Fix #24991: Weaken narrowing for ==

src/compiler/checker.ts

@@ -14168,6 +14168,32 @@ namespace ts {
             return strictNullChecks ? getGlobalNonNullableTypeInstantiation(type) : type;
         }
 
+
+        /**
+         * Is source potentially coercible to target type under `==`.
+         * Assumes that `source` is a constituent of a union, hence
+         * the boolean literal flag on the LHS, but not on the RHS.
+         *
+         * This does not fully replicate the semantics of `==`. The
+         * intention is to catch cases that are clearly not right.
+         *
+         * Comparing (string | number) to number should not remove the
+         * string element.
+         *
+         * Comparing (string | number) to 1 will remove the string
+         * element, though this is not sound. This is a pragmatic
+         * choice.
+         *
+         * @see narrowTypeByEquality
+         *
+         * @param source
+         * @param target
+         */
+        function isCoercibleUnderDoubleEquals(source: Type, target: Type): boolean {
+            return ((source.flags & (TypeFlags.Number | TypeFlags.String | TypeFlags.BooleanLiteral)) !== 0)
+                && ((target.flags & (TypeFlags.Number | TypeFlags.String | TypeFlags.Boolean)) !== 0);
+        }
+
         /**
          * Return true if type was inferred from an object literal, written as an object type literal, or is the shape of a module
          * with no call or construct signatures.
@@ -16511,7 +16537,10 @@ namespace ts {
                     return type;
                 }
                 if (assumeTrue) {
-                    const narrowedType = filterType(type, t => areTypesComparable(t, valueType));
+                    const filterFn: (t: Type) => boolean = operator === SyntaxKind.EqualsEqualsToken ?
+                        (t => areTypesComparable(t, valueType) || isCoercibleUnderDoubleEquals(t, valueType)) :
+                        t => areTypesComparable(t, valueType);
+                    const narrowedType = filterType(type, filterFn);
                     return narrowedType.flags & TypeFlags.Never ? type : replacePrimitivesWithLiterals(narrowedType, valueType);
                 }
                 if (isUnitType(valueType)) {

