Don't allow generic narrowing when contextually typed by a binding pattern

src/compiler/checker.ts

@@ -24124,12 +24124,12 @@ namespace ts {
             return !!(type.flags & TypeFlags.Instantiable || type.flags & TypeFlags.UnionOrIntersection && some((<UnionOrIntersectionType>type).types, containsGenericType));
         }
 
-        function hasContextualTypeWithNoGenericTypes(node: Node) {
+        function hasNonBindingPatternContextualTypeWithNoGenericTypes(node: Node) {
             // Computing the contextual type for a child of a JSX element involves resolving the type of the
             // element's tag name, so we exclude that here to avoid circularities.
             const contextualType = (isIdentifier(node) || isPropertyAccessExpression(node) || isElementAccessExpression(node)) &&
                 !((isJsxOpeningElement(node.parent) || isJsxSelfClosingElement(node.parent)) && node.parent.tagName === node) &&
-                getContextualType(node);
+                getContextualType(node, ContextFlags.SkipBindingPatterns);
             return contextualType && !someType(contextualType, containsGenericType);
         }
 
@@ -24143,7 +24143,7 @@ namespace ts {
             // 'string | undefined' to give control flow analysis the opportunity to narrow to type 'string'.
             const substituteConstraints = !(checkMode && checkMode & CheckMode.Inferential) &&
                 someType(type, isGenericTypeWithUnionConstraint) &&
-                (isConstraintPosition(reference) || hasContextualTypeWithNoGenericTypes(reference));
+                (isConstraintPosition(reference) || hasNonBindingPatternContextualTypeWithNoGenericTypes(reference));
             return substituteConstraints ? mapType(type, t => t.flags & TypeFlags.Instantiable ? getBaseConstraintOrType(t) : t) : type;
         }
 

tests/baselines/reference/genericObjectSpreadResultInSwitch.js

@@ -0,0 +1,67 @@
+//// [genericObjectSpreadResultInSwitch.ts]
+type Params = {
+    foo: string;
+} & ({ tag: 'a'; type: number } | { tag: 'b'; type: string });
+
+const getType = <P extends Params>(params: P) => {
+    const {
+        // Omit
+        foo,
+
+        ...rest
+    } = params;
+
+    return rest;
+};
+
+declare const params: Params;
+
+switch (params.tag) {
+    case 'a': {
+        // TS 4.2: number
+        // TS 4.3: string | number
+        const result = getType(params).type;
+
+        break;
+    }
+    case 'b': {
+        // TS 4.2: string
+        // TS 4.3: string | number
+        const result = getType(params).type;
+
+        break;
+    }
+}
+
+//// [genericObjectSpreadResultInSwitch.js]
+var __rest = (this && this.__rest) || function (s, e) {
+    var t = {};
+    for (var p in s) if (Object.prototype.hasOwnProperty.call(s, p) && e.indexOf(p) < 0)
+        t[p] = s[p];
+    if (s != null && typeof Object.getOwnPropertySymbols === "function")
+        for (var i = 0, p = Object.getOwnPropertySymbols(s); i < p.length; i++) {
+            if (e.indexOf(p[i]) < 0 && Object.prototype.propertyIsEnumerable.call(s, p[i]))
+                t[p[i]] = s[p[i]];
+        }
+    return t;
+};
+var getType = function (params) {
+    var 
+    // Omit
+    foo = params.foo, rest = __rest(params, ["foo"]);
+    return rest;
+};
+switch (params.tag) {
+    case 'a': {
+        // TS 4.2: number
+        // TS 4.3: string | number
+        var result = getType(params).type;
+        break;
+    }
+    case 'b': {
+        // TS 4.2: string
+        // TS 4.3: string | number
+        var result = getType(params).type;
+        break;
+    }
+}

