Improve error messages for discriminated union incompatible members

src/compiler/checker.ts

@@ -23098,6 +23098,41 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 }
             }
             if (reportErrors) {
+                // Check if we're dealing with a discriminated union case where we can provide better error messages
+                if (target.flags & TypeFlags.Union && source.flags & TypeFlags.Object) {
+                    const discriminantProps = findDiscriminantProperties(getPropertiesOfType(source), target);
+                    if (discriminantProps && discriminantProps.length > 0) {
+                        // For each discriminant property, check if the source property type contains values
+                        // not present in any constituent of the target union
+                        for (const prop of discriminantProps) {
+                            const sourcePropType = getTypeOfSymbol(prop);
+                            if (sourcePropType.flags & TypeFlags.Union) {
+                                // Get all discriminant values from the target union for this property
+                                const targetDiscriminantTypes: Type[] = [];
+                                for (const constituent of (target as UnionType).types) {
+                                    const constituentPropType = getTypeOfPropertyOfType(constituent, prop.escapedName);
+                                    if (constituentPropType) {
+                                        forEachType(constituentPropType, t => targetDiscriminantTypes.push(t));
+                                    }
+                                }
+                                const targetDiscriminantUnion = getUnionType(targetDiscriminantTypes);
+                                // Find source types that are not assignable to the target discriminant union
+                                const incompatibleTypes: Type[] = [];
+                                for (const sourceType of (sourcePropType as UnionType).types) {
+                                    if (!isTypeAssignableTo(sourceType, targetDiscriminantUnion)) {
+                                        incompatibleTypes.push(sourceType);
+                                    }
+                                }
+                                if (incompatibleTypes.length > 0) {
+                                    const incompatibleTypesStr = incompatibleTypes.map(t => typeToString(t)).join(" | ");
+                                    const propName = symbolToString(prop);
+                                    reportError(Diagnostics.Type_0_is_not_assignable_to_type_1_The_type_s_2_are_not_present_in_the_discriminator_3_of_the_target, typeToString(source), typeToString(target), incompatibleTypesStr, propName);
+                                    return Ternary.False;
+                                }
+                            }
+                        }
+                    }
+                }
                 // Elaborate only if we can find a best matching type in the target union
                 const bestMatchingType = getBestMatchingType(source, target, isRelatedTo);
                 if (bestMatchingType) {

src/compiler/diagnosticMessages.json

@@ -4023,6 +4023,14 @@
         "category": "Error",
         "code": 2882
     },
+    "Type '{0}' is not assignable to type '{1}'. The types of '{2}' are incompatible between these types.": {
+        "category": "Error",
+        "code": 2883
+    },
+    "Type '{0}' is not assignable to type '{1}'. The type(s) '{2}' are not present in the discriminator '{3}' of the target.": {
+        "category": "Error",
+        "code": 2884
+    },
 
     "Import declaration '{0}' is using private name '{1}'.": {
         "category": "Error",

tests/baselines/reference/discriminatedUnionIncompatibleMemberErrorMessage.errors.txt

