From 1cc8ce16562f94f156c0e97404c0d96ada1eb9f9 Mon Sep 17 00:00:00 2001
From: Jason Cobb <jason.e.cobb@gmail.com>
Date: Thu, 2 Jan 2020 16:27:32 -0500
Subject: [PATCH 1/5] Don't prohibit interface inheritance

---
 libsolidity/analysis/TypeChecker.cpp | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/libsolidity/analysis/TypeChecker.cpp b/libsolidity/analysis/TypeChecker.cpp
index 2c867b962..646edeb1a 100644
--- a/libsolidity/analysis/TypeChecker.cpp
+++ b/libsolidity/analysis/TypeChecker.cpp
@@ -237,9 +237,6 @@ void TypeChecker::endVisit(InheritanceSpecifier const& _inheritance)
 	auto base = dynamic_cast<ContractDefinition const*>(&dereference(_inheritance.name()));
 	solAssert(base, "Base contract not available.");
 
-	if (m_scope->isInterface())
-		m_errorReporter.typeError(_inheritance.location(), "Interfaces cannot inherit.");
-
 	if (base->isLibrary())
 		m_errorReporter.typeError(_inheritance.location(), "Libraries cannot be inherited from.");
 

From 573a054d5d304f9363f212199f658c34b1f0729c Mon Sep 17 00:00:00 2001
From: Jason Cobb <jason.e.cobb@gmail.com>
Date: Thu, 2 Jan 2020 16:32:35 -0500
Subject: [PATCH 2/5] Prohibit interfaces inheriting from non-interfaces

---
 libsolidity/analysis/TypeChecker.cpp                         | 3 +++
 .../syntaxTests/inheritance/interface/contract_base.sol      | 5 +++++
 2 files changed, 8 insertions(+)
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/contract_base.sol

diff --git a/libsolidity/analysis/TypeChecker.cpp b/libsolidity/analysis/TypeChecker.cpp
index 646edeb1a..153f4a774 100644
--- a/libsolidity/analysis/TypeChecker.cpp
+++ b/libsolidity/analysis/TypeChecker.cpp
@@ -237,6 +237,9 @@ void TypeChecker::endVisit(InheritanceSpecifier const& _inheritance)
 	auto base = dynamic_cast<ContractDefinition const*>(&dereference(_inheritance.name()));
 	solAssert(base, "Base contract not available.");
 
+	if (m_scope->isInterface() && !base->isInterface())
+		m_errorReporter.typeError(_inheritance.location(), "Interfaces can only inherit from other interfaces.");
+
 	if (base->isLibrary())
 		m_errorReporter.typeError(_inheritance.location(), "Libraries cannot be inherited from.");
 
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/contract_base.sol b/test/libsolidity/syntaxTests/inheritance/interface/contract_base.sol
new file mode 100644
index 000000000..1afb921c5
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/contract_base.sol
@@ -0,0 +1,5 @@
+contract C {}
+interface I is C {}
+
+// ----
+// TypeError: (29-30): Interfaces can only inherit from other interfaces.

From 595f569e97e012ae00fc5259afb0bf4c1474f5e2 Mon Sep 17 00:00:00 2001
From: Jason Cobb <jason.e.cobb@gmail.com>
Date: Thu, 2 Jan 2020 16:28:07 -0500
Subject: [PATCH 3/5] Update tests for interface inheritance

---
 .../interface_inheritance_conversions.sol     | 39 +++++++++++++++++++
 .../interface/diamond/diamond_no_relist.sol   | 10 +++++
 .../interface/diamond/diamond_with_relist.sol | 15 +++++++
 .../interface/implementation/complete.sol     | 17 ++++++++
 .../interface/implementation/partial.sol      | 19 +++++++++
 .../linearization/invalid/lists_a.sol         |  8 ++++
 .../linearization/invalid/lists_b.sol         |  8 ++++
 .../linearization/invalid/lists_both.sol      |  8 ++++
 .../interface/linearization/valid.sol         |  7 ++++
 .../interface/multiple_parents.sol            | 15 +++++++
 .../interface/overrides_multiple.sol          | 31 +++++++++++++++
 .../interface/overrides_single.sol            | 14 +++++++
 .../inheritance/interface/single_parent.sol   |  5 +++
 .../418_interface_inheritance.sol             |  2 -
 14 files changed, 196 insertions(+), 2 deletions(-)
 create mode 100644 test/libsolidity/semanticTests/interface_inheritance_conversions.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/diamond/diamond_no_relist.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/diamond/diamond_with_relist.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/implementation/complete.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/implementation/partial.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_a.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_b.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_both.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/linearization/valid.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/multiple_parents.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/overrides_multiple.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/overrides_single.sol
 create mode 100644 test/libsolidity/syntaxTests/inheritance/interface/single_parent.sol