tests/baselines/reference/narrowByEquality.errors.txt

@@ -0,0 +1,71 @@
+tests/cases/compiler/narrowByEquality.ts(53,15): error TS2322: Type 'string | number' is not assignable to type 'number'.
+  Type 'string' is not assignable to type 'number'.
+tests/cases/compiler/narrowByEquality.ts(54,9): error TS2322: Type 'string | number' is not assignable to type 'number'.
+  Type 'string' is not assignable to type 'number'.
+
+
+==== tests/cases/compiler/narrowByEquality.ts (2 errors) ====
+    declare let x: number | string | boolean
+    declare let n: number;
+    declare let s: string;
+    declare let b: boolean;
+
+    if (x == n) {
+        x;
+    }
+
+    if (x == s) {
+        x;
+    }
+
+    if (x == b) {
+        x;
+    }
+
+    if (x == 1) {
+        x;
+    }
+
+    if (x == "") {
+        x;
+    }
+
+    if (x == "foo") {
+        x;
+    }
+
+    if (x == true) {
+        x;
+    }
+
+    if (x == false) {
+        x;
+    }
+
+    declare let xAndObj: number | string | boolean | object
+
+    if (xAndObj == {}) {
+        xAndObj;
+    }
+
+    if (x == xAndObj) {
+        x;
+        xAndObj;
+    }
+
+    // Repro from #24991
+
+    function test(level: number | string):number {
+        if (level == +level) {
+            const q2: number = level; // error
+                  ~~
+!!! error TS2322: Type 'string | number' is not assignable to type 'number'.
+!!! error TS2322:   Type 'string' is not assignable to type 'number'.
+            return level;
+            ~~~~~~~~~~~~~
+!!! error TS2322: Type 'string | number' is not assignable to type 'number'.
+!!! error TS2322:   Type 'string' is not assignable to type 'number'.
+        }
+        return 0;
+    }
+

tests/baselines/reference/narrowByEquality.js

@@ -0,0 +1,101 @@
+//// [narrowByEquality.ts]
+declare let x: number | string | boolean
+declare let n: number;
+declare let s: string;
+declare let b: boolean;
+
+if (x == n) {
+    x;
+}
+
+if (x == s) {
+    x;
+}
+
+if (x == b) {
+    x;
+}
+
+if (x == 1) {
+    x;
+}
+
+if (x == "") {
+    x;
+}
+
+if (x == "foo") {
+    x;
+}
+
+if (x == true) {
+    x;
+}
+
+if (x == false) {
+    x;
+}
+
+declare let xAndObj: number | string | boolean | object
+
+if (xAndObj == {}) {
+    xAndObj;
+}
+
+if (x == xAndObj) {
+    x;
+    xAndObj;
+}
+
+// Repro from #24991
+
+function test(level: number | string):number {
+    if (level == +level) {
+        const q2: number = level; // error
+        return level;
+    }
+    return 0;
+}
+
+
+//// [narrowByEquality.js]
+"use strict";
+if (x == n) {
+    x;
+}
+if (x == s) {
+    x;
+}
+if (x == b) {
+    x;
+}
+if (x == 1) {
+    x;
+}
+if (x == "") {
+    x;
+}
+if (x == "foo") {
+    x;
+}
+if (x == true) {
+    x;
+}
+if (x == false) {
+    x;
+}
+if (xAndObj == {}) {
+    xAndObj;
+}
+if (x == xAndObj) {
+    x;
+    xAndObj;
+}
+// Repro from #24991
+function test(level) {
+    if (level == +level) {
+        var q2 = level; // error
+        return level;
+    }
+    return 0;
+}

tests/baselines/reference/narrowByEquality.symbols

@@ -0,0 +1,113 @@
+=== tests/cases/compiler/narrowByEquality.ts ===
+declare let x: number | string | boolean
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+
+declare let n: number;
+>n : Symbol(n, Decl(narrowByEquality.ts, 1, 11))
+
+declare let s: string;
+>s : Symbol(s, Decl(narrowByEquality.ts, 2, 11))
+
+declare let b: boolean;
+>b : Symbol(b, Decl(narrowByEquality.ts, 3, 11))
+
+if (x == n) {
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+>n : Symbol(n, Decl(narrowByEquality.ts, 1, 11))
+
+    x;
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+}
+
+if (x == s) {
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+>s : Symbol(s, Decl(narrowByEquality.ts, 2, 11))
+
+    x;
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+}
+
+if (x == b) {
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+>b : Symbol(b, Decl(narrowByEquality.ts, 3, 11))
+
+    x;
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+}
+
+if (x == 1) {
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+
+    x;
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+}
+
+if (x == "") {
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+
+    x;
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+}
+
+if (x == "foo") {
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+
+    x;
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+}
+
+if (x == true) {
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+
+    x;
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+}
+
+if (x == false) {
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+
+    x;
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+}
+
+declare let xAndObj: number | string | boolean | object
+>xAndObj : Symbol(xAndObj, Decl(narrowByEquality.ts, 37, 11))
+
+if (xAndObj == {}) {
+>xAndObj : Symbol(xAndObj, Decl(narrowByEquality.ts, 37, 11))
+
+    xAndObj;
+>xAndObj : Symbol(xAndObj, Decl(narrowByEquality.ts, 37, 11))
+}
+
+if (x == xAndObj) {
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+>xAndObj : Symbol(xAndObj, Decl(narrowByEquality.ts, 37, 11))
+
+    x;
+>x : Symbol(x, Decl(narrowByEquality.ts, 0, 11))
+
+    xAndObj;
+>xAndObj : Symbol(xAndObj, Decl(narrowByEquality.ts, 37, 11))
+}
+
+// Repro from #24991
+
+function test(level: number | string):number {
+>test : Symbol(test, Decl(narrowByEquality.ts, 46, 1))
+>level : Symbol(level, Decl(narrowByEquality.ts, 50, 14))
+
+    if (level == +level) {
+>level : Symbol(level, Decl(narrowByEquality.ts, 50, 14))
+>level : Symbol(level, Decl(narrowByEquality.ts, 50, 14))
+
+        const q2: number = level; // error
+>q2 : Symbol(q2, Decl(narrowByEquality.ts, 52, 13))
+>level : Symbol(level, Decl(narrowByEquality.ts, 50, 14))
+
+        return level;
+>level : Symbol(level, Decl(narrowByEquality.ts, 50, 14))
+    }
+    return 0;
+}
+