Merge pull request #18438 from Microsoft/unionIntersectionUnit

Remove empty intersection types in unit types

src/compiler/checker.ts

@@ -7298,6 +7298,22 @@ namespace ts {
             return binarySearchTypes(types, type) >= 0;
         }
 
+        // Return true if the given intersection type contains (a) more than one unit type or (b) an object
+        // type and a nullable type (null or undefined).
+        function isEmptyIntersectionType(type: IntersectionType) {
+            let combined: TypeFlags = 0;
+            for (const t of type.types) {
+                if (t.flags & TypeFlags.Unit && combined & TypeFlags.Unit) {
+                    return true;
+                }
+                combined |= t.flags;
+                if (combined & TypeFlags.Nullable && combined & (TypeFlags.Object | TypeFlags.NonPrimitive)) {
+                    return true;
+                }
+            }
+            return false;
+        }
+
         function addTypeToUnion(typeSet: TypeSet, type: Type) {
             const flags = type.flags;
             if (flags & TypeFlags.Union) {
@@ -7311,7 +7327,11 @@ namespace ts {
                 if (flags & TypeFlags.Null) typeSet.containsNull = true;
                 if (!(flags & TypeFlags.ContainsWideningType)) typeSet.containsNonWideningType = true;
             }
-            else if (!(flags & TypeFlags.Never)) {
+            else if (!(flags & TypeFlags.Never || flags & TypeFlags.Intersection && isEmptyIntersectionType(<IntersectionType>type))) {
+                // We ignore 'never' types in unions. Likewise, we ignore intersections of unit types as they are
+                // another form of 'never' (in that they have an empty value domain). We could in theory turn
+                // intersections of unit types into 'never' upon construction, but deferring the reduction makes it
+                // easier to reason about their origin.
                 if (flags & TypeFlags.String) typeSet.containsString = true;
                 if (flags & TypeFlags.Number) typeSet.containsNumber = true;
                 if (flags & TypeFlags.StringOrNumberLiteral) typeSet.containsStringOrNumberLiteral = true;
@@ -10048,7 +10068,7 @@ namespace ts {
         }
 
         function isUnitType(type: Type): boolean {
-            return (type.flags & (TypeFlags.Literal | TypeFlags.Undefined | TypeFlags.Null)) !== 0;
+            return !!(type.flags & TypeFlags.Unit);
         }
 
         function isLiteralType(type: Type): boolean {

src/compiler/types.ts

@@ -3216,6 +3216,7 @@ namespace ts {
         /* @internal */
         Nullable = Undefined | Null,
         Literal = StringLiteral | NumberLiteral | BooleanLiteral,
+        Unit = Literal | Nullable,
         StringOrNumberLiteral = StringLiteral | NumberLiteral,
         /* @internal */
         DefinitelyFalsy = StringLiteral | NumberLiteral | BooleanLiteral | Void | Undefined | Null,

tests/baselines/reference/intersectionOfUnionOfUnitTypes.js

@@ -0,0 +1,47 @@
+//// [intersectionOfUnionOfUnitTypes.ts]
+// @strict
+
+const enum E { A, B, C, D, E, F }
+
+let x0: ('a' | 'b' | 'c') & ('a' | 'b' | 'c');  // 'a' | 'b' | 'c'
+let x1: ('a' | 'b' | 'c') & ('b' | 'c' | 'd');  // 'b' | 'c'
+let x2: ('a' | 'b' | 'c') & ('c' | 'd' | 'e');  // 'c'
+let x3: ('a' | 'b' | 'c') & ('d' | 'e' | 'f');  // never
+let x4: ('a' | 'b' | 'c') & ('b' | 'c' | 'd') & ('c' | 'd' | 'e');  // 'c'
+let x5: ('a' | 'b' | 'c') & ('b' | 'c' | 'd') & ('c' | 'd' | 'e') & ('d' | 'e' | 'f');  // never
+
+let y0: (0 | 1 | 2) & (0 | 1 | 2);  // 0 | 1 | 2
+let y1: (0 | 1 | 2) & (1 | 2 | 3);  // 1 | 2
+let y2: (0 | 1 | 2) & (2 | 3 | 4);  // 2
+let y3: (0 | 1 | 2) & (3 | 4 | 5);  // never
+let y4: (0 | 1 | 2) & (1 | 2 | 3) & (2 | 3 | 4);  // 2
+let y5: (0 | 1 | 2) & (1 | 2 | 3) & (2 | 3 | 4) & (3 | 4 | 5);  // never
+
+let z0: (E.A | E.B | E.C) & (E.A | E.B | E.C);  // E.A | E.B | E.C
+let z1: (E.A | E.B | E.C) & (E.B | E.C | E.D);  // E.B | E.C
+let z2: (E.A | E.B | E.C) & (E.C | E.D | E.E);  // E.C
+let z3: (E.A | E.B | E.C) & (E.D | E.E | E.F);  // never
+let z4: (E.A | E.B | E.C) & (E.B | E.C | E.D) & (E.C | E.D | E.E);  // E.C
+let z5: (E.A | E.B | E.C) & (E.B | E.C | E.D) & (E.C | E.D | E.E) & (E.D | E.E | E.F);  // never
+
+
+//// [intersectionOfUnionOfUnitTypes.js]
+// @strict
+var x0; // 'a' | 'b' | 'c'
+var x1; // 'b' | 'c'
+var x2; // 'c'
+var x3; // never
+var x4; // 'c'
+var x5; // never
+var y0; // 0 | 1 | 2
+var y1; // 1 | 2
+var y2; // 2
+var y3; // never
+var y4; // 2
+var y5; // never
+var z0; // E.A | E.B | E.C
+var z1; // E.B | E.C
+var z2; // E.C
+var z3; // never
+var z4; // E.C
+var z5; // never

