Make visibility optional, but it has to be consistent.

test/libsolidity/syntaxTests/constructor/constructible_abstract_base.sol

@@ -0,0 +1,6 @@
+abstract contract C {
+	constructor() {}
+}
+contract D is C {
+	constructor() { }
+}

test/libsolidity/syntaxTests/constructor/constructible_internal_constructor.sol

@@ -1,6 +1,8 @@
 abstract contract C {
-	constructor() {}
+	constructor() internal {}
 }
 contract D is C {
 	constructor() { }
 }
+// ----
+// Warning 2462: (23-48): Visibility for constructor is ignored. If you want the contract to be non-deployable, making it "abstract" is sufficient.

test/libsolidity/syntaxTests/constructor/external_constructor.sol

@@ -2,4 +2,4 @@ contract test {
 	constructor() external {}
 }
 // ----
-// SyntaxError 2462: (17-42): Visibility specified for constructor. If you want the contract to be non-deployable, make it "abstract".
+// TypeError 9239: (17-42): Constructor cannot have visibility.

test/libsolidity/syntaxTests/constructor/inconstructible_abstract_contract.sol

@@ -0,0 +1,8 @@
+abstract contract C {
+	constructor() {}
+}
+contract D {
+	function f() public { C c = new C(); c; }
+}
+// ----
+// TypeError 4614: (84-89): Cannot instantiate an abstract contract.

test/libsolidity/syntaxTests/constructor/inconstructible_abstract_contract_inverted.sol

@@ -0,0 +1,13 @@
+// Previously, the type information for A was not yet available at the point of
+// "new A".
+contract B {
+	A a;
+	constructor() {
+		a = new A(address(this));
+	}
+}
+abstract contract A {
+	constructor(address) {}
+}
+// ----
+// TypeError 4614: (134-139): Cannot instantiate an abstract contract.

test/libsolidity/syntaxTests/constructor/inconstructible_internal_constructor.sol

@@ -1,8 +1,8 @@
-abstract contract C {
-	constructor() {}
+contract C {
+	constructor() internal {}
 }
 contract D {
 	function f() public { C c = new C(); c; }
 }
 // ----
-// TypeError 4614: (84-89): Cannot instantiate an abstract contract.
+// DeclarationError 1845: (14-39): Non-abstract contracts cannot have internal constructors. Remove the "internal" keyword and make the contract abstract to fix this.

test/libsolidity/syntaxTests/constructor/inconstructible_internal_constructor_inverted.sol

@@ -6,8 +6,8 @@ contract B {
 		a = new A(address(this));
 	}
 }
-abstract contract A {
-	constructor(address) {}
+contract A {
+	constructor(address) internal {}
 }
 // ----
-// TypeError 4614: (134-139): Cannot instantiate an abstract contract.
+// DeclarationError 1845: (175-207): Non-abstract contracts cannot have internal constructors. Remove the "internal" keyword and make the contract abstract to fix this.

test/libsolidity/syntaxTests/constructor/internal_constructor_non_abstract.sol

@@ -0,0 +1,5 @@
+contract C {
+	constructor() internal {}
+}
+// ----
+// DeclarationError 1845: (14-39): Non-abstract contracts cannot have internal constructors. Remove the "internal" keyword and make the contract abstract to fix this.

test/libsolidity/syntaxTests/constructor/public_constructor_abstract.sol

@@ -0,0 +1,5 @@
+abstract contract C {
+	constructor() public {}
+}
+// ----
+// DeclarationError 8295: (23-46): Abstract contracts cannot have public constructors. Remove the "public" keyword to fix this.

test/libsolidity/syntaxTests/constructor/public_constructor_non_abstract.sol

@@ -0,0 +1,5 @@
+contract C {
+	constructor() public {}
+}
+// ----
+// Warning 2462: (14-37): Visibility for constructor is ignored. If you want the contract to be non-deployable, making it "abstract" is sufficient.