Improve narrowing of generic types in control flow analysis

tests/baselines/reference/assignmentNonObjectTypeConstraints.types

@@ -40,7 +40,7 @@ function bar<T extends A | B>(x: T) {
 
     var y: A | B = x;  // Ok
 >y : A | B
->x : T
+>x : A | B
 }
 
 bar(new A);

tests/baselines/reference/conditionalTypes1.errors.txt

@@ -2,17 +2,11 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(12,5): error TS23
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(17,5): error TS2322: Type 'T' is not assignable to type 'NonNullable<T>'.
   Type 'string | undefined' is not assignable to type 'NonNullable<T>'.
     Type 'undefined' is not assignable to type 'NonNullable<T>'.
-tests/cases/conformance/types/conditional/conditionalTypes1.ts(18,9): error TS2322: Type 'T' is not assignable to type 'string'.
-  Type 'string | undefined' is not assignable to type 'string'.
-    Type 'undefined' is not assignable to type 'string'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(24,5): error TS2322: Type 'T[keyof T] | undefined' is not assignable to type 'NonNullable<Partial<T>[keyof T]>'.
   Type 'undefined' is not assignable to type 'NonNullable<Partial<T>[keyof T]>'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(29,5): error TS2322: Type 'T["x"]' is not assignable to type 'NonNullable<T["x"]>'.
   Type 'string | undefined' is not assignable to type 'NonNullable<T["x"]>'.
     Type 'undefined' is not assignable to type 'NonNullable<T["x"]>'.
-tests/cases/conformance/types/conditional/conditionalTypes1.ts(30,9): error TS2322: Type 'T["x"]' is not assignable to type 'string'.
-  Type 'string | undefined' is not assignable to type 'string'.
-    Type 'undefined' is not assignable to type 'string'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(103,5): error TS2322: Type 'FunctionProperties<T>' is not assignable to type 'T'.
   'T' could be instantiated with an arbitrary type which could be unrelated to 'FunctionProperties<T>'.
 tests/cases/conformance/types/conditional/conditionalTypes1.ts(104,5): error TS2322: Type 'NonFunctionProperties<T>' is not assignable to type 'T'.
@@ -63,7 +57,7 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
       Type 'boolean' is not assignable to type 'true'.
 
 
-==== tests/cases/conformance/types/conditional/conditionalTypes1.ts (22 errors) ====
+==== tests/cases/conformance/types/conditional/conditionalTypes1.ts (20 errors) ====
     type T00 = Exclude<"a" | "b" | "c" | "d", "a" | "c" | "f">;  // "b" | "d"
     type T01 = Extract<"a" | "b" | "c" | "d", "a" | "c" | "f">;  // "a" | "c"
 
@@ -88,10 +82,6 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
 !!! error TS2322:   Type 'string | undefined' is not assignable to type 'NonNullable<T>'.
 !!! error TS2322:     Type 'undefined' is not assignable to type 'NonNullable<T>'.
         let s1: string = x;  // Error
-            ~~
-!!! error TS2322: Type 'T' is not assignable to type 'string'.
-!!! error TS2322:   Type 'string | undefined' is not assignable to type 'string'.
-!!! error TS2322:     Type 'undefined' is not assignable to type 'string'.
         let s2: string = y;
     }
 
@@ -111,10 +101,6 @@ tests/cases/conformance/types/conditional/conditionalTypes1.ts(288,43): error TS
 !!! error TS2322:   Type 'string | undefined' is not assignable to type 'NonNullable<T["x"]>'.
 !!! error TS2322:     Type 'undefined' is not assignable to type 'NonNullable<T["x"]>'.
         let s1: string = x;  // Error
-            ~~
-!!! error TS2322: Type 'T["x"]' is not assignable to type 'string'.
-!!! error TS2322:   Type 'string | undefined' is not assignable to type 'string'.
-!!! error TS2322:     Type 'undefined' is not assignable to type 'string'.
         let s2: string = y;
     }
 

tests/baselines/reference/conditionalTypes1.types

@@ -51,7 +51,7 @@ function f2<T extends string | undefined>(x: T, y: NonNullable<T>) {
 
     let s1: string = x;  // Error
 >s1 : string
->x : T
+>x : string
 
     let s2: string = y;
 >s2 : string
@@ -92,7 +92,7 @@ function f4<T extends { x: string | undefined }>(x: T["x"], y: NonNullable<T["x"
 
     let s1: string = x;  // Error
 >s1 : string
->x : T["x"]
+>x : string
 
     let s2: string = y;
 >s2 : string
@@ -476,11 +476,11 @@ function f21<T extends number | string>(x: T, y: ZeroOf<T>) {
 
