Report error using type parameter from merged declaration

Author: rbuckton

src/compiler/checker.ts

@@ -17594,6 +17594,11 @@ namespace ts {
                     error(node.name, Diagnostics.All_declarations_of_0_must_have_identical_modifiers, declarationNameToString(node.name));
                 }
             }
+            if (type.flags & TypeFlags.TypeParameter && !(<TypeParameter>type).isThisType && type.symbol) {
+                if (!isTypeParameterInScope(<TypeParameter>type, node)) {
+                    error(node.name, Diagnostics.Type_parameter_0_cannot_be_referenced_outside_of_a_declaration_that_defines_it, symbolToString(type.symbol));
+                }
+            }
             if (node.kind !== SyntaxKind.PropertyDeclaration && node.kind !== SyntaxKind.PropertySignature) {
                 // We know we don't have a binding pattern or computed name here
                 checkExportsOnMergedDeclarations(node);
@@ -17608,6 +17613,19 @@ namespace ts {
             }
         }
 
+        function isTypeParameterInScope(typeParameter: TypeParameter, node: Node) {
+            const parents = map(filter(typeParameter.symbol.declarations, isTypeParameter), node => node.parent);
+            while (node) {
+                if (isFunctionLike(node) || isClassLike(node) || node.kind === SyntaxKind.InterfaceDeclaration || node.kind === SyntaxKind.TypeAliasDeclaration) {
+                    if (contains(parents, node)) {
+                        return true;
+                    }
+                }
+                node = node.parent;
+            }
+            return false;
+        }
+
         function areDeclarationFlagsIdentical(left: Declaration, right: Declaration) {
             if ((left.kind === SyntaxKind.Parameter && right.kind === SyntaxKind.VariableDeclaration) ||
                 (left.kind === SyntaxKind.VariableDeclaration && right.kind === SyntaxKind.Parameter)) {

src/compiler/diagnosticMessages.json

@@ -1127,6 +1127,10 @@
         "category": "Error",
         "code": 2366
     },
+    "Type parameter '{0}' cannot be referenced outside of a declaration that defines it.": {
+        "category": "Error",
+        "code": 2367
+    },
     "Type parameter name cannot be '{0}'": {
         "category": "Error",
         "code": 2368

tests/baselines/reference/genericDefaultsErrors.errors.txt

@@ -31,9 +31,10 @@ tests/cases/compiler/genericDefaultsErrors.ts(35,32): error TS2344: Type 'number
 tests/cases/compiler/genericDefaultsErrors.ts(38,15): error TS2707: Generic type 'i09<T, U, V>' requires between 2 and 3 type arguments.
 tests/cases/compiler/genericDefaultsErrors.ts(39,15): error TS2707: Generic type 'i09<T, U, V>' requires between 2 and 3 type arguments.
 tests/cases/compiler/genericDefaultsErrors.ts(42,15): error TS2707: Generic type 'i09<T, U, V>' requires between 2 and 3 type arguments.
+tests/cases/compiler/genericDefaultsErrors.ts(44,17): error TS2367: Type parameter 'T' cannot be referenced outside of a declaration that defines it.
 
 
-==== tests/cases/compiler/genericDefaultsErrors.ts (31 errors) ====
+==== tests/cases/compiler/genericDefaultsErrors.ts (32 errors) ====
 
     declare const x: any;
 
@@ -139,4 +140,9 @@ tests/cases/compiler/genericDefaultsErrors.ts(42,15): error TS2707: Generic type
     type i09t03 = i09<1, 2, 3>; // ok
     type i09t04 = i09<1, 2, 3, 4>; // error
                   ~~~~~~~~~~~~~~~
-!!! error TS2707: Generic type 'i09<T, U, V>' requires between 2 and 3 type arguments.
+!!! error TS2707: Generic type 'i09<T, U, V>' requires between 2 and 3 type arguments.
+    
+    interface i10 { x: T; } // error
+                    ~
+!!! error TS2367: Type parameter 'T' cannot be referenced outside of a declaration that defines it.
+    interface i10<T = number> {}

tests/baselines/reference/genericDefaultsErrors.js

@@ -40,7 +40,10 @@ type i09t00 = i09; // error
 type i09t01 = i09<1>; // error
 type i09t02 = i09<1, 2>; // ok
 type i09t03 = i09<1, 2, 3>; // ok
-type i09t04 = i09<1, 2, 3, 4>; // error
+type i09t04 = i09<1, 2, 3, 4>; // error
+
+interface i10 { x: T; } // error
+interface i10<T = number> {}
 
 //// [genericDefaultsErrors.js]
 f11(); // ok
@@ -101,3 +104,8 @@ declare type i09t01 = i09<1>;
 declare type i09t02 = i09<1, 2>;
 declare type i09t03 = i09<1, 2, 3>;
 declare type i09t04 = i09<1, 2, 3, 4>;
+interface i10 {
+    x: T;
+}
+interface i10<T = number> {
+}

tests/cases/compiler/genericDefaultsErrors.ts

@@ -40,4 +40,7 @@ type i09t00 = i09; // error
 type i09t01 = i09<1>; // error
 type i09t02 = i09<1, 2>; // ok
 type i09t03 = i09<1, 2, 3>; // ok
-type i09t04 = i09<1, 2, 3, 4>; // error
+type i09t04 = i09<1, 2, 3, 4>; // error
+
+interface i10 { x: T; } // error
+interface i10<T = number> {}