Stop elaborating errors when relating intersection constituents

src/compiler/checker.ts

@@ -5521,8 +5521,20 @@ namespace ts {
                     // on either side that need to be prioritized. For example, A | B = (A | B) & (C | D) or
                     // A & B = (A & B) | (C & D).
                     if (source.flags & TypeFlags.Intersection) {
-                        // If target is a union type the following check will report errors so we suppress them here
-                        if (result = someTypeRelatedToType(<IntersectionType>source, target, reportErrors && !(target.flags & TypeFlags.Union))) {
+                        // Check to see if any constituents of the intersection are immediately related to the target.
+                        //
+                        // Don't report errors though. Checking whether a constituent is related to the source is not actually
+                        // useful and leads to some confusing error messages. Instead it is better to let the below checks
+                        // take care of this, or to not elaborate at all. For instance,
+                        //
+                        //    - For an object type (such as 'C = A & B'), users are usually more interested in structural errors.
+                        //
+                        //    - For a union type (such as '(A | B) = (C & D)'), it's better to hold onto the whole intersection
+                        //          than to report that 'D' is not assignable to 'A' or 'B'.
+                        //
+                        //    - For a primitive type or type parameter (such as 'number = A & B') there is no point in
+                        //          breaking the intersection apart.
+                        if (result = someTypeRelatedToType(<IntersectionType>source, target, /*reportErrors*/ false)) {
                             return result;
                         }
                     }

src/compiler/tsc.ts

@@ -239,8 +239,8 @@ namespace ts {
     function isJSONSupported() {
         return typeof JSON === "object" && typeof JSON.parse === "function";
     }
-    
-    function isWatchSet(options: CompilerOptions) { 
+
+    function isWatchSet(options: CompilerOptions) {
         // Firefox has Object.prototype.watch
         return options.watch && options.hasOwnProperty("watch");
     }

tests/baselines/reference/errorMessagesIntersectionTypes01.errors.txt

@@ -0,0 +1,26 @@
+tests/cases/compiler/errorMessagesIntersectionTypes01.ts(14,5): error TS2322: Type '{ fooProp: string; } & Bar' is not assignable to type 'FooBar'.
+  Types of property 'fooProp' are incompatible.
+    Type 'string' is not assignable to type 'boolean'.
+
+
+==== tests/cases/compiler/errorMessagesIntersectionTypes01.ts (1 errors) ====
+    interface Foo {
+        fooProp: boolean;
+    }
+
+    interface Bar {
+        barProp: string;
+    }
+
+    interface FooBar extends Foo, Bar {
+    }
+
+    declare function mixBar<T>(obj: T): T & Bar;
+
+    let fooBar: FooBar = mixBar({
+        ~~~~~~
+!!! error TS2322: Type '{ fooProp: string; } & Bar' is not assignable to type 'FooBar'.
+!!! error TS2322:   Types of property 'fooProp' are incompatible.
+!!! error TS2322:     Type 'string' is not assignable to type 'boolean'.
+        fooProp: "frizzlebizzle"
+    });

tests/baselines/reference/errorMessagesIntersectionTypes01.js

@@ -0,0 +1,22 @@
+//// [errorMessagesIntersectionTypes01.ts]
+interface Foo {
+    fooProp: boolean;
+}
+
+interface Bar {
+    barProp: string;
+}
+
+interface FooBar extends Foo, Bar {
+}
+
+declare function mixBar<T>(obj: T): T & Bar;
+
+let fooBar: FooBar = mixBar({
+    fooProp: "frizzlebizzle"
+});
+
+//// [errorMessagesIntersectionTypes01.js]
+var fooBar = mixBar({
+    fooProp: "frizzlebizzle"
+});

tests/baselines/reference/errorMessagesIntersectionTypes02.errors.txt

@@ -0,0 +1,28 @@
+tests/cases/compiler/errorMessagesIntersectionTypes02.ts(14,5): error TS2322: Type '{ fooProp: string; } & Bar' is not assignable to type 'FooBar'.
+  Types of property 'fooProp' are incompatible.
+    Type 'string' is not assignable to type '"hello" | "world"'.
+      Type 'string' is not assignable to type '"world"'.
+
+
+==== tests/cases/compiler/errorMessagesIntersectionTypes02.ts (1 errors) ====
+    interface Foo {
+        fooProp: "hello" | "world";
+    }
+
+    interface Bar {
+        barProp: string;
+    }
+
+    interface FooBar extends Foo, Bar {
+    }
+
+    declare function mixBar<T>(obj: T): T & Bar;
+
+    let fooBar: FooBar = mixBar({
+        ~~~~~~
+!!! error TS2322: Type '{ fooProp: string; } & Bar' is not assignable to type 'FooBar'.
+!!! error TS2322:   Types of property 'fooProp' are incompatible.
+!!! error TS2322:     Type 'string' is not assignable to type '"hello" | "world"'.
+!!! error TS2322:       Type 'string' is not assignable to type '"world"'.
+        fooProp: "frizzlebizzle"
+    });

tests/baselines/reference/errorMessagesIntersectionTypes02.js

@@ -0,0 +1,22 @@
+//// [errorMessagesIntersectionTypes02.ts]
+interface Foo {
+    fooProp: "hello" | "world";
+}
+
+interface Bar {
+    barProp: string;
+}
+
+interface FooBar extends Foo, Bar {
+}
+
+declare function mixBar<T>(obj: T): T & Bar;
+
+let fooBar: FooBar = mixBar({
+    fooProp: "frizzlebizzle"
+});
+
+//// [errorMessagesIntersectionTypes02.js]
+var fooBar = mixBar({
+    fooProp: "frizzlebizzle"
+});