tests/baselines/reference/intersectionOfUnionOfUnitTypes.symbols

@@ -0,0 +1,156 @@
+=== tests/cases/conformance/types/intersection/intersectionOfUnionOfUnitTypes.ts ===
+// @strict
+
+const enum E { A, B, C, D, E, F }
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>A : Symbol(E.A, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 14))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>D : Symbol(E.D, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 23))
+>E : Symbol(E.E, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 26))
+>F : Symbol(E.F, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 29))
+
+let x0: ('a' | 'b' | 'c') & ('a' | 'b' | 'c');  // 'a' | 'b' | 'c'
+>x0 : Symbol(x0, Decl(intersectionOfUnionOfUnitTypes.ts, 4, 3))
+
+let x1: ('a' | 'b' | 'c') & ('b' | 'c' | 'd');  // 'b' | 'c'
+>x1 : Symbol(x1, Decl(intersectionOfUnionOfUnitTypes.ts, 5, 3))
+
+let x2: ('a' | 'b' | 'c') & ('c' | 'd' | 'e');  // 'c'
+>x2 : Symbol(x2, Decl(intersectionOfUnionOfUnitTypes.ts, 6, 3))
+
+let x3: ('a' | 'b' | 'c') & ('d' | 'e' | 'f');  // never
+>x3 : Symbol(x3, Decl(intersectionOfUnionOfUnitTypes.ts, 7, 3))
+
+let x4: ('a' | 'b' | 'c') & ('b' | 'c' | 'd') & ('c' | 'd' | 'e');  // 'c'
+>x4 : Symbol(x4, Decl(intersectionOfUnionOfUnitTypes.ts, 8, 3))
+
+let x5: ('a' | 'b' | 'c') & ('b' | 'c' | 'd') & ('c' | 'd' | 'e') & ('d' | 'e' | 'f');  // never
+>x5 : Symbol(x5, Decl(intersectionOfUnionOfUnitTypes.ts, 9, 3))
+
+let y0: (0 | 1 | 2) & (0 | 1 | 2);  // 0 | 1 | 2
+>y0 : Symbol(y0, Decl(intersectionOfUnionOfUnitTypes.ts, 11, 3))
+
+let y1: (0 | 1 | 2) & (1 | 2 | 3);  // 1 | 2
+>y1 : Symbol(y1, Decl(intersectionOfUnionOfUnitTypes.ts, 12, 3))
+
+let y2: (0 | 1 | 2) & (2 | 3 | 4);  // 2
+>y2 : Symbol(y2, Decl(intersectionOfUnionOfUnitTypes.ts, 13, 3))
+
+let y3: (0 | 1 | 2) & (3 | 4 | 5);  // never
+>y3 : Symbol(y3, Decl(intersectionOfUnionOfUnitTypes.ts, 14, 3))
+
+let y4: (0 | 1 | 2) & (1 | 2 | 3) & (2 | 3 | 4);  // 2
+>y4 : Symbol(y4, Decl(intersectionOfUnionOfUnitTypes.ts, 15, 3))
+
+let y5: (0 | 1 | 2) & (1 | 2 | 3) & (2 | 3 | 4) & (3 | 4 | 5);  // never
+>y5 : Symbol(y5, Decl(intersectionOfUnionOfUnitTypes.ts, 16, 3))
+
+let z0: (E.A | E.B | E.C) & (E.A | E.B | E.C);  // E.A | E.B | E.C
+>z0 : Symbol(z0, Decl(intersectionOfUnionOfUnitTypes.ts, 18, 3))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>A : Symbol(E.A, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 14))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>A : Symbol(E.A, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 14))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+
+let z1: (E.A | E.B | E.C) & (E.B | E.C | E.D);  // E.B | E.C
+>z1 : Symbol(z1, Decl(intersectionOfUnionOfUnitTypes.ts, 19, 3))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>A : Symbol(E.A, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 14))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>D : Symbol(E.D, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 23))
+
+let z2: (E.A | E.B | E.C) & (E.C | E.D | E.E);  // E.C
+>z2 : Symbol(z2, Decl(intersectionOfUnionOfUnitTypes.ts, 20, 3))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>A : Symbol(E.A, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 14))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>D : Symbol(E.D, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 23))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>E : Symbol(E.E, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 26))
+
+let z3: (E.A | E.B | E.C) & (E.D | E.E | E.F);  // never
+>z3 : Symbol(z3, Decl(intersectionOfUnionOfUnitTypes.ts, 21, 3))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>A : Symbol(E.A, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 14))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>D : Symbol(E.D, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 23))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>E : Symbol(E.E, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 26))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>F : Symbol(E.F, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 29))
+
+let z4: (E.A | E.B | E.C) & (E.B | E.C | E.D) & (E.C | E.D | E.E);  // E.C
+>z4 : Symbol(z4, Decl(intersectionOfUnionOfUnitTypes.ts, 22, 3))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>A : Symbol(E.A, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 14))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>D : Symbol(E.D, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 23))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>D : Symbol(E.D, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 23))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>E : Symbol(E.E, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 26))
+
+let z5: (E.A | E.B | E.C) & (E.B | E.C | E.D) & (E.C | E.D | E.E) & (E.D | E.E | E.F);  // never
+>z5 : Symbol(z5, Decl(intersectionOfUnionOfUnitTypes.ts, 23, 3))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>A : Symbol(E.A, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 14))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>B : Symbol(E.B, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 17))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>D : Symbol(E.D, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 23))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>C : Symbol(E.C, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 20))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>D : Symbol(E.D, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 23))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>E : Symbol(E.E, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 26))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>D : Symbol(E.D, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 23))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>E : Symbol(E.E, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 26))
+>E : Symbol(E, Decl(intersectionOfUnionOfUnitTypes.ts, 0, 0))
+>F : Symbol(E.F, Decl(intersectionOfUnionOfUnitTypes.ts, 2, 29))
+

