6 files changed, 89 insertions, 9 deletions

diff --git a/Changelog.md b/Changelog.md
index db425291..44e35f5f 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -11,6 +11,7 @@ Features:
    variables.
  * Syntax Checker: Deprecate the ``var`` keyword (and mark it an error as experimental 0.5.0 feature).
  * Type Checker: Issue warning for using ``public`` visibility for interface functions.
+ * Type Checker: Allow `this.f.selector` to be a pure expression.
 
 Bugfixes:
  * Parser: Disallow event declarations with no parameter list.
diff --git a/libsolidity/analysis/ViewPureChecker.cpp b/libsolidity/analysis/ViewPureChecker.cpp
index 6257ac6d..13c3ab68 100644
--- a/libsolidity/analysis/ViewPureChecker.cpp
+++ b/libsolidity/analysis/ViewPureChecker.cpp
@@ -275,6 +275,22 @@ void ViewPureChecker::endVisit(FunctionCall const& _functionCall)
 	reportMutability(mut, _functionCall.location());
 }
 
+bool ViewPureChecker::visit(MemberAccess const& _memberAccess)
+{
+	// Catch the special case of `this.f.selector` which is a pure expression.
+	ASTString const& member = _memberAccess.memberName();
+	if (
+		_memberAccess.expression().annotation().type->category() == Type::Category::Function &&
+		member == "selector"
+	)
+		if (auto const* expr = dynamic_cast<MemberAccess const*>(&_memberAccess.expression()))
+			if (auto const* exprInt = dynamic_cast<Identifier const*>(&expr->expression()))
+				if (exprInt->name() == "this")
+					// Do not continue visiting.
+					return false;
+	return true;
+}
+
 void ViewPureChecker::endVisit(MemberAccess const& _memberAccess)
 {
 	StateMutability mutability = StateMutability::Pure;
diff --git a/libsolidity/analysis/ViewPureChecker.h b/libsolidity/analysis/ViewPureChecker.h
index fec060b6..0b882cd8 100644
--- a/libsolidity/analysis/ViewPureChecker.h
+++ b/libsolidity/analysis/ViewPureChecker.h
@@ -56,6 +56,7 @@ private:
 	virtual bool visit(ModifierDefinition const& _modifierDef) override;
 	virtual void endVisit(ModifierDefinition const& _modifierDef) override;
 	virtual void endVisit(Identifier const& _identifier) override;
+	virtual bool visit(MemberAccess const& _memberAccess) override;
 	virtual void endVisit(MemberAccess const& _memberAccess) override;
 	virtual void endVisit(IndexAccess const& _indexAccess) override;
 	virtual void endVisit(ModifierInvocation const& _modifier) override;
diff --git a/libsolidity/codegen/ExpressionCompiler.cpp b/libsolidity/codegen/ExpressionCompiler.cpp
index bb8c4a94..8e1cf019 100644
--- a/libsolidity/codegen/ExpressionCompiler.cpp
+++ b/libsolidity/codegen/ExpressionCompiler.cpp
@@ -1014,6 +1014,30 @@ bool ExpressionCompiler::visit(MemberAccess const& _memberAccess)
 			_memberAccess.expression().accept(*this);
 		return false;
 	}
+	// Another special case for `this.f.selector` which does not need the address.
+	// There are other uses of `.selector` which do need the address, but we want this
+	// specific use to be a pure expression.
+	if (
+		_memberAccess.expression().annotation().type->category() == Type::Category::Function &&
+		member == "selector"
+	)
+		if (auto const* expr = dynamic_cast<MemberAccess const*>(&_memberAccess.expression()))
+			if (auto const* exprInt = dynamic_cast<Identifier const*>(&expr->expression()))
+				if (exprInt->name() == "this")
+					if (Declaration const* declaration = expr->annotation().referencedDeclaration)
+					{
+						u256 identifier;
+						if (auto const* variable = dynamic_cast<VariableDeclaration const*>(declaration))
+							identifier = FunctionType(*variable).externalIdentifier();
+						else if (auto const* function = dynamic_cast<FunctionDefinition const*>(declaration))
+							identifier = FunctionType(*function).externalIdentifier();
+						else
+							solAssert(false, "Contract member is neither variable nor function.");
+						m_context << identifier;
+						/// need to store store it as bytes4
+						utils().leftShiftNumberOnStack(224);
+						return false;
+					}
 
 	_memberAccess.expression().accept(*this);
 	switch (_memberAccess.expression().annotation().type->category())
diff --git a/test/libsolidity/SolidityNameAndTypeResolution.cpp b/test/libsolidity/SolidityNameAndTypeResolution.cpp
index 1953195c..3f72e847 100644
--- a/test/libsolidity/SolidityNameAndTypeResolution.cpp
+++ b/test/libsolidity/SolidityNameAndTypeResolution.cpp
@@ -6904,15 +6904,7 @@ BOOST_AUTO_TEST_CASE(function_types_sig)
 	CHECK_ERROR(text, TypeError, "Member \"selector\" not found");
 	text = R"(
 		contract C {
-			function f() view external returns (bytes4) {
-				return this.f.selector;
-			}
-		}
-	)";
-	CHECK_SUCCESS_NO_WARNINGS(text);
-	text = R"(
-		contract C {
-			function f() view external returns (bytes4) {
+			function f() pure external returns (bytes4) {
 				return this.f.selector;
 			}
 		}
diff --git a/test/libsolidity/ViewPureChecker.cpp b/test/libsolidity/ViewPureChecker.cpp
index e91e713c..0fee95c5 100644
--- a/test/libsolidity/ViewPureChecker.cpp
+++ b/test/libsolidity/ViewPureChecker.cpp
@@ -326,6 +326,52 @@ BOOST_AUTO_TEST_CASE(function_types)
 	CHECK_SUCCESS_NO_WARNINGS(text);
 }
 
+BOOST_AUTO_TEST_CASE(selector)
+{
+	string text = R"(
+		contract C {
+			function f() payable public {
+			}
+			function g() pure public returns (bytes4) {
+				return this.f.selector;
+			}
+		}
+	)";
+	CHECK_SUCCESS_NO_WARNINGS(text);
+}
+
+BOOST_AUTO_TEST_CASE(selector_complex)
+{
+	string text = R"(
+		contract C {
+			function f(C c) pure public returns (C) {
+				return c;
+			}
+			function g() pure public returns (bytes4) {
+				// By passing `this`, we read from the state, even if f itself is pure.
+				return f(this).f.selector;
+			}
+		}
+	)";
+	CHECK_ERROR(text, TypeError, "reads from the environment or state and thus requires \"view\"");
+}
+
+BOOST_AUTO_TEST_CASE(selector_complex2)
+{
+	string text = R"(
+		contract C {
+				function f() payable public returns (C) {
+				return this;
+			}
+			function g() pure public returns (bytes4) {
+				C x = C(0x123);
+				return x.f.selector;
+			}
+		}
+	)";
+	CHECK_SUCCESS_NO_WARNINGS(text);
+}
+
 BOOST_AUTO_TEST_CASE(creation)
 {
 	string text = R"(