     let z1: number | string = y;
 >z1 : string | number
->y : ZeroOf<T>
+>y : "" | 0
 
     let z2: 0 | "" = y;
 >z2 : "" | 0
->y : ZeroOf<T>
+>y : "" | 0
 
     x = y;  // Error
 >x = y : ZeroOf<T>

tests/baselines/reference/controlFlowGenericTypes.errors.txt

@@ -0,0 +1,152 @@
+tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(49,15): error TS2345: Argument of type 'undefined' is not assignable to parameter of type 'Box<T>'.
+tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(55,15): error TS2345: Argument of type 'undefined' is not assignable to parameter of type 'Box<unknown>'.
+tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(81,11): error TS2339: Property 'foo' does not exist on type 'MyUnion'.
+  Property 'foo' does not exist on type 'AA'.
+tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(90,44): error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts(91,11): error TS2339: Property 'foo' does not exist on type 'MyUnion'.
+  Property 'foo' does not exist on type 'AA'.
+
+
+==== tests/cases/conformance/controlFlow/controlFlowGenericTypes.ts (5 errors) ====
+    function f1<T extends string | undefined>(x: T, y: { a: T }, z: [T]): string {
+        if (x) {
+            x;
+            x.length;
+            return x;
+        }
+        if (y.a) {
+            y.a.length;
+            return y.a;
+        }
+        if (z[0]) {
+            z[0].length;
+            return z[0];
+        }
+        return "hello";
+    }
+
+    function f2<T>(x: Extract<T, string | undefined> | null): string {
+        if (x) {
+            x;
+            x.length;
+            return x;
+        }
+        return "hello";
+    }
+
+    interface Box<T> {
+        item: T;
+    }
+
+    declare function isBox(x: any): x is Box<unknown>;
+    declare function isUndefined(x: unknown): x is undefined;
+    declare function unbox<T>(x: Box<T>): T;
+
+    function g1<T extends Box<T> | undefined>(x: T) {
+        if (isBox(x)) {
+            unbox(x);
+        }
+    }
+
+    function g2<T extends Box<T> | undefined>(x: T) {
+        if (!isUndefined(x)) {
+            unbox(x);
+        }
+    }
+
+    function g3<T extends Box<T> | undefined>(x: T) {
+        if (!isBox(x)) {
+            unbox(x);  // Error
+                  ~
+!!! error TS2345: Argument of type 'undefined' is not assignable to parameter of type 'Box<T>'.
+        }
+    }
+
+    function g4<T extends Box<T> | undefined>(x: T) {
+        if (isUndefined(x)) {
+            unbox(x);  // Error
+                  ~
+!!! error TS2345: Argument of type 'undefined' is not assignable to parameter of type 'Box<unknown>'.
+        }
+    }
+
+    // Repro from #13995
+
+    declare function takeA(val: 'A'): void;
+    export function bounceAndTakeIfA<AB extends 'A' | 'B'>(value: AB): AB {
+        if (value === 'A') {
+            takeA(value);
+            return value;
+        }
+        else {
+            return value;
+        }
+    }
+
+    // Repro from #13995
+
+    type Common = { id: number };
+    type AA = { tag: 'A', id: number };
+    type BB = { tag: 'B', id: number, foo: number };
+
+    type MyUnion = AA | BB;
+
+    const fn = (value: MyUnion) => {
+        value.foo;  // Error
+              ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'MyUnion'.
+!!! error TS2339:   Property 'foo' does not exist on type 'AA'.
+        if ('foo' in value) {
+            value.foo;
+        }
+        if (value.tag === 'B') {
+            value.foo;
+        }
+    };
+
+    const fn2 = <T extends MyUnion>(value: T): MyUnion => {
+                                               ~~~~~~~
+!!! error TS2355: A function whose declared type is neither 'void' nor 'any' must return a value.
+        value.foo;  // Error
+              ~~~
+!!! error TS2339: Property 'foo' does not exist on type 'MyUnion'.
+!!! error TS2339:   Property 'foo' does not exist on type 'AA'.
+        if ('foo' in value) {
+            value.foo;
+        }
+        if (value.tag === 'B') {
+            value.foo;
+        }
+    };
+
+    // Repro from #13995
+
+    type A1 = {
+        testable: true
+        doTest: () => void
+    }
+    type B1 = {
+        testable: false
+    };
+
+    type Union = A1 | B1
+
+    function notWorking<T extends Union>(object: T) {
+        if (!object.testable) return;
+        object.doTest();
+    }
+
+    // Repro from #42939
+
+    interface A {
+        a: number | null;
+    };
+
+    function get<K extends keyof A>(key: K, obj: A): number {
+        const value = obj[key];
+        if (value !== null) {
+            return value;
+        }
+        return 0;
+    };
+