tests/baselines/reference/genericObjectSpreadResultInSwitch.symbols

@@ -0,0 +1,70 @@
+=== tests/cases/compiler/genericObjectSpreadResultInSwitch.ts ===
+type Params = {
+>Params : Symbol(Params, Decl(genericObjectSpreadResultInSwitch.ts, 0, 0))
+
+    foo: string;
+>foo : Symbol(foo, Decl(genericObjectSpreadResultInSwitch.ts, 0, 15))
+
+} & ({ tag: 'a'; type: number } | { tag: 'b'; type: string });
+>tag : Symbol(tag, Decl(genericObjectSpreadResultInSwitch.ts, 2, 6))
+>type : Symbol(type, Decl(genericObjectSpreadResultInSwitch.ts, 2, 16))
+>tag : Symbol(tag, Decl(genericObjectSpreadResultInSwitch.ts, 2, 35))
+>type : Symbol(type, Decl(genericObjectSpreadResultInSwitch.ts, 2, 45))
+
+const getType = <P extends Params>(params: P) => {
+>getType : Symbol(getType, Decl(genericObjectSpreadResultInSwitch.ts, 4, 5))
+>P : Symbol(P, Decl(genericObjectSpreadResultInSwitch.ts, 4, 17))
+>Params : Symbol(Params, Decl(genericObjectSpreadResultInSwitch.ts, 0, 0))
+>params : Symbol(params, Decl(genericObjectSpreadResultInSwitch.ts, 4, 35))
+>P : Symbol(P, Decl(genericObjectSpreadResultInSwitch.ts, 4, 17))
+
+    const {
+        // Omit
+        foo,
+>foo : Symbol(foo, Decl(genericObjectSpreadResultInSwitch.ts, 5, 11))
+
+        ...rest
+>rest : Symbol(rest, Decl(genericObjectSpreadResultInSwitch.ts, 7, 12))
+
+    } = params;
+>params : Symbol(params, Decl(genericObjectSpreadResultInSwitch.ts, 4, 35))
+
+    return rest;
+>rest : Symbol(rest, Decl(genericObjectSpreadResultInSwitch.ts, 7, 12))
+
+};
+
+declare const params: Params;
+>params : Symbol(params, Decl(genericObjectSpreadResultInSwitch.ts, 15, 13))
+>Params : Symbol(Params, Decl(genericObjectSpreadResultInSwitch.ts, 0, 0))
+
+switch (params.tag) {
+>params.tag : Symbol(tag, Decl(genericObjectSpreadResultInSwitch.ts, 2, 6), Decl(genericObjectSpreadResultInSwitch.ts, 2, 35))
+>params : Symbol(params, Decl(genericObjectSpreadResultInSwitch.ts, 15, 13))
+>tag : Symbol(tag, Decl(genericObjectSpreadResultInSwitch.ts, 2, 6), Decl(genericObjectSpreadResultInSwitch.ts, 2, 35))
+
+    case 'a': {
+        // TS 4.2: number
+        // TS 4.3: string | number
+        const result = getType(params).type;
+>result : Symbol(result, Decl(genericObjectSpreadResultInSwitch.ts, 21, 13))
+>getType(params).type : Symbol(type, Decl(genericObjectSpreadResultInSwitch.ts, 2, 16))
+>getType : Symbol(getType, Decl(genericObjectSpreadResultInSwitch.ts, 4, 5))
+>params : Symbol(params, Decl(genericObjectSpreadResultInSwitch.ts, 15, 13))
+>type : Symbol(type, Decl(genericObjectSpreadResultInSwitch.ts, 2, 16))
+
+        break;
+    }
+    case 'b': {
+        // TS 4.2: string
+        // TS 4.3: string | number
+        const result = getType(params).type;
+>result : Symbol(result, Decl(genericObjectSpreadResultInSwitch.ts, 28, 13))
+>getType(params).type : Symbol(type, Decl(genericObjectSpreadResultInSwitch.ts, 2, 45))
+>getType : Symbol(getType, Decl(genericObjectSpreadResultInSwitch.ts, 4, 5))
+>params : Symbol(params, Decl(genericObjectSpreadResultInSwitch.ts, 15, 13))
+>type : Symbol(type, Decl(genericObjectSpreadResultInSwitch.ts, 2, 45))
+
+        break;
+    }
+}

tests/baselines/reference/genericObjectSpreadResultInSwitch.types

@@ -0,0 +1,73 @@
+=== tests/cases/compiler/genericObjectSpreadResultInSwitch.ts ===
+type Params = {
+>Params : Params
+
+    foo: string;
+>foo : string
+
+} & ({ tag: 'a'; type: number } | { tag: 'b'; type: string });
+>tag : "a"
+>type : number
+>tag : "b"
+>type : string
+
+const getType = <P extends Params>(params: P) => {
+>getType : <P extends Params>(params: P) => Omit<P, "foo">
+><P extends Params>(params: P) => {    const {        // Omit        foo,        ...rest    } = params;    return rest;} : <P extends Params>(params: P) => Omit<P, "foo">
+>params : P
+
+    const {
+        // Omit
+        foo,
+>foo : string
+
+        ...rest
+>rest : Omit<P, "foo">
+
+    } = params;
+>params : P
+
+    return rest;
+>rest : Omit<P, "foo">
+
+};
+
+declare const params: Params;
+>params : Params
+
+switch (params.tag) {
+>params.tag : "a" | "b"
+>params : Params
+>tag : "a" | "b"
+
+    case 'a': {
+>'a' : "a"
+
+        // TS 4.2: number
+        // TS 4.3: string | number
+        const result = getType(params).type;
+>result : number
+>getType(params).type : number
+>getType(params) : Omit<{ foo: string; } & { tag: "a"; type: number; }, "foo">
+>getType : <P extends Params>(params: P) => Omit<P, "foo">
+>params : { foo: string; } & { tag: "a"; type: number; }
+>type : number
+
+        break;
+    }
+    case 'b': {
+>'b' : "b"
+
+        // TS 4.2: string
+        // TS 4.3: string | number
+        const result = getType(params).type;
+>result : string
+>getType(params).type : string
+>getType(params) : Omit<{ foo: string; } & { tag: "b"; type: string; }, "foo">
+>getType : <P extends Params>(params: P) => Omit<P, "foo">
+>params : { foo: string; } & { tag: "b"; type: string; }
+>type : string
+
+        break;
+    }
+}

tests/baselines/reference/restInvalidArgumentType.types

@@ -84,7 +84,7 @@ function f<T extends { b: string }>(p1: T, p2: T[]) {
 
     var {...r5} = k;   // Error, index
 >r5 : any
->k : string | number | symbol
+>k : keyof T
 
     var {...r6} = mapped_generic; // Error, generic mapped object type
 >r6 : { [P in keyof T]: T[P]; }

tests/cases/compiler/genericObjectSpreadResultInSwitch.ts

@@ -0,0 +1,33 @@
+type Params = {
+    foo: string;
+} & ({ tag: 'a'; type: number } | { tag: 'b'; type: string });
+
+const getType = <P extends Params>(params: P) => {
+    const {
+        // Omit
+        foo,
+
+        ...rest
+    } = params;
+
+    return rest;
+};
+
+declare const params: Params;
+
+switch (params.tag) {
+    case 'a': {
+        // TS 4.2: number
+        // TS 4.3: string | number
+        const result = getType(params).type;
+
+        break;
+    }
+    case 'b': {
+        // TS 4.2: string
+        // TS 4.3: string | number
+        const result = getType(params).type;
+
+        break;
+    }
+}