@@ -0,0 +1,58 @@
+discriminatedUnionIncompatibleMemberErrorMessage.ts(12,7): error TS2322: Type '{ discriminator: UnionType; }' is not assignable to type 'Discriminated'.
+  Type '{ discriminator: UnionType; }' is not assignable to type 'Discriminated'. The type(s) '"baz"' are not present in the discriminator 'discriminator' of the target.
+discriminatedUnionIncompatibleMemberErrorMessage.ts(17,7): error TS2322: Type '{ discriminator: WiderUnion; }' is not assignable to type 'Discriminated'.
+  Type '{ discriminator: WiderUnion; }' is not assignable to type 'Discriminated'. The type(s) '"baz" | "qux"' are not present in the discriminator 'discriminator' of the target.
+discriminatedUnionIncompatibleMemberErrorMessage.ts(26,7): error TS2322: Type '{ kind: ShapeKind; }' is not assignable to type 'Shape'.
+  Type '{ kind: ShapeKind; }' is not assignable to type 'Shape'. The type(s) '"triangle"' are not present in the discriminator 'kind' of the target.
+discriminatedUnionIncompatibleMemberErrorMessage.ts(33,7): error TS2322: Type '{ discriminator: UnionType; }' is not assignable to type 'Discriminated2'.
+  Type '{ discriminator: UnionType; }' is not assignable to type 'Discriminated2'. The type(s) '"baz"' are not present in the discriminator 'discriminator' of the target.
+
+
+==== discriminatedUnionIncompatibleMemberErrorMessage.ts (4 errors) ====
+    // Test case for issue #62737
+    // Improve error messages when assigning objects with wider discriminator unions to discriminated union types
+
+    // Basic case - single incompatible type
+    type Discriminated =
+      | { discriminator: "foo" }
+      | { discriminator: "bar" };
+
+    type UnionType = "foo" | "bar" | "baz";
+
+    const obj = { discriminator: "foo" as UnionType };
+    const err: Discriminated = obj; // Error: "baz" is not present in discriminator
+          ~~~
+!!! error TS2322: Type '{ discriminator: UnionType; }' is not assignable to type 'Discriminated'.
+!!! error TS2322:   Type '{ discriminator: UnionType; }' is not assignable to type 'Discriminated'. The type(s) '"baz"' are not present in the discriminator 'discriminator' of the target.
+
+    // Multiple incompatible types
+    type WiderUnion = "foo" | "bar" | "baz" | "qux";
+    const obj2 = { discriminator: "foo" as WiderUnion };
+    const err2: Discriminated = obj2; // Error: "baz" | "qux" are not present in discriminator
+          ~~~~
+!!! error TS2322: Type '{ discriminator: WiderUnion; }' is not assignable to type 'Discriminated'.
+!!! error TS2322:   Type '{ discriminator: WiderUnion; }' is not assignable to type 'Discriminated'. The type(s) '"baz" | "qux"' are not present in the discriminator 'discriminator' of the target.
+
+    // Different discriminator property name
+    type Shape =
+      | { kind: "circle"; radius: number }
+      | { kind: "square"; side: number };
+
+    type ShapeKind = "circle" | "square" | "triangle";
+    const shape = { kind: "circle" as ShapeKind };
+    const err3: Shape = shape; // Error: "triangle" is not present in discriminator "kind"
+          ~~~~
+!!! error TS2322: Type '{ kind: ShapeKind; }' is not assignable to type 'Shape'.
+!!! error TS2322:   Type '{ kind: ShapeKind; }' is not assignable to type 'Shape'. The type(s) '"triangle"' are not present in the discriminator 'kind' of the target.
+
+    // From issue #62603
+    type Discriminated2 =
+      | { discriminator: "foo" }
+      | { discriminator: "bar" };
+
+    const unexpectedError: Discriminated2 = { discriminator: "foo" } as { discriminator: UnionType };
+          ~~~~~~~~~~~~~~~
+!!! error TS2322: Type '{ discriminator: UnionType; }' is not assignable to type 'Discriminated2'.
+!!! error TS2322:   Type '{ discriminator: UnionType; }' is not assignable to type 'Discriminated2'. The type(s) '"baz"' are not present in the discriminator 'discriminator' of the target.
+
+

tests/baselines/reference/discriminatedUnionIncompatibleMemberErrorMessage.js

@@ -0,0 +1,50 @@
+//// [tests/cases/compiler/discriminatedUnionIncompatibleMemberErrorMessage.ts] ////
+
+//// [discriminatedUnionIncompatibleMemberErrorMessage.ts]
+// Test case for issue #62737
+// Improve error messages when assigning objects with wider discriminator unions to discriminated union types
+
+// Basic case - single incompatible type
+type Discriminated =
+  | { discriminator: "foo" }
+  | { discriminator: "bar" };
+
+type UnionType = "foo" | "bar" | "baz";
+
+const obj = { discriminator: "foo" as UnionType };
+const err: Discriminated = obj; // Error: "baz" is not present in discriminator
+
+// Multiple incompatible types
+type WiderUnion = "foo" | "bar" | "baz" | "qux";
+const obj2 = { discriminator: "foo" as WiderUnion };
+const err2: Discriminated = obj2; // Error: "baz" | "qux" are not present in discriminator
+
+// Different discriminator property name
+type Shape =
+  | { kind: "circle"; radius: number }
+  | { kind: "square"; side: number };
+
+type ShapeKind = "circle" | "square" | "triangle";
+const shape = { kind: "circle" as ShapeKind };
+const err3: Shape = shape; // Error: "triangle" is not present in discriminator "kind"
+
+// From issue #62603
+type Discriminated2 =
+  | { discriminator: "foo" }
+  | { discriminator: "bar" };
+
+const unexpectedError: Discriminated2 = { discriminator: "foo" } as { discriminator: UnionType };
+
+
+
+//// [discriminatedUnionIncompatibleMemberErrorMessage.js]
+"use strict";
+// Test case for issue #62737
+// Improve error messages when assigning objects with wider discriminator unions to discriminated union types
+var obj = { discriminator: "foo" };
+var err = obj; // Error: "baz" is not present in discriminator
+var obj2 = { discriminator: "foo" };
+var err2 = obj2; // Error: "baz" | "qux" are not present in discriminator
+var shape = { kind: "circle" };
+var err3 = shape; // Error: "triangle" is not present in discriminator "kind"
+var unexpectedError = { discriminator: "foo" };

tests/baselines/reference/discriminatedUnionIncompatibleMemberErrorMessage.symbols

