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Allow weak type detection for intersection sources (#26668)
Previously, intersections were only allowed as targets, but this was just an artifact of the original implementation, which operated inside the structural part of isRelatedTo. Removing this restriction catches subtle bugs in React user code, where a function named `create` returns a mapped type whose types are all branded numbers. The display of these properties, for some original type `T`, is not `number & { __ }` but the much-less-obvious `RegisteredStyle<T>`.
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‎src/compiler/checker.ts‎

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@@ -11178,7 +11178,7 @@ namespace ts {
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}
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if (relation !== comparableRelation &&
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!(source.flags & TypeFlags.UnionOrIntersection) &&
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!(source.flags & TypeFlags.Union) &&
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!(target.flags & TypeFlags.Union) &&
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!isIntersectionConstituent &&
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source !== globalObjectType &&

‎tests/baselines/reference/intersectionAsWeakTypeSource.errors.txt‎

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tests/cases/conformance/types/intersection/intersectionAsWeakTypeSource.ts(8,7): error TS2559: Type 'XY' has no properties in common with type 'Z'.
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tests/cases/conformance/types/intersection/intersectionAsWeakTypeSource.ts(18,7): error TS2322: Type 'Brand<{ view: number; styleMedia: string; }>' is not assignable to type 'ViewStyle'.
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Property 'view' is missing in type 'Number & { __brand: { view: number; styleMedia: string; }; }'.
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==== tests/cases/conformance/types/intersection/intersectionAsWeakTypeSource.ts (2 errors) ====
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interface X { x: string }
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interface Y { y: number }
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interface Z { z?: boolean }
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type XY = X & Y;
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const xy: XY = {x: 'x', y: 10};
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const z1: Z = xy; // error, {xy} doesn't overlap with {z}
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~~
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!!! error TS2559: Type 'XY' has no properties in common with type 'Z'.
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interface ViewStyle {
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view: number
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styleMedia: string
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}
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type Brand<T> = number & { __brand: T }
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declare function create<T extends { [s: string]: ViewStyle }>(styles: T): { [P in keyof T]: Brand<T[P]> };
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const wrapped = create({ first: { view: 0, styleMedia: "???" } });
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const vs: ViewStyle = wrapped.first // error, first is a branded number
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~~
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!!! error TS2322: Type 'Brand<{ view: number; styleMedia: string; }>' is not assignable to type 'ViewStyle'.
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!!! error TS2322: Property 'view' is missing in type 'Number & { __brand: { view: number; styleMedia: string; }; }'.
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‎tests/baselines/reference/intersectionAsWeakTypeSource.js‎

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//// [intersectionAsWeakTypeSource.ts]
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interface X { x: string }
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interface Y { y: number }
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interface Z { z?: boolean }
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type XY = X & Y;
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const xy: XY = {x: 'x', y: 10};
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const z1: Z = xy; // error, {xy} doesn't overlap with {z}
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interface ViewStyle {
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view: number
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styleMedia: string
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}
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type Brand<T> = number & { __brand: T }
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declare function create<T extends { [s: string]: ViewStyle }>(styles: T): { [P in keyof T]: Brand<T[P]> };
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const wrapped = create({ first: { view: 0, styleMedia: "???" } });
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const vs: ViewStyle = wrapped.first // error, first is a branded number
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//// [intersectionAsWeakTypeSource.js]
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var xy = { x: 'x', y: 10 };
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var z1 = xy; // error, {xy} doesn't overlap with {z}
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var wrapped = create({ first: { view: 0, styleMedia: "???" } });
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var vs = wrapped.first; // error, first is a branded number

