclarationError(
+			3530_error,
+			_variable.location(),
+			"The type contains a (nested) mapping and therefore cannot be a constant."
+		);
 
 	_variable.annotation().type = type;
 }

libsolidity/analysis/TypeChecker.cpp

@@ -530,6 +530,15 @@ bool TypeChecker::visit(VariableDeclaration const& _variable)
 	}
 	if (_variable.isConstant())
 	{
+		if (!varType->isValueType())
+		{
+			bool allowed = false;
+			if (auto arrayType = dynamic_cast<ArrayType const*>(varType))
+				allowed = arrayType->isByteArray();
+			if (!allowed)
+				m_errorReporter.fatalTypeError(9259_error, _variable.location(), "Constants of non-value type not yet implemented.");
+		}
+
 		if (!_variable.value())
 			m_errorReporter.typeError(4266_error, _variable.location(), "Uninitialized \"constant\" variable.");
 		else if (!*_variable.value()->annotation().isPure)

libsolidity/ast/Types.h

@@ -1124,6 +1124,8 @@ public:
 	bool containsNestedMapping() const override
 	{
 		solAssert(nameable(), "Called for a non nameable type.");
+		// DeclarationTypeChecker::endVisit(VariableDeclaration const&)
+		// assumes that this will never be true.
 		solAssert(!underlyingType().containsNestedMapping(), "");
 		return false;
 	}