tests/baselines/reference/intersectionOfUnionOfUnitTypes.types

@@ -0,0 +1,156 @@
+=== tests/cases/conformance/types/intersection/intersectionOfUnionOfUnitTypes.ts ===
+// @strict
+
+const enum E { A, B, C, D, E, F }
+>E : E
+>A : E.A
+>B : E.B
+>C : E.C
+>D : E.D
+>E : E.E
+>F : E.F
+
+let x0: ('a' | 'b' | 'c') & ('a' | 'b' | 'c');  // 'a' | 'b' | 'c'
+>x0 : "a" | "b" | "c"
+
+let x1: ('a' | 'b' | 'c') & ('b' | 'c' | 'd');  // 'b' | 'c'
+>x1 : "b" | "c"
+
+let x2: ('a' | 'b' | 'c') & ('c' | 'd' | 'e');  // 'c'
+>x2 : "c"
+
+let x3: ('a' | 'b' | 'c') & ('d' | 'e' | 'f');  // never
+>x3 : never
+
+let x4: ('a' | 'b' | 'c') & ('b' | 'c' | 'd') & ('c' | 'd' | 'e');  // 'c'
+>x4 : "c"
+
+let x5: ('a' | 'b' | 'c') & ('b' | 'c' | 'd') & ('c' | 'd' | 'e') & ('d' | 'e' | 'f');  // never
+>x5 : never
+
+let y0: (0 | 1 | 2) & (0 | 1 | 2);  // 0 | 1 | 2
+>y0 : 0 | 1 | 2
+
+let y1: (0 | 1 | 2) & (1 | 2 | 3);  // 1 | 2
+>y1 : 1 | 2
+
+let y2: (0 | 1 | 2) & (2 | 3 | 4);  // 2
+>y2 : 2
+
+let y3: (0 | 1 | 2) & (3 | 4 | 5);  // never
+>y3 : never
+
+let y4: (0 | 1 | 2) & (1 | 2 | 3) & (2 | 3 | 4);  // 2
+>y4 : 2
+
+let y5: (0 | 1 | 2) & (1 | 2 | 3) & (2 | 3 | 4) & (3 | 4 | 5);  // never
+>y5 : never
+
+let z0: (E.A | E.B | E.C) & (E.A | E.B | E.C);  // E.A | E.B | E.C
+>z0 : E.A | E.B | E.C
+>E : any
+>A : E.A
+>E : any
+>B : E.B
+>E : any
+>C : E.C
+>E : any
+>A : E.A
+>E : any
+>B : E.B
+>E : any
+>C : E.C
+
+let z1: (E.A | E.B | E.C) & (E.B | E.C | E.D);  // E.B | E.C
+>z1 : E.B | E.C
+>E : any
+>A : E.A
+>E : any
+>B : E.B
+>E : any
+>C : E.C
+>E : any
+>B : E.B
+>E : any
+>C : E.C
+>E : any
+>D : E.D
+
+let z2: (E.A | E.B | E.C) & (E.C | E.D | E.E);  // E.C
+>z2 : E.C
+>E : any
+>A : E.A
+>E : any
+>B : E.B
+>E : any
+>C : E.C
+>E : any
+>C : E.C
+>E : any
+>D : E.D
+>E : any
+>E : E.E
+
+let z3: (E.A | E.B | E.C) & (E.D | E.E | E.F);  // never
