Allow identical discriminants when discriminating, rather than trying to unify identical union members

Author: weswigham

src/compiler/checker.ts

@@ -12972,26 +12972,6 @@ namespace ts {
             return true;
         }
 
-        function removeStructurallyIdenticalTypes(types: Type[]): void {
-            const len = types.length;
-            if (len === 0 || isSetOfLiteralsFromSameEnum(types)) {
-                return;
-            }
-            let i = len;
-            while (i > 0) {
-                i--;
-                const source = types[i];
-                for (const target of types) {
-                    if (source !== target) {
-                        if (isTypeIdenticalTo(source, target)) {
-                            orderedRemoveItemAt(types, i);
-                            break;
-                        }
-                    }
-                }
-            }
-        }
-
         function removeRedundantLiteralTypes(types: Type[], includes: TypeFlags) {
             let i = types.length;
             while (i > 0) {
@@ -13026,7 +13006,7 @@ namespace ts {
             const typeSet: Type[] = [];
             const includes = addTypesToUnion(typeSet, 0, types);
             if (unionReduction !== UnionReduction.None) {
-                if (includes & TypeFlags.AnyOrUnknown && unionReduction !== UnionReduction.Exact) {
+                if (includes & TypeFlags.AnyOrUnknown) {
                     return includes & TypeFlags.Any ? includes & TypeFlags.IncludesWildcard ? wildcardType : anyType : unknownType;
                 }
                 switch (unionReduction) {
@@ -13040,9 +13020,6 @@ namespace ts {
                             return errorType;
                         }
                         break;
-                    case UnionReduction.Exact:
-                        removeStructurallyIdenticalTypes(typeSet);
-                        break;
                 }
                 if (typeSet.length === 0) {
                     return includes & TypeFlags.Null ? includes & TypeFlags.IncludesNonWideningType ? nullType : nullWideningType :
@@ -18266,11 +18243,6 @@ namespace ts {
         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) {
-            const reduced = getUnionType(target.types, UnionReduction.Exact, target.aliasSymbol, target.aliasTypeArguments);
-            if (!(reduced.flags & TypeFlags.Union)) {
-                return reduced;
-            }
-            target = reduced as UnionType;
             // 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)[];
@@ -18291,9 +18263,19 @@ namespace ts {
                     i++;
                 }
             }
-            const match = discriminable.indexOf(/*searchElement*/ true);
+            let match = discriminable.indexOf(/*searchElement*/ true);
+            if (match === -1) {
+                return defaultValue;
+            }
             // make sure exactly 1 matches before returning it
-            return match === -1 || discriminable.indexOf(/*searchElement*/ true, match + 1) !== -1 ? defaultValue : target.types[match];
+            let nextMatch = discriminable.indexOf(/*searchElement*/ true, match + 1);
+            while (nextMatch !== -1) {
+                if (!isTypeIdenticalTo(target.types[match], target.types[nextMatch])) {
+                    return defaultValue;
+                }
+                nextMatch = discriminable.indexOf(/*searchElement*/ true, nextMatch + 1);
+            }
+            return target.types[match];
         }
 
         /**

src/compiler/types.ts

@@ -4185,7 +4185,6 @@ namespace ts {
         None = 0,
         Literal,
         Subtype,
-        Exact,
     }
 
     /* @internal */

tests/baselines/reference/contextualTypeWithUnionTypeObjectLiteral.errors.txt

@@ -23,10 +23,10 @@ tests/cases/conformance/types/union/contextualTypeWithUnionTypeObjectLiteral.ts(
     Types of property 'prop' are incompatible.
       Type 'string | number' is not assignable to type 'number'.
         Type 'string' is not assignable to type 'number'.
-tests/cases/conformance/types/union/contextualTypeWithUnionTypeObjectLiteral.ts(58,5): error TS2322: Type '(a: string, b: number) => string | number' is not assignable to type '((a: string, b: number) => number) | ((a: string, b: number) => string)'.
-  Type '(a: string, b: number) => string | number' is not assignable to type '(a: string, b: number) => number'.
-    Type 'string | number' is not assignable to type 'number'.
-      Type 'string' is not assignable to type 'number'.
+tests/cases/conformance/types/union/contextualTypeWithUnionTypeObjectLiteral.ts(58,5): error TS2322: Type '(a: string, b: number) => string | number' is not assignable to type '((a: string, b: number) => string) | ((a: string, b: number) => number)'.
+  Type '(a: string, b: number) => string | number' is not assignable to type '(a: string, b: number) => string'.
+    Type 'string | number' is not assignable to type 'string'.
+      Type 'number' is not assignable to type 'string'.
 
 
 ==== tests/cases/conformance/types/union/contextualTypeWithUnionTypeObjectLiteral.ts (6 errors) ====
@@ -119,9 +119,9 @@ tests/cases/conformance/types/union/contextualTypeWithUnionTypeObjectLiteral.ts(
     var i11Ori21: I11 | I21 = { // Like i1 and i2 both
         commonMethodDifferentReturnType: (a, b) => strOrNumber,
         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-!!! error TS2322: Type '(a: string, b: number) => string | number' is not assignable to type '((a: string, b: number) => number) | ((a: string, b: number) => string)'.
-!!! error TS2322:   Type '(a: string, b: number) => string | number' is not assignable to type '(a: string, b: number) => number'.
-!!! error TS2322:     Type 'string | number' is not assignable to type 'number'.
-!!! error TS2322:       Type 'string' is not assignable to type 'number'.
+!!! error TS2322: Type '(a: string, b: number) => string | number' is not assignable to type '((a: string, b: number) => string) | ((a: string, b: number) => number)'.
+!!! error TS2322:   Type '(a: string, b: number) => string | number' is not assignable to type '(a: string, b: number) => string'.
+!!! error TS2322:     Type 'string | number' is not assignable to type 'string'.
+!!! error TS2322:       Type 'number' is not assignable to type 'string'.
 !!! related TS6500 tests/cases/conformance/types/union/contextualTypeWithUnionTypeObjectLiteral.ts:35:5: The expected type comes from property 'commonMethodDifferentReturnType' which is declared here on type 'I11 | I21'
     };