diff --git a/test/libsolidity/semanticTests/interface_inheritance_conversions.sol b/test/libsolidity/semanticTests/interface_inheritance_conversions.sol
new file mode 100644
index 000000000..4241afe81
--- /dev/null
+++ b/test/libsolidity/semanticTests/interface_inheritance_conversions.sol
@@ -0,0 +1,39 @@
+interface Parent {
+    function parentFun() external returns (uint256);
+}
+
+interface SubA is Parent {
+    function subAFun() external returns (uint256);
+}
+
+interface SubB is Parent {
+    function subBFun() external returns (uint256);
+}
+
+contract Impl is SubA, SubB {
+    function parentFun() override external returns (uint256) { return 1; }
+    function subAFun() override external returns (uint256) { return 2; }
+    function subBFun() override external returns (uint256) { return 3; }
+}
+
+contract C {
+    function convertParent() public returns (uint256) {
+        Parent p = new Impl();
+        return p.parentFun();
+    }
+
+    function convertSubA() public returns (uint256, uint256) {
+        SubA sa = new Impl();
+        return (sa.parentFun(), sa.subAFun());
+    }
+
+    function convertSubB() public returns (uint256, uint256) {
+        SubB sb = new Impl();
+        return (sb.parentFun(), sb.subBFun());
+    }
+}
+
+// ----
+// convertParent() -> 1
+// convertSubA() -> 1, 2
+// convertSubB() -> 1, 3
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/diamond/diamond_no_relist.sol b/test/libsolidity/syntaxTests/inheritance/interface/diamond/diamond_no_relist.sol
new file mode 100644
index 000000000..5d525fde4
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/diamond/diamond_no_relist.sol
@@ -0,0 +1,10 @@
+interface Parent {
+    function test() external returns (uint256);
+}
+
+interface SubA is Parent {}
+interface SubB is Parent {}
+
+contract C is SubA, SubB {
+    function test() external override returns (uint256) { return 42; }
+}
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/diamond/diamond_with_relist.sol b/test/libsolidity/syntaxTests/inheritance/interface/diamond/diamond_with_relist.sol
new file mode 100644
index 000000000..30ee63cc7
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/diamond/diamond_with_relist.sol
@@ -0,0 +1,15 @@
+interface Parent {
+    function test() external returns (uint256);
+}
+
+interface SubA is Parent {
+    function test() external override returns (uint256);
+}
+
+interface SubB is Parent {
+    function test() external override returns (uint256);
+}
+
+contract C is SubA, SubB {
+    function test() external override(SubA, SubB) returns (uint256) { return 42; }
+}
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/implementation/complete.sol b/test/libsolidity/syntaxTests/inheritance/interface/implementation/complete.sol
new file mode 100644
index 000000000..09334dafc
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/implementation/complete.sol
@@ -0,0 +1,17 @@
+interface ParentA {
+    function testA() external returns (uint256);
+}
+
+interface ParentB {
+    function testB() external returns (uint256);
+}
+
+interface Sub is ParentA, ParentB {
+    function testSub() external returns (uint256);
+}
+
+contract SubImpl is Sub {
+    function testA() external override returns (uint256) { return 12; }
+    function testB() external override(ParentB) returns (uint256) { return 42; }
+    function testSub() external override returns (uint256) { return 99; }
+}
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/implementation/partial.sol b/test/libsolidity/syntaxTests/inheritance/interface/implementation/partial.sol
new file mode 100644
index 000000000..378c3c41b
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/implementation/partial.sol
@@ -0,0 +1,19 @@
+interface ParentA {
+    function testA() external returns (uint256);
+}
+
+interface ParentB {
+    function testB() external returns (uint256);
+}
+
+interface Sub is ParentA, ParentB {
+    function testSub() external returns (uint256);
+}
+
+contract SubImpl is Sub {
+    function testA() external override returns (uint256) { return 12; }
+    function testSub() external override returns (uint256) { return 99; }
+}
+
+// ----
+// TypeError: (234-407): Contract "SubImpl" should be marked as abstract.
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_a.sol b/test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_a.sol
new file mode 100644
index 000000000..0528af074
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_a.sol
@@ -0,0 +1,8 @@
+interface ParentA {}
+interface ParentB {}
+interface Sub is ParentA, ParentB {}
+
+contract ListsA is Sub, ParentA {}
+
+// ----
+// TypeError: (80-114): Linearization of inheritance graph impossible
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_b.sol b/test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_b.sol
new file mode 100644
index 000000000..cea97889c
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_b.sol
@@ -0,0 +1,8 @@
+interface ParentA {}
+interface ParentB {}
+interface Sub is ParentA, ParentB {}
+
+contract ListsB is Sub, ParentB {}
+
+// ----
+// TypeError: (80-114): Linearization of inheritance graph impossible