+>z3 : never
+>E : any
+>A : E.A
+>E : any
+>B : E.B
+>E : any
+>C : E.C
+>E : any
+>D : E.D
+>E : any
+>E : E.E
+>E : any
+>F : E.F
+
+let z4: (E.A | E.B | E.C) & (E.B | E.C | E.D) & (E.C | E.D | E.E);  // E.C
+>z4 : E.C
+>E : any
+>A : E.A
+>E : any
+>B : E.B
+>E : any
+>C : E.C
+>E : any
+>B : E.B
+>E : any
+>C : E.C
+>E : any
+>D : E.D
+>E : any
+>C : E.C
+>E : any
+>D : E.D
+>E : any
+>E : E.E
+
+let z5: (E.A | E.B | E.C) & (E.B | E.C | E.D) & (E.C | E.D | E.E) & (E.D | E.E | E.F);  // never
+>z5 : never
+>E : any
+>A : E.A
+>E : any
+>B : E.B
+>E : any
+>C : E.C
+>E : any
+>B : E.B
+>E : any
+>C : E.C
+>E : any
+>D : E.D
+>E : any
+>C : E.C
+>E : any
+>D : E.D
+>E : any
+>E : E.E
+>E : any
+>D : E.D
+>E : any
+>E : E.E
+>E : any
+>F : E.F
+

tests/baselines/reference/keyofAndIndexedAccess.types

@@ -205,7 +205,7 @@ type Q40 = (Shape | Options)["visible"];  // boolean | "yes" | "no"
 >Options : Options
 
 type Q41 = (Shape & Options)["visible"];  // true & "yes" | true & "no" | false & "yes" | false & "no"
->Q41 : (true & "yes") | (true & "no") | (false & "yes") | (false & "no")
+>Q41 : never
 >Shape : Shape
 >Options : Options
 

tests/baselines/reference/restUnion2.js

@@ -13,7 +13,7 @@ declare const nullAndUndefinedUnion: null | undefined;
 var rest4: { };
 var {...rest4 } = nullAndUndefinedUnion;
 
-declare const unionWithIntersection: ({ n: number } & { s: string }) & undefined | null;
+declare const unionWithIntersection: ({ n: number } & { s: string }) & undefined;
 var rest5: { n: number, s: string };
 var {...rest5 } = unionWithIntersection;
 

tests/baselines/reference/restUnion2.symbols

@@ -35,7 +35,7 @@ var {...rest4 } = nullAndUndefinedUnion;
 >rest4 : Symbol(rest4, Decl(restUnion2.ts, 11, 3), Decl(restUnion2.ts, 12, 5))
 >nullAndUndefinedUnion : Symbol(nullAndUndefinedUnion, Decl(restUnion2.ts, 10, 13))
 
