Improve error message to suggest indexed access types.

src/compiler/checker.ts

@@ -1240,7 +1240,21 @@ namespace ts {
         function checkAndReportErrorForUsingTypeAsNamespace(errorLocation: Node, name: __String, meaning: SymbolFlags): boolean {
             if (meaning === SymbolFlags.Namespace) {
                 const symbol = resolveSymbol(resolveName(errorLocation, name, SymbolFlags.Type & ~SymbolFlags.Value, /*nameNotFoundMessage*/undefined, /*nameArg*/ undefined));
+                const parent = errorLocation.parent;
                 if (symbol) {
+                    if (isQualifiedName(parent)) {
+                        const propName = parent.right.escapedText;
+                        const propType = getPropertyOfType(getDeclaredTypeOfSymbol(symbol), propName);
+                        if (propType) {
+                            error(
+                                parent,
+                                Diagnostics.Cannot_access_0_1_because_0_is_a_type_but_not_a_namespace_Did_you_mean_to_retrieve_the_type_of_the_property_1_in_0_with_0_1,
+                                unescapeLeadingUnderscores(name),
+                                unescapeLeadingUnderscores(propName),
+                            );
+                            return true;
+                        }
+                    }
                     error(errorLocation, Diagnostics._0_only_refers_to_a_type_but_is_being_used_as_a_namespace_here, unescapeLeadingUnderscores(name));
                     return true;
                 }

src/compiler/diagnosticMessages.json

@@ -2192,6 +2192,10 @@
         "category": "Error",
         "code": 2712
     },
+    "Cannot access '{0}.{1}' because '{0}' is a type, but not a namespace. Did you mean to retrieve the type of the property '{1}' in '{0}' with '{0}[\"{1}\"]'?": {
+        "category": "Error",
+        "code": 2713
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

tests/baselines/reference/errorForUsingPropertyOfTypeAsType01.errors.txt

@@ -0,0 +1,55 @@
+tests/cases/compiler/errorForUsingPropertyOfTypeAsType01.ts(6,12): error TS2713: Cannot access 'Foo.bar' because 'Foo' is a type, but not a namespace. Did you mean to retrieve the type of the property 'bar' in 'Foo' with 'Foo["bar"]'?
+tests/cases/compiler/errorForUsingPropertyOfTypeAsType01.ts(14,12): error TS2503: Cannot find namespace 'Foo'.
+tests/cases/compiler/errorForUsingPropertyOfTypeAsType01.ts(22,12): error TS2713: Cannot access 'Foo.bar' because 'Foo' is a type, but not a namespace. Did you mean to retrieve the type of the property 'bar' in 'Foo' with 'Foo["bar"]'?
+tests/cases/compiler/errorForUsingPropertyOfTypeAsType01.ts(29,12): error TS2713: Cannot access 'Foo.bar' because 'Foo' is a type, but not a namespace. Did you mean to retrieve the type of the property 'bar' in 'Foo' with 'Foo["bar"]'?
+tests/cases/compiler/errorForUsingPropertyOfTypeAsType01.ts(36,12): error TS2702: 'Foo' only refers to a type, but is being used as a namespace here.
+
+
+==== tests/cases/compiler/errorForUsingPropertyOfTypeAsType01.ts (5 errors) ====
+    namespace Test1 {
+        export interface Foo {
+            bar: string;
+        }
+
+        var x: Foo.bar = "";
+               ~~~~~~~
+!!! error TS2713: Cannot access 'Foo.bar' because 'Foo' is a type, but not a namespace. Did you mean to retrieve the type of the property 'bar' in 'Foo' with 'Foo["bar"]'?
+    }
+
+    namespace Test2 {
+        export class Foo {
+            bar: string;
+        }
+
+        var x: Foo.bar = "";
+               ~~~
+!!! error TS2503: Cannot find namespace 'Foo'.
+    }
+
+    namespace Test3 {
+        export type Foo = {
+            bar: string;
+        }
+
+        var x: Foo.bar = "";
+               ~~~~~~~
+!!! error TS2713: Cannot access 'Foo.bar' because 'Foo' is a type, but not a namespace. Did you mean to retrieve the type of the property 'bar' in 'Foo' with 'Foo["bar"]'?
+    }
+
+    namespace Test4 {
+        export type Foo = { bar: number }
+                        | { bar: string }
+
+        var x: Foo.bar = "";
+               ~~~~~~~
+!!! error TS2713: Cannot access 'Foo.bar' because 'Foo' is a type, but not a namespace. Did you mean to retrieve the type of the property 'bar' in 'Foo' with 'Foo["bar"]'?
+    }
+
+    namespace Test5 {
+        export type Foo = { bar: number }
+                        | { wat: string }
+
+        var x: Foo.bar = "";
+               ~~~
+!!! error TS2702: 'Foo' only refers to a type, but is being used as a namespace here.
+    }

tests/baselines/reference/errorForUsingPropertyOfTypeAsType01.js

@@ -0,0 +1,66 @@
+//// [errorForUsingPropertyOfTypeAsType01.ts]
+namespace Test1 {
+    export interface Foo {
+        bar: string;
+    }
+
+    var x: Foo.bar = "";
+}
+
+namespace Test2 {
+    export class Foo {
+        bar: string;
+    }
+
+    var x: Foo.bar = "";
+}
+
+namespace Test3 {
+    export type Foo = {
+        bar: string;
+    }
+
+    var x: Foo.bar = "";
+}
+
+namespace Test4 {
+    export type Foo = { bar: number }
+                    | { bar: string }
+
+    var x: Foo.bar = "";
+}
+
+namespace Test5 {
+    export type Foo = { bar: number }
+                    | { wat: string }
+
+    var x: Foo.bar = "";
+}
+
+//// [errorForUsingPropertyOfTypeAsType01.js]
+var Test1;
+(function (Test1) {
+    var x = "";
+})(Test1 || (Test1 = {}));
+var Test2;
+(function (Test2) {
+    var Foo = (function () {
+        function Foo() {
+        }
+        return Foo;
+    }());
+    Test2.Foo = Foo;
+    var x = "";
+})(Test2 || (Test2 = {}));
+var Test3;
+(function (Test3) {
+    var x = "";
+})(Test3 || (Test3 = {}));
+var Test4;
+(function (Test4) {
+    var x = "";
+})(Test4 || (Test4 = {}));
+var Test5;
+(function (Test5) {
+    var x = "";
+})(Test5 || (Test5 = {}));

tests/cases/compiler/errorForUsingPropertyOfTypeAsType01.ts

@@ -0,0 +1,37 @@
+namespace Test1 {
+    export interface Foo {
+        bar: string;
+    }
+
+    var x: Foo.bar = "";
+}
+
+namespace Test2 {
+    export class Foo {
+        bar: string;
+    }
+
+    var x: Foo.bar = "";
+}
+
+namespace Test3 {
+    export type Foo = {
+        bar: string;
+    }
+
+    var x: Foo.bar = "";
+}
+
+namespace Test4 {
+    export type Foo = { bar: number }
+                    | { bar: string }
+
+    var x: Foo.bar = "";
+}
+
+namespace Test5 {
+    export type Foo = { bar: number }
+                    | { wat: string }
+
+    var x: Foo.bar = "";
+}