Adds error message for incompatible assignment of identically named type

Fixes issue #12050

src/compiler/checker.ts

@@ -6824,9 +6824,15 @@ namespace ts {
                 }
 
                 if (!message) {
-                    message = relation === comparableRelation ?
-                        Diagnostics.Type_0_is_not_comparable_to_type_1 :
-                        Diagnostics.Type_0_is_not_assignable_to_type_1;
+                    if (relation === comparableRelation) {
+                        message = Diagnostics.Type_0_is_not_comparable_to_type_1;
+                    }
+                    else if (sourceType === targetType) {
+                        message = Diagnostics.Type_0_is_not_assignable_to_type_1_Two_different_types_with_this_name_exist_but_they_are_unrelated;
+                    }
+                    else {
+                        message = Diagnostics.Type_0_is_not_assignable_to_type_1;
+                    }
                 }
 
                 reportError(message, sourceType, targetType);

src/compiler/diagnosticMessages.json

@@ -3170,5 +3170,9 @@
     "Adding a tsconfig.json file will help organize projects that contain both TypeScript and JavaScript files. Learn more at https://aka.ms/tsconfig": {
         "category": "Error",
         "code": 90009
+    },
+    "Type '{0}' is not assignable to type '{1}'. Two different types with this name exist, but they are unrelated.": {
+        "category": "Error",
+        "code": 90010
     }
 }

tests/baselines/reference/incompatibleAssignmentOfIdenticallyNamedTypes.errors.txt

@@ -0,0 +1,15 @@
+tests/cases/compiler/incompatibleAssignmentOfIdenticallyNamedTypes.ts(6,9): error TS90010: Type 'T' is not assignable to type 'T'. Two different types with this name exist, but they are unrelated.
+
+
+==== tests/cases/compiler/incompatibleAssignmentOfIdenticallyNamedTypes.ts (1 errors) ====
+    interface T { }
+    declare const a: T;
+    class Foo<T> {
+        x: T;
+        fn() {
+            this.x = a;
+            ~~~~~~
+!!! error TS90010: Type 'T' is not assignable to type 'T'. Two different types with this name exist, but they are unrelated.
+        }
+    }
+    

tests/baselines/reference/incompatibleAssignmentOfIdenticallyNamedTypes.js

@@ -0,0 +1,20 @@
+//// [incompatibleAssignmentOfIdenticallyNamedTypes.ts]
+interface T { }
+declare const a: T;
+class Foo<T> {
+    x: T;
+    fn() {
+        this.x = a;
+    }
+}
+
+
+//// [incompatibleAssignmentOfIdenticallyNamedTypes.js]
+var Foo = (function () {
+    function Foo() {
+    }
+    Foo.prototype.fn = function () {
+        this.x = a;
+    };
+    return Foo;
+}());

tests/cases/compiler/incompatibleAssignmentOfIdenticallyNamedTypes.ts

@@ -0,0 +1,8 @@
+interface T { }
+declare const a: T;
+class Foo<T> {
+    x: T;
+    fn() {
+        this.x = a;
+    }
+}