-declare const unionWithIntersection: ({ n: number } & { s: string }) & undefined | null;
+declare const unionWithIntersection: ({ n: number } & { s: string }) & undefined;
 >unionWithIntersection : Symbol(unionWithIntersection, Decl(restUnion2.ts, 14, 13))
 >n : Symbol(n, Decl(restUnion2.ts, 14, 39))
 >s : Symbol(s, Decl(restUnion2.ts, 14, 55))

tests/baselines/reference/restUnion2.types

@@ -37,11 +37,10 @@ var {...rest4 } = nullAndUndefinedUnion;
 >rest4 : {}
 >nullAndUndefinedUnion : null | undefined
 
-declare const unionWithIntersection: ({ n: number } & { s: string }) & undefined | null;
->unionWithIntersection : ({ n: number; } & { s: string; } & undefined) | null
+declare const unionWithIntersection: ({ n: number } & { s: string }) & undefined;
+>unionWithIntersection : { n: number; } & { s: string; } & undefined
 >n : number
 >s : string
->null : null
 
 var rest5: { n: number, s: string };
 >rest5 : { n: number; s: string; }
@@ -50,5 +49,5 @@ var rest5: { n: number, s: string };
 
 var {...rest5 } = unionWithIntersection;
 >rest5 : { n: number; s: string; }
->unionWithIntersection : ({ n: number; } & { s: string; } & undefined) | null
+>unionWithIntersection : { n: number; } & { s: string; } & undefined
 

tests/cases/compiler/restUnion2.ts

@@ -14,6 +14,6 @@ declare const nullAndUndefinedUnion: null | undefined;
 var rest4: { };
 var {...rest4 } = nullAndUndefinedUnion;
 
-declare const unionWithIntersection: ({ n: number } & { s: string }) & undefined | null;
+declare const unionWithIntersection: ({ n: number } & { s: string }) & undefined;
 var rest5: { n: number, s: string };
 var {...rest5 } = unionWithIntersection;

tests/cases/conformance/types/intersection/intersectionOfUnionOfUnitTypes.ts

@@ -0,0 +1,24 @@
+// @strict
+
+const enum E { A, B, C, D, E, F }
+
+let x0: ('a' | 'b' | 'c') & ('a' | 'b' | 'c');  // 'a' | 'b' | 'c'
+let x1: ('a' | 'b' | 'c') & ('b' | 'c' | 'd');  // 'b' | 'c'
+let x2: ('a' | 'b' | 'c') & ('c' | 'd' | 'e');  // 'c'
+let x3: ('a' | 'b' | 'c') & ('d' | 'e' | 'f');  // never
+let x4: ('a' | 'b' | 'c') & ('b' | 'c' | 'd') & ('c' | 'd' | 'e');  // 'c'
+let x5: ('a' | 'b' | 'c') & ('b' | 'c' | 'd') & ('c' | 'd' | 'e') & ('d' | 'e' | 'f');  // never
+
+let y0: (0 | 1 | 2) & (0 | 1 | 2);  // 0 | 1 | 2
+let y1: (0 | 1 | 2) & (1 | 2 | 3);  // 1 | 2
+let y2: (0 | 1 | 2) & (2 | 3 | 4);  // 2
+let y3: (0 | 1 | 2) & (3 | 4 | 5);  // never
+let y4: (0 | 1 | 2) & (1 | 2 | 3) & (2 | 3 | 4);  // 2
+let y5: (0 | 1 | 2) & (1 | 2 | 3) & (2 | 3 | 4) & (3 | 4 | 5);  // never
+
+let z0: (E.A | E.B | E.C) & (E.A | E.B | E.C);  // E.A | E.B | E.C
+let z1: (E.A | E.B | E.C) & (E.B | E.C | E.D);  // E.B | E.C
+let z2: (E.A | E.B | E.C) & (E.C | E.D | E.E);  // E.C
+let z3: (E.A | E.B | E.C) & (E.D | E.E | E.F);  // never
+let z4: (E.A | E.B | E.C) & (E.B | E.C | E.D) & (E.C | E.D | E.E);  // E.C
+let z5: (E.A | E.B | E.C) & (E.B | E.C | E.D) & (E.C | E.D | E.E) & (E.D | E.E | E.F);  // never