Disallow partial matches for discriminant properties when generating error messages

src/compiler/checker.ts

@@ -16882,20 +16882,24 @@ namespace ts {
         }
 
         function getBestMatchingType(source: Type, target: UnionOrIntersectionType, isRelatedTo = compareTypesAssignable) {
-            return findMatchingDiscriminantType(source, target, isRelatedTo) ||
+            return findMatchingDiscriminantType(source, target, isRelatedTo, /*skipPartial*/ true) ||
                 findMatchingTypeReferenceOrTypeAliasReference(source, target) ||
                 findBestTypeForObjectLiteral(source, target) ||
                 findBestTypeForInvokable(source, target) ||
                 findMostOverlappyType(source, target);
         }
 
-        function discriminateTypeByDiscriminableItems(target: UnionType, discriminators: [() => Type, __String][], related: (source: Type, target: Type) => boolean | Ternary): Type | undefined;
-        function discriminateTypeByDiscriminableItems(target: UnionType, discriminators: [() => Type, __String][], related: (source: Type, target: Type) => boolean | Ternary, defaultValue: Type): Type;
-        function discriminateTypeByDiscriminableItems(target: UnionType, discriminators: [() => Type, __String][], related: (source: Type, target: Type) => boolean | Ternary, defaultValue?: Type) {
+        function discriminateTypeByDiscriminableItems(target: UnionType, discriminators: [() => Type, __String][], related: (source: Type, target: Type) => boolean | Ternary, defaultValue?: undefined, skipPartial?: boolean): Type | undefined;
+        function discriminateTypeByDiscriminableItems(target: UnionType, discriminators: [() => Type, __String][], related: (source: Type, target: Type) => boolean | Ternary, defaultValue: Type, skipPartial?: boolean): Type;
+        function discriminateTypeByDiscriminableItems(target: UnionType, discriminators: [() => Type, __String][], related: (source: Type, target: Type) => boolean | Ternary, defaultValue?: Type, skipPartial?: boolean) {
             // undefined=unknown, true=discriminated, false=not discriminated
             // The state of each type progresses from left to right. Discriminated types stop at 'true'.
             const discriminable = target.types.map(_ => undefined) as (boolean | undefined)[];
             for (const [getDiscriminatingType, propertyName] of discriminators) {
+                const targetProp = getUnionOrIntersectionProperty(target, propertyName);
+                if (skipPartial && targetProp && getCheckFlags(targetProp) & CheckFlags.ReadPartial) {
+                    continue;
+                }
                 let i = 0;
                 for (const type of target.types) {
                     const targetType = getTypeOfPropertyOfType(type, propertyName);
@@ -37852,13 +37856,13 @@ namespace ts {
         }
 
         // Keep this up-to-date with the same logic within `getApparentTypeOfContextualType`, since they should behave similarly
-        function findMatchingDiscriminantType(source: Type, target: Type, isRelatedTo: (source: Type, target: Type) => Ternary) {
+        function findMatchingDiscriminantType(source: Type, target: Type, isRelatedTo: (source: Type, target: Type) => Ternary, skipPartial?: boolean) {
             if (target.flags & TypeFlags.Union && source.flags & (TypeFlags.Intersection | TypeFlags.Object)) {
                 const sourceProperties = getPropertiesOfType(source);
                 if (sourceProperties) {
                     const sourcePropertiesFiltered = findDiscriminantProperties(sourceProperties, target);
                     if (sourcePropertiesFiltered) {
-                        return discriminateTypeByDiscriminableItems(<UnionType>target, map(sourcePropertiesFiltered, p => ([() => getTypeOfSymbol(p), p.escapedName] as [() => Type, __String])), isRelatedTo);
+                        return discriminateTypeByDiscriminableItems(<UnionType>target, map(sourcePropertiesFiltered, p => ([() => getTypeOfSymbol(p), p.escapedName] as [() => Type, __String])), isRelatedTo, /*defaultValue*/ undefined, skipPartial);
                     }
                 }
             }

