support 'in' type guard of intersections (microsoft#37106)

Author: pizzacat83

src/compiler/checker.ts

@@ -20241,7 +20241,7 @@ namespace ts {
             }
 
             function narrowByInKeyword(type: Type, literal: LiteralExpression, assumeTrue: boolean) {
-                if (type.flags & (TypeFlags.Union | TypeFlags.Object) || isThisTypeParameter(type)) {
+                if (type.flags & (TypeFlags.Union | TypeFlags.Object | TypeFlags.Intersection) || isThisTypeParameter(type)) {
                     const propName = escapeLeadingUnderscores(literal.text);
                     return filterType(type, t => isTypePresencePossible(t, propName, assumeTrue));
                 }

tests/baselines/reference/inKeywordTypeguard.errors.txt

@@ -156,4 +156,13 @@ tests/cases/compiler/inKeywordTypeguard.ts(94,26): error TS2339: Property 'a' do
 !!! error TS2339: Property 'a' does not exist on type 'never'.
             }
         }
-    }
+    }
+    
+    function positiveIntersectionTest(x: { a: string } & { b: string }) {
+        if ("a" in x) {
+            let s: string = x.a;
+        } else {
+            let n: never = x;
+        }
+    }
+    

tests/baselines/reference/inKeywordTypeguard.js

@@ -95,7 +95,16 @@ class UnreachableCodeDetection {
             let y = this.a;
         }
     }
-}
+}
+
+function positiveIntersectionTest(x: { a: string } & { b: string }) {
+    if ("a" in x) {
+        let s: string = x.a;
+    } else {
+        let n: never = x;
+    }
+}
+
 
 //// [inKeywordTypeguard.js]
 var A = /** @class */ (function () {
@@ -228,3 +237,11 @@ var UnreachableCodeDetection = /** @class */ (function () {
     };
     return UnreachableCodeDetection;
 }());
+function positiveIntersectionTest(x) {
+    if ("a" in x) {
+        var s = x.a;
+    }
+    else {
+        var n = x;
+    }
+}

tests/baselines/reference/inKeywordTypeguard.symbols

@@ -239,3 +239,26 @@ class UnreachableCodeDetection {
         }
     }
 }
+
+function positiveIntersectionTest(x: { a: string } & { b: string }) {
+>positiveIntersectionTest : Symbol(positiveIntersectionTest, Decl(inKeywordTypeguard.ts, 96, 1))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 98, 34))
+>a : Symbol(a, Decl(inKeywordTypeguard.ts, 98, 38))
+>b : Symbol(b, Decl(inKeywordTypeguard.ts, 98, 54))
+
+    if ("a" in x) {
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 98, 34))
+
+        let s: string = x.a;
+>s : Symbol(s, Decl(inKeywordTypeguard.ts, 100, 11))
+>x.a : Symbol(a, Decl(inKeywordTypeguard.ts, 98, 38))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 98, 34))
+>a : Symbol(a, Decl(inKeywordTypeguard.ts, 98, 38))
+
+    } else {
+        let n: never = x;
+>n : Symbol(n, Decl(inKeywordTypeguard.ts, 102, 11))
+>x : Symbol(x, Decl(inKeywordTypeguard.ts, 98, 34))
+    }
+}
+

tests/baselines/reference/inKeywordTypeguard.types

@@ -297,3 +297,28 @@ class UnreachableCodeDetection {
         }
     }
 }
+
+function positiveIntersectionTest(x: { a: string } & { b: string }) {
+>positiveIntersectionTest : (x: { a: string; } & { b: string; }) => void
+>x : { a: string; } & { b: string; }
+>a : string
+>b : string
+
+    if ("a" in x) {
+>"a" in x : boolean
+>"a" : "a"
+>x : { a: string; } & { b: string; }
+
+        let s: string = x.a;
+>s : string
+>x.a : string
+>x : { a: string; } & { b: string; }
+>a : string
+
+    } else {
+        let n: never = x;
+>n : never
+>x : never
+    }
+}
+

tests/cases/compiler/inKeywordTypeguard.ts

@@ -94,4 +94,12 @@ class UnreachableCodeDetection {
             let y = this.a;
         }
     }
-}
+}
+
+function positiveIntersectionTest(x: { a: string } & { b: string }) {
+    if ("a" in x) {
+        let s: string = x.a;
+    } else {
+        let n: never = x;
+    }
+}