@@ -0,0 +1,86 @@
+//// [tests/cases/compiler/discriminatedUnionIncompatibleMemberErrorMessage.ts] ////
+
+=== discriminatedUnionIncompatibleMemberErrorMessage.ts ===
+// Test case for issue #62737
+// Improve error messages when assigning objects with wider discriminator unions to discriminated union types
+
+// Basic case - single incompatible type
+type Discriminated =
+>Discriminated : Symbol(Discriminated, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 0, 0))
+
+  | { discriminator: "foo" }
+>discriminator : Symbol(discriminator, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 5, 5))
+
+  | { discriminator: "bar" };
+>discriminator : Symbol(discriminator, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 6, 5))
+
+type UnionType = "foo" | "bar" | "baz";
+>UnionType : Symbol(UnionType, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 6, 29))
+
+const obj = { discriminator: "foo" as UnionType };
+>obj : Symbol(obj, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 10, 5))
+>discriminator : Symbol(discriminator, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 10, 13))
+>UnionType : Symbol(UnionType, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 6, 29))
+
+const err: Discriminated = obj; // Error: "baz" is not present in discriminator
+>err : Symbol(err, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 11, 5))
+>Discriminated : Symbol(Discriminated, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 0, 0))
+>obj : Symbol(obj, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 10, 5))
+
+// Multiple incompatible types
+type WiderUnion = "foo" | "bar" | "baz" | "qux";
+>WiderUnion : Symbol(WiderUnion, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 11, 31))
+
+const obj2 = { discriminator: "foo" as WiderUnion };
+>obj2 : Symbol(obj2, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 15, 5))
+>discriminator : Symbol(discriminator, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 15, 14))
+>WiderUnion : Symbol(WiderUnion, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 11, 31))
+
+const err2: Discriminated = obj2; // Error: "baz" | "qux" are not present in discriminator
+>err2 : Symbol(err2, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 16, 5))
+>Discriminated : Symbol(Discriminated, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 0, 0))
+>obj2 : Symbol(obj2, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 15, 5))
+
+// Different discriminator property name
+type Shape =
+>Shape : Symbol(Shape, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 16, 33))
+
+  | { kind: "circle"; radius: number }
+>kind : Symbol(kind, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 20, 5))
+>radius : Symbol(radius, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 20, 21))
+
+  | { kind: "square"; side: number };
+>kind : Symbol(kind, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 21, 5))
+>side : Symbol(side, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 21, 21))
+
+type ShapeKind = "circle" | "square" | "triangle";
+>ShapeKind : Symbol(ShapeKind, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 21, 37))
+
+const shape = { kind: "circle" as ShapeKind };
+>shape : Symbol(shape, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 24, 5))
+>kind : Symbol(kind, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 24, 15))
+>ShapeKind : Symbol(ShapeKind, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 21, 37))
+
+const err3: Shape = shape; // Error: "triangle" is not present in discriminator "kind"
+>err3 : Symbol(err3, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 25, 5))
+>Shape : Symbol(Shape, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 16, 33))
+>shape : Symbol(shape, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 24, 5))
+
+// From issue #62603
+type Discriminated2 =
+>Discriminated2 : Symbol(Discriminated2, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 25, 26))
+
+  | { discriminator: "foo" }
+>discriminator : Symbol(discriminator, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 29, 5))
+
+  | { discriminator: "bar" };
+>discriminator : Symbol(discriminator, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 30, 5))
+
+const unexpectedError: Discriminated2 = { discriminator: "foo" } as { discriminator: UnionType };
+>unexpectedError : Symbol(unexpectedError, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 32, 5))
+>Discriminated2 : Symbol(Discriminated2, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 25, 26))
+>discriminator : Symbol(discriminator, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 32, 41))
+>discriminator : Symbol(discriminator, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 32, 69))
+>UnionType : Symbol(UnionType, Decl(discriminatedUnionIncompatibleMemberErrorMessage.ts, 6, 29))
+
+