tests/baselines/reference/unionErrorMessageOnMatchingDiscriminant.errors.txt

@@ -0,0 +1,28 @@
+tests/cases/compiler/unionErrorMessageOnMatchingDiscriminant.ts(21,5): error TS2322: Type 'null' is not assignable to type '{ a: string; }'.
+
+
+==== tests/cases/compiler/unionErrorMessageOnMatchingDiscriminant.ts (1 errors) ====
+    type A = {
+        type: 'a',
+        data: { a: string }
+    };
+
+    type B = {
+        type: 'b',
+        data: null
+    };
+
+    type C = {
+        type: 'c',
+        payload: string
+    };
+
+    type Union = A | B | C;
+
+    // error
+    const foo: Union = {
+        type: 'a',
+        data: null
+        ~~~~
+!!! error TS2322: Type 'null' is not assignable to type '{ a: string; }'.
+    };

tests/baselines/reference/unionErrorMessageOnMatchingDiscriminant.js

@@ -0,0 +1,31 @@
+//// [unionErrorMessageOnMatchingDiscriminant.ts]
+type A = {
+    type: 'a',
+    data: { a: string }
+};
+
+type B = {
+    type: 'b',
+    data: null
+};
+
+type C = {
+    type: 'c',
+    payload: string
+};
+
+type Union = A | B | C;
+
+// error
+const foo: Union = {
+    type: 'a',
+    data: null
+};
+
+//// [unionErrorMessageOnMatchingDiscriminant.js]
+"use strict";
+// error
+var foo = {
+    type: 'a',
+    data: null
+};

tests/baselines/reference/unionErrorMessageOnMatchingDiscriminant.symbols

@@ -0,0 +1,53 @@
+=== tests/cases/compiler/unionErrorMessageOnMatchingDiscriminant.ts ===
+type A = {
+>A : Symbol(A, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 0, 0))
+
+    type: 'a',
+>type : Symbol(type, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 0, 10))
+
+    data: { a: string }
+>data : Symbol(data, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 1, 14))
+>a : Symbol(a, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 2, 11))
+
+};
+
+type B = {
+>B : Symbol(B, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 3, 2))
+
+    type: 'b',
+>type : Symbol(type, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 5, 10))
+
+    data: null
+>data : Symbol(data, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 6, 14))
+
+};
+
+type C = {
+>C : Symbol(C, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 8, 2))
+
+    type: 'c',
+>type : Symbol(type, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 10, 10))
+
+    payload: string
+>payload : Symbol(payload, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 11, 14))
+
+};
+
+type Union = A | B | C;
+>Union : Symbol(Union, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 13, 2))
+>A : Symbol(A, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 0, 0))
+>B : Symbol(B, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 3, 2))
+>C : Symbol(C, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 8, 2))
+
+// error
+const foo: Union = {
+>foo : Symbol(foo, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 18, 5))
+>Union : Symbol(Union, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 13, 2))
+
+    type: 'a',
+>type : Symbol(type, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 18, 20))
+
+    data: null
+>data : Symbol(data, Decl(unionErrorMessageOnMatchingDiscriminant.ts, 19, 14))
+
+};

tests/baselines/reference/unionErrorMessageOnMatchingDiscriminant.types

@@ -0,0 +1,53 @@
+=== tests/cases/compiler/unionErrorMessageOnMatchingDiscriminant.ts ===
+type A = {
+>A : A
+
+    type: 'a',
+>type : "a"
+
+    data: { a: string }
+>data : { a: string; }
+>a : string
+
+};
+
+type B = {
+>B : B
+
+    type: 'b',
+>type : "b"
+
+    data: null
+>data : null
+>null : null
+
+};
+
+type C = {
+>C : C
+
+    type: 'c',
+>type : "c"
+
+    payload: string
+>payload : string
+
+};
+
+type Union = A | B | C;
+>Union : Union
+
+// error
+const foo: Union = {
+>foo : Union
+>{    type: 'a',    data: null} : { type: "a"; data: null; }
+
+    type: 'a',
+>type : "a"
+>'a' : "a"
+
+    data: null
+>data : null
+>null : null
+
+};

tests/cases/compiler/unionErrorMessageOnMatchingDiscriminant.ts

@@ -0,0 +1,23 @@
+// @strict: true
+type A = {
+    type: 'a',
+    data: { a: string }
+};
+
+type B = {
+    type: 'b',
+    data: null
+};
+
+type C = {
+    type: 'c',
+    payload: string
+};
+
+type Union = A | B | C;
+
+// error
+const foo: Union = {
+    type: 'a',
+    data: null
+};