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_both.sol b/test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_both.sol
new file mode 100644
index 000000000..8bf99bd91
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/linearization/invalid/lists_both.sol
@@ -0,0 +1,8 @@
+interface ParentA {}
+interface ParentB {}
+interface Sub is ParentA, ParentB {}
+
+contract ListsBoth is Sub, ParentA, ParentB {}
+
+// ----
+// TypeError: (80-126): Linearization of inheritance graph impossible
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/linearization/valid.sol b/test/libsolidity/syntaxTests/inheritance/interface/linearization/valid.sol
new file mode 100644
index 000000000..4f725e1b7
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/linearization/valid.sol
@@ -0,0 +1,7 @@
+interface ParentA {}
+interface ParentB {}
+interface Sub is ParentA, ParentB {}
+
+contract ListsA is ParentA, Sub {}
+contract ListsB is ParentB, Sub {}
+contract ListsBoth is ParentA, ParentB, Sub {}
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/multiple_parents.sol b/test/libsolidity/syntaxTests/inheritance/interface/multiple_parents.sol
new file mode 100644
index 000000000..f3dcf6f0e
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/multiple_parents.sol
@@ -0,0 +1,15 @@
+interface SuperA {
+    function test() external returns (uint256);
+    function testA() external returns (int128);
+}
+
+interface SuperB {
+    function test() external returns (uint256);
+    function testB() external returns (int256);
+}
+
+interface Sub is SuperA, SuperB {
+}
+
+// ----
+// TypeError: (236-271): Derived contract must override function "test". Two or more base classes define function with same name and parameter types.
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/overrides_multiple.sol b/test/libsolidity/syntaxTests/inheritance/interface/overrides_multiple.sol
new file mode 100644
index 000000000..36d76f3a8
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/overrides_multiple.sol
@@ -0,0 +1,31 @@
+interface SuperA {
+    function test1() external returns (uint256);
+    function test2() external returns (uint256);
+    function test3() external returns (uint256);
+    function test4() external returns (uint256);
+    function test5() external returns (uint256);
+}
+
+interface SuperB {
+    function test1() external returns (uint256);
+    function test2() external returns (uint256);
+    function test3() external returns (uint256);
+    function test4() external returns (uint256);
+    function test5() external returns (uint256);
+}
+
+interface Sub is SuperA, SuperB {
+    function test1() external returns (uint256);
+    function test2() external override returns (uint256);
+    function test3() external override(SuperA) returns (uint256);
+    function test4() external override(SuperB) returns (uint256);
+    function test5() external override(SuperA, SuperB) returns (uint256);
+}
+
+// ----
+// TypeError: (572-616): Overriding function is missing "override" specifier.
+// TypeError: (572-616): Overriding function is missing "override" specifier.
+// TypeError: (572-616): Function needs to specify overridden contracts "SuperA" and "SuperB".
+// TypeError: (647-655): Function needs to specify overridden contracts "SuperA" and "SuperB".
+// TypeError: (705-721): Function needs to specify overridden contract "SuperB".
+// TypeError: (771-787): Function needs to specify overridden contract "SuperA".
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/overrides_single.sol b/test/libsolidity/syntaxTests/inheritance/interface/overrides_single.sol
new file mode 100644
index 000000000..1a68d980b
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/overrides_single.sol
@@ -0,0 +1,14 @@
+interface Super {
+    function test1() external returns (uint256);
+    function test2() external returns (uint256);
+    function test3() external returns (uint256);
+}
+
+interface Sub is Super {
+    function test1() external returns (uint256);
+    function test2() external override returns (uint256);
+    function test3() external override(Super) returns (uint256);
+}
+
+// ----
+// TypeError: (197-241): Overriding function is missing "override" specifier.
diff --git a/test/libsolidity/syntaxTests/inheritance/interface/single_parent.sol b/test/libsolidity/syntaxTests/inheritance/interface/single_parent.sol
new file mode 100644
index 000000000..d6a7c747e
--- /dev/null
+++ b/test/libsolidity/syntaxTests/inheritance/interface/single_parent.sol
@@ -0,0 +1,5 @@
+interface Super {
+    function test() external returns (uint256);
+}
+
+interface Sub is Super {}
diff --git a/test/libsolidity/syntaxTests/nameAndTypeResolution/418_interface_inheritance.sol b/test/libsolidity/syntaxTests/nameAndTypeResolution/418_interface_inheritance.sol
index 92683cda3..53a45636e 100644
--- a/test/libsolidity/syntaxTests/nameAndTypeResolution/418_interface_inheritance.sol
+++ b/test/libsolidity/syntaxTests/nameAndTypeResolution/418_interface_inheritance.sol
@@ -2,5 +2,3 @@ interface A {
 }
 interface I is A {
 }
-// ----
-// TypeError: (31-32): Interfaces cannot inherit.