tests/baselines/reference/discriminatedUnionIncompatibleMemberErrorMessage.types

@@ -0,0 +1,131 @@
+//// [tests/cases/compiler/discriminatedUnionIncompatibleMemberErrorMessage.ts] ////
+
+=== discriminatedUnionIncompatibleMemberErrorMessage.ts ===
+// Test case for issue #62737
+// Improve error messages when assigning objects with wider discriminator unions to discriminated union types
+
+// Basic case - single incompatible type
+type Discriminated =
+>Discriminated : Discriminated
+>              : ^^^^^^^^^^^^^
+
+  | { discriminator: "foo" }
+>discriminator : "foo"
+>              : ^^^^^
+
+  | { discriminator: "bar" };
+>discriminator : "bar"
+>              : ^^^^^
+
+type UnionType = "foo" | "bar" | "baz";
+>UnionType : UnionType
+>          : ^^^^^^^^^
+
+const obj = { discriminator: "foo" as UnionType };
+>obj : { discriminator: UnionType; }
+>    : ^^^^^^^^^^^^^^^^^         ^^^
+>{ discriminator: "foo" as UnionType } : { discriminator: UnionType; }
+>                                      : ^^^^^^^^^^^^^^^^^         ^^^
+>discriminator : UnionType
+>              : ^^^^^^^^^
+>"foo" as UnionType : UnionType
+>                   : ^^^^^^^^^
+>"foo" : "foo"
+>      : ^^^^^
+
+const err: Discriminated = obj; // Error: "baz" is not present in discriminator
+>err : Discriminated
+>    : ^^^^^^^^^^^^^
+>obj : { discriminator: UnionType; }
+>    : ^^^^^^^^^^^^^^^^^         ^^^
+
+// Multiple incompatible types
+type WiderUnion = "foo" | "bar" | "baz" | "qux";
+>WiderUnion : WiderUnion
+>           : ^^^^^^^^^^
+
+const obj2 = { discriminator: "foo" as WiderUnion };
+>obj2 : { discriminator: WiderUnion; }
+>     : ^^^^^^^^^^^^^^^^^          ^^^
+>{ discriminator: "foo" as WiderUnion } : { discriminator: WiderUnion; }
+>                                       : ^^^^^^^^^^^^^^^^^          ^^^
+>discriminator : WiderUnion
+>              : ^^^^^^^^^^
+>"foo" as WiderUnion : WiderUnion
+>                    : ^^^^^^^^^^
+>"foo" : "foo"
+>      : ^^^^^
+
+const err2: Discriminated = obj2; // Error: "baz" | "qux" are not present in discriminator
+>err2 : Discriminated
+>     : ^^^^^^^^^^^^^
+>obj2 : { discriminator: WiderUnion; }
+>     : ^^^^^^^^^^^^^^^^^          ^^^
+
+// Different discriminator property name
+type Shape =
+>Shape : Shape
+>      : ^^^^^
+
+  | { kind: "circle"; radius: number }
+>kind : "circle"
+>     : ^^^^^^^^
+>radius : number
+>       : ^^^^^^
+
+  | { kind: "square"; side: number };
+>kind : "square"
+>     : ^^^^^^^^
+>side : number
+>     : ^^^^^^
+
+type ShapeKind = "circle" | "square" | "triangle";
+>ShapeKind : ShapeKind
+>          : ^^^^^^^^^
+
+const shape = { kind: "circle" as ShapeKind };
+>shape : { kind: ShapeKind; }
+>      : ^^^^^^^^         ^^^
+>{ kind: "circle" as ShapeKind } : { kind: ShapeKind; }
+>                                : ^^^^^^^^         ^^^
+>kind : ShapeKind
+>     : ^^^^^^^^^
+>"circle" as ShapeKind : ShapeKind
+>                      : ^^^^^^^^^
+>"circle" : "circle"
+>         : ^^^^^^^^
+
+const err3: Shape = shape; // Error: "triangle" is not present in discriminator "kind"
+>err3 : Shape
+>     : ^^^^^
+>shape : { kind: ShapeKind; }
+>      : ^^^^^^^^         ^^^
+
+// From issue #62603
+type Discriminated2 =
+>Discriminated2 : Discriminated2
+>               : ^^^^^^^^^^^^^^
+
+  | { discriminator: "foo" }
+>discriminator : "foo"
+>              : ^^^^^
+
+  | { discriminator: "bar" };
+>discriminator : "bar"
+>              : ^^^^^
+
+const unexpectedError: Discriminated2 = { discriminator: "foo" } as { discriminator: UnionType };
+>unexpectedError : Discriminated2
+>                : ^^^^^^^^^^^^^^
+>{ discriminator: "foo" } as { discriminator: UnionType } : { discriminator: UnionType; }
+>                                                         : ^^^^^^^^^^^^^^^^^         ^^^
+>{ discriminator: "foo" } : { discriminator: "foo"; }
+>                         : ^^^^^^^^^^^^^^^^^^^^^^^^^
+>discriminator : "foo"
+>              : ^^^^^
+>"foo" : "foo"
+>      : ^^^^^
+>discriminator : UnionType
+>              : ^^^^^^^^^
+
+