‎tests/baselines/reference/intersectionAsWeakTypeSource.symbols‎

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=== tests/cases/conformance/types/intersection/intersectionAsWeakTypeSource.ts ===
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interface X { x: string }
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>X : Symbol(X, Decl(intersectionAsWeakTypeSource.ts, 0, 0))
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>x : Symbol(X.x, Decl(intersectionAsWeakTypeSource.ts, 0, 13))
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interface Y { y: number }
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>Y : Symbol(Y, Decl(intersectionAsWeakTypeSource.ts, 0, 25))
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>y : Symbol(Y.y, Decl(intersectionAsWeakTypeSource.ts, 1, 13))
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interface Z { z?: boolean }
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>Z : Symbol(Z, Decl(intersectionAsWeakTypeSource.ts, 1, 25))
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>z : Symbol(Z.z, Decl(intersectionAsWeakTypeSource.ts, 2, 13))
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type XY = X & Y;
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>XY : Symbol(XY, Decl(intersectionAsWeakTypeSource.ts, 2, 27))
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>X : Symbol(X, Decl(intersectionAsWeakTypeSource.ts, 0, 0))
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>Y : Symbol(Y, Decl(intersectionAsWeakTypeSource.ts, 0, 25))
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const xy: XY = {x: 'x', y: 10};
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>xy : Symbol(xy, Decl(intersectionAsWeakTypeSource.ts, 5, 5))
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>XY : Symbol(XY, Decl(intersectionAsWeakTypeSource.ts, 2, 27))
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>x : Symbol(x, Decl(intersectionAsWeakTypeSource.ts, 5, 16))
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>y : Symbol(y, Decl(intersectionAsWeakTypeSource.ts, 5, 23))
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const z1: Z = xy; // error, {xy} doesn't overlap with {z}
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>z1 : Symbol(z1, Decl(intersectionAsWeakTypeSource.ts, 7, 5))
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>Z : Symbol(Z, Decl(intersectionAsWeakTypeSource.ts, 1, 25))
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>xy : Symbol(xy, Decl(intersectionAsWeakTypeSource.ts, 5, 5))
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interface ViewStyle {
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>ViewStyle : Symbol(ViewStyle, Decl(intersectionAsWeakTypeSource.ts, 7, 17))
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view: number
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>view : Symbol(ViewStyle.view, Decl(intersectionAsWeakTypeSource.ts, 10, 21))
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styleMedia: string
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>styleMedia : Symbol(ViewStyle.styleMedia, Decl(intersectionAsWeakTypeSource.ts, 11, 16))
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}
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type Brand<T> = number & { __brand: T }
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>Brand : Symbol(Brand, Decl(intersectionAsWeakTypeSource.ts, 13, 1))
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>T : Symbol(T, Decl(intersectionAsWeakTypeSource.ts, 14, 11))
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>__brand : Symbol(__brand, Decl(intersectionAsWeakTypeSource.ts, 14, 26))
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>T : Symbol(T, Decl(intersectionAsWeakTypeSource.ts, 14, 11))
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declare function create<T extends { [s: string]: ViewStyle }>(styles: T): { [P in keyof T]: Brand<T[P]> };
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>create : Symbol(create, Decl(intersectionAsWeakTypeSource.ts, 14, 39))
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>T : Symbol(T, Decl(intersectionAsWeakTypeSource.ts, 15, 24))
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>s : Symbol(s, Decl(intersectionAsWeakTypeSource.ts, 15, 37))
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>ViewStyle : Symbol(ViewStyle, Decl(intersectionAsWeakTypeSource.ts, 7, 17))
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>styles : Symbol(styles, Decl(intersectionAsWeakTypeSource.ts, 15, 62))
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>T : Symbol(T, Decl(intersectionAsWeakTypeSource.ts, 15, 24))
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>P : Symbol(P, Decl(intersectionAsWeakTypeSource.ts, 15, 77))
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>T : Symbol(T, Decl(intersectionAsWeakTypeSource.ts, 15, 24))
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>Brand : Symbol(Brand, Decl(intersectionAsWeakTypeSource.ts, 13, 1))
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>T : Symbol(T, Decl(intersectionAsWeakTypeSource.ts, 15, 24))
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>P : Symbol(P, Decl(intersectionAsWeakTypeSource.ts, 15, 77))
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const wrapped = create({ first: { view: 0, styleMedia: "???" } });
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>wrapped : Symbol(wrapped, Decl(intersectionAsWeakTypeSource.ts, 16, 5))
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>create : Symbol(create, Decl(intersectionAsWeakTypeSource.ts, 14, 39))
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>first : Symbol(first, Decl(intersectionAsWeakTypeSource.ts, 16, 24))
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>view : Symbol(view, Decl(intersectionAsWeakTypeSource.ts, 16, 33))
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>styleMedia : Symbol(styleMedia, Decl(intersectionAsWeakTypeSource.ts, 16, 42))
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const vs: ViewStyle = wrapped.first // error, first is a branded number
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>vs : Symbol(vs, Decl(intersectionAsWeakTypeSource.ts, 17, 5))
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>ViewStyle : Symbol(ViewStyle, Decl(intersectionAsWeakTypeSource.ts, 7, 17))
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>wrapped.first : Symbol(first, Decl(intersectionAsWeakTypeSource.ts, 16, 24))
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>wrapped : Symbol(wrapped, Decl(intersectionAsWeakTypeSource.ts, 16, 5))
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>first : Symbol(first, Decl(intersectionAsWeakTypeSource.ts, 16, 24))
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‎tests/baselines/reference/intersectionAsWeakTypeSource.types‎