From f492a0a3ef46e5466c717e314be459ade5b03981 Mon Sep 17 00:00:00 2001
From: Jason Cobb <jason.e.cobb@gmail.com>
Date: Tue, 21 Jan 2020 08:32:04 -0500
Subject: [PATCH 4/5] Update changelog for interface inheritance

---
 Changelog.md | 1 +
 1 file changed, 1 insertion(+)

diff --git a/Changelog.md b/Changelog.md
index 0c700d1a6..44e515d16 100644
--- a/Changelog.md
+++ b/Changelog.md
@@ -3,6 +3,7 @@
 Language Features:
  * Allow accessing external functions via contract and interface names to obtain their selector.
  * Inline Assembly: Support literals ``true`` and ``false``.
+ * Allow interfaces to inherit from other interfaces
 
 
 Compiler Features:

From 86145adac8362f55e4cd1ccc903f02f909b67b06 Mon Sep 17 00:00:00 2001
From: Jason Cobb <jason.e.cobb@gmail.com>
Date: Tue, 21 Jan 2020 08:42:58 -0500
Subject: [PATCH 5/5] Update documentation for interface inheritance

---
 docs/contracts/interfaces.rst | 25 +++++++++++++++++++++++--
 1 file changed, 23 insertions(+), 2 deletions(-)

diff --git a/docs/contracts/interfaces.rst b/docs/contracts/interfaces.rst
index 69d4917cc..727809460 100644
--- a/docs/contracts/interfaces.rst
+++ b/docs/contracts/interfaces.rst
@@ -8,7 +8,7 @@ Interfaces
 
 Interfaces are similar to abstract contracts, but they cannot have any functions implemented. There are further restrictions:
 
-- They cannot inherit other contracts or interfaces.
+- They cannot inherit from other contracts, but they can inherit from other interfaces.
 - All declared functions must be external.
 - They cannot declare a constructor.
 - They cannot declare state variables.
@@ -37,10 +37,31 @@ they can be overridden. This does not automatically mean that an overriding func
 can be overridden again - this is only possible if the overriding
 function is marked ``virtual``.
 
+Interfaces can inherit from other interfaces. This has the same rules as normal
+inheritance.
+
+::
+
+    pragma solidity >0.6.1 <0.7.0;
+
+    interface ParentA {
+        function test() external returns (uint256);
+    }
+
+    interface ParentB {
+        function test() external returns (uint256);
+    }
+
+    interface SubInterface is ParentA, ParentB {
+        // Must redefine test in order to assert that the parent
+        // meanings are compatible.
+        function test() external override(ParentA, ParentB) returns (uint256);
+    }
+
 Types defined inside interfaces and other contract-like structures
 can be accessed from other contracts: ``Token.TokenType`` or ``Token.Coin``.
 
 .. warning:
 
     Interfaces have supported ``enum`` types since :doc:`Solidity version 0.5.0 <050-breaking-changes>`, make
-    sure the pragma version specifies this version as a minimum.
\ No newline at end of file
+    sure the pragma version specifies this version as a minimum.