Allow super to access method signatures

Previously, super was only allowed to access method *declarations*. But
method signatures can come from interfaces that merge with classes, and
should also be accessible as methods on super.

src/compiler/checker.ts

@@ -12337,12 +12337,15 @@ namespace ts {
                 // - In a static member function or static member accessor
                 //   where this references the constructor function object of a derived class,
                 //   a super property access is permitted and must specify a public static member function of the base class.
-                if (languageVersion < ScriptTarget.ES2015 && getDeclarationKindFromSymbol(prop) !== SyntaxKind.MethodDeclaration) {
-                    // `prop` refers to a *property* declared in the super class
-                    // rather than a *method*, so it does not satisfy the above criteria.
+                if (languageVersion < ScriptTarget.ES2015) {
+                    const propKind = getDeclarationKindFromSymbol(prop);
+                    if (propKind !== SyntaxKind.MethodDeclaration && propKind !== SyntaxKind.MethodSignature) {
+                        // `prop` refers to a *property* declared in the super class
+                        // rather than a *method*, so it does not satisfy the above criteria.
 
-                    error(errorNode, Diagnostics.Only_public_and_protected_methods_of_the_base_class_are_accessible_via_the_super_keyword);
-                    return false;
+                        error(errorNode, Diagnostics.Only_public_and_protected_methods_of_the_base_class_are_accessible_via_the_super_keyword);
+                        return false;
+                    }
                 }
 
                 if (flags & ModifierFlags.Abstract) {

tests/baselines/reference/superHasMethodsFromMergedInterface.js

@@ -0,0 +1,37 @@
+//// [superHasMethodsFromMergedInterface.ts]
+class C { m1() { } }
+interface C { m2(): void }
+class Sub extends C {
+    m3() {
+        super.m2();
+    }
+}
+
+
+//// [superHasMethodsFromMergedInterface.js]
+var __extends = (this && this.__extends) || (function () {
+    var extendStatics = Object.setPrototypeOf ||
+        ({ __proto__: [] } instanceof Array && function (d, b) { d.__proto__ = b; }) ||
+        function (d, b) { for (var p in b) if (b.hasOwnProperty(p)) d[p] = b[p]; };
+    return function (d, b) {
+        extendStatics(d, b);
+        function __() { this.constructor = d; }
+        d.prototype = b === null ? Object.create(b) : (__.prototype = b.prototype, new __());
+    };
+})();
+var C = (function () {
+    function C() {
+    }
+    C.prototype.m1 = function () { };
+    return C;
+}());
+var Sub = (function (_super) {
+    __extends(Sub, _super);
+    function Sub() {
+        return _super !== null && _super.apply(this, arguments) || this;
+    }
+    Sub.prototype.m3 = function () {
+        _super.prototype.m2.call(this);
+    };
+    return Sub;
+}(C));

tests/baselines/reference/superHasMethodsFromMergedInterface.symbols

@@ -0,0 +1,23 @@
+=== tests/cases/compiler/superHasMethodsFromMergedInterface.ts ===
+class C { m1() { } }
+>C : Symbol(C, Decl(superHasMethodsFromMergedInterface.ts, 0, 0), Decl(superHasMethodsFromMergedInterface.ts, 0, 20))
+>m1 : Symbol(C.m1, Decl(superHasMethodsFromMergedInterface.ts, 0, 9))
+
+interface C { m2(): void }
+>C : Symbol(C, Decl(superHasMethodsFromMergedInterface.ts, 0, 0), Decl(superHasMethodsFromMergedInterface.ts, 0, 20))
+>m2 : Symbol(C.m2, Decl(superHasMethodsFromMergedInterface.ts, 1, 13))
+
+class Sub extends C {
+>Sub : Symbol(Sub, Decl(superHasMethodsFromMergedInterface.ts, 1, 26))
+>C : Symbol(C, Decl(superHasMethodsFromMergedInterface.ts, 0, 0), Decl(superHasMethodsFromMergedInterface.ts, 0, 20))
+
+    m3() {
+>m3 : Symbol(Sub.m3, Decl(superHasMethodsFromMergedInterface.ts, 2, 21))
+
+        super.m2();
+>super.m2 : Symbol(C.m2, Decl(superHasMethodsFromMergedInterface.ts, 1, 13))
+>super : Symbol(C, Decl(superHasMethodsFromMergedInterface.ts, 0, 0), Decl(superHasMethodsFromMergedInterface.ts, 0, 20))
+>m2 : Symbol(C.m2, Decl(superHasMethodsFromMergedInterface.ts, 1, 13))
+    }
+}
+

tests/baselines/reference/superHasMethodsFromMergedInterface.types

@@ -0,0 +1,24 @@
+=== tests/cases/compiler/superHasMethodsFromMergedInterface.ts ===
+class C { m1() { } }
+>C : C
+>m1 : () => void
+
+interface C { m2(): void }
+>C : C
+>m2 : () => void
+
+class Sub extends C {
+>Sub : Sub
+>C : C
+
+    m3() {
+>m3 : () => void
+
+        super.m2();
+>super.m2() : void
+>super.m2 : () => void
+>super : C
+>m2 : () => void
+    }
+}
+

tests/cases/compiler/superHasMethodsFromMergedInterface.ts

@@ -0,0 +1,7 @@
+class C { m1() { } }
+interface C { m2(): void }
+class Sub extends C {
+    m3() {
+        super.m2();
+    }
+}