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=== tests/cases/conformance/types/intersection/intersectionAsWeakTypeSource.ts ===
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interface X { x: string }
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>x : string
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interface Y { y: number }
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>y : number
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interface Z { z?: boolean }
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>z : boolean
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type XY = X & Y;
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>XY : XY
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const xy: XY = {x: 'x', y: 10};
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>xy : XY
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>{x: 'x', y: 10} : { x: string; y: number; }
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>x : string
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>'x' : "x"
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>y : number
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>10 : 10
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const z1: Z = xy; // error, {xy} doesn't overlap with {z}
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>z1 : Z
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>xy : XY
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interface ViewStyle {
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view: number
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>view : number
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styleMedia: string
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>styleMedia : string
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}
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type Brand<T> = number & { __brand: T }
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>Brand : Brand<T>
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>__brand : T
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declare function create<T extends { [s: string]: ViewStyle }>(styles: T): { [P in keyof T]: Brand<T[P]> };
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>create : <T extends { [s: string]: ViewStyle; }>(styles: T) => { [P in keyof T]: Brand<T[P]>; }
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>s : string
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>styles : T
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const wrapped = create({ first: { view: 0, styleMedia: "???" } });
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>wrapped : { first: Brand<{ view: number; styleMedia: string; }>; }
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>create({ first: { view: 0, styleMedia: "???" } }) : { first: Brand<{ view: number; styleMedia: string; }>; }
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>create : <T extends { [s: string]: ViewStyle; }>(styles: T) => { [P in keyof T]: Brand<T[P]>; }
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>{ first: { view: 0, styleMedia: "???" } } : { first: { view: number; styleMedia: string; }; }
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>first : { view: number; styleMedia: string; }
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>{ view: 0, styleMedia: "???" } : { view: number; styleMedia: string; }
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>view : number
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>0 : 0
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>styleMedia : string
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>"???" : "???"
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const vs: ViewStyle = wrapped.first // error, first is a branded number
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>vs : ViewStyle
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>wrapped.first : Brand<{ view: number; styleMedia: string; }>
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>wrapped : { first: Brand<{ view: number; styleMedia: string; }>; }
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>first : Brand<{ view: number; styleMedia: string; }>
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‎tests/cases/conformance/types/intersection/intersectionAsWeakTypeSource.ts‎

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interface X { x: string }
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interface Y { y: number }
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interface Z { z?: boolean }
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type XY = X & Y;
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const xy: XY = {x: 'x', y: 10};
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const z1: Z = xy; // error, {xy} doesn't overlap with {z}
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interface ViewStyle {
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view: number
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styleMedia: string
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}
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type Brand<T> = number & { __brand: T }
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declare function create<T extends { [s: string]: ViewStyle }>(styles: T): { [P in keyof T]: Brand<T[P]> };
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const wrapped = create({ first: { view: 0, styleMedia: "???" } });
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const vs: ViewStyle = wrapped.first // error, first is a branded number

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