tests/baselines/reference/errorMessagesIntersectionTypes03.errors.txt

@@ -0,0 +1,42 @@
+tests/cases/compiler/errorMessagesIntersectionTypes03.ts(17,5): error TS2322: Type 'A & B' is not assignable to type 'T'.
+tests/cases/compiler/errorMessagesIntersectionTypes03.ts(18,5): error TS2322: Type 'A & B' is not assignable to type 'U'.
+tests/cases/compiler/errorMessagesIntersectionTypes03.ts(19,5): error TS2322: Type 'A & B' is not assignable to type 'V'.
+tests/cases/compiler/errorMessagesIntersectionTypes03.ts(22,5): error TS2322: Type 'T & B' is not assignable to type 'U'.
+tests/cases/compiler/errorMessagesIntersectionTypes03.ts(23,5): error TS2322: Type 'T & B' is not assignable to type 'V'.
+
+
+==== tests/cases/compiler/errorMessagesIntersectionTypes03.ts (5 errors) ====
+    interface A {
+        a;
+    }
+
+    interface B {
+        b;
+    }
+
+    function f<T, U extends A, V extends U>(): void {
+        let t: T;
+        let u: U;
+        let v: V;
+
+        let a_and_b: A & B;
+        let t_and_b: T & B;
+
+        t = a_and_b;
+        ~
+!!! error TS2322: Type 'A & B' is not assignable to type 'T'.
+        u = a_and_b;
+        ~
+!!! error TS2322: Type 'A & B' is not assignable to type 'U'.
+        v = a_and_b;
+        ~
+!!! error TS2322: Type 'A & B' is not assignable to type 'V'.
+
+        t = t_and_b;
+        u = t_and_b;
+        ~
+!!! error TS2322: Type 'T & B' is not assignable to type 'U'.
+        v = t_and_b;
+        ~
+!!! error TS2322: Type 'T & B' is not assignable to type 'V'.
+    }

tests/baselines/reference/errorMessagesIntersectionTypes03.js

@@ -0,0 +1,40 @@
+//// [errorMessagesIntersectionTypes03.ts]
+interface A {
+    a;
+}
+
+interface B {
+    b;
+}
+
+function f<T, U extends A, V extends U>(): void {
+    let t: T;
+    let u: U;
+    let v: V;
+
+    let a_and_b: A & B;
+    let t_and_b: T & B;
+
+    t = a_and_b;
+    u = a_and_b;
+    v = a_and_b;
+
+    t = t_and_b;
+    u = t_and_b;
+    v = t_and_b;
+}
+
+//// [errorMessagesIntersectionTypes03.js]
+function f() {
+    var t;
+    var u;
+    var v;
+    var a_and_b;
+    var t_and_b;
+    t = a_and_b;
+    u = a_and_b;
+    v = a_and_b;
+    t = t_and_b;
+    u = t_and_b;
+    v = t_and_b;
+}

tests/baselines/reference/errorMessagesIntersectionTypes04.errors.txt

@@ -0,0 +1,37 @@
+tests/cases/compiler/errorMessagesIntersectionTypes04.ts(17,5): error TS2322: Type 'A & B' is not assignable to type 'number'.
+tests/cases/compiler/errorMessagesIntersectionTypes04.ts(18,5): error TS2322: Type 'A & B' is not assignable to type 'boolean'.
+tests/cases/compiler/errorMessagesIntersectionTypes04.ts(19,5): error TS2322: Type 'A & B' is not assignable to type 'string'.
+tests/cases/compiler/errorMessagesIntersectionTypes04.ts(21,5): error TS2322: Type 'number & boolean' is not assignable to type 'string'.
+
+
+==== tests/cases/compiler/errorMessagesIntersectionTypes04.ts (4 errors) ====
+    interface A {
+        a;
+    }
+
+    interface B {
+        b;
+    }
+
+    function f<T, U extends A, V extends U>(): void {
+        let num: number;
+        let bool: boolean;
+        let str: string;
+
+        let a_and_b: A & B;
+        let num_and_bool: number & boolean;
+
+        num = a_and_b;
+        ~~~
+!!! error TS2322: Type 'A & B' is not assignable to type 'number'.
+        bool = a_and_b;
+        ~~~~
+!!! error TS2322: Type 'A & B' is not assignable to type 'boolean'.
+        str = a_and_b;
+        ~~~
+!!! error TS2322: Type 'A & B' is not assignable to type 'string'.
+
+        str = num_and_bool;
+        ~~~
+!!! error TS2322: Type 'number & boolean' is not assignable to type 'string'.
+    }

tests/baselines/reference/errorMessagesIntersectionTypes04.js

@@ -0,0 +1,36 @@
+//// [errorMessagesIntersectionTypes04.ts]
+interface A {
+    a;
+}
+
+interface B {
+    b;
+}
+
+function f<T, U extends A, V extends U>(): void {
+    let num: number;
+    let bool: boolean;
+    let str: string;
+
+    let a_and_b: A & B;
+    let num_and_bool: number & boolean;
+
+    num = a_and_b;
+    bool = a_and_b;
+    str = a_and_b;
+
+    str = num_and_bool;
+}
+
+//// [errorMessagesIntersectionTypes04.js]
+function f() {
+    var num;
+    var bool;
+    var str;
+    var a_and_b;
+    var num_and_bool;
+    num = a_and_b;
+    bool = a_and_b;
+    str = a_and_b;
+    str = num_and_bool;
+}