Fix for relating covered discriminants in unions

src/compiler/checker.ts

@@ -17273,7 +17273,11 @@ namespace ts {
                             result &= signaturesRelatedTo(source, type, SignatureKind.Construct, /*reportStructuralErrors*/ false);
                             if (result) {
                                 result &= indexTypesRelatedTo(source, type, IndexKind.String, /*sourceIsPrimitive*/ false, /*reportStructuralErrors*/ false, IntersectionState.None);
-                                if (result) {
+                                // Comparing numeric index types when both `source` and `type` are tuples is unnecessary as the
+                                // element types should be sufficiently covered by `propertiesRelatedTo`. It also causes problems
+                                // with index type assignability as the types for the excluded discriminants are still included
+                                // in the index type.
+                                if (result && !(isTupleType(source) && isTupleType(type))) {
                                     result &= indexTypesRelatedTo(source, type, IndexKind.Number, /*sourceIsPrimitive*/ false, /*reportStructuralErrors*/ false, IntersectionState.None);
                                 }
                             }
@@ -17498,6 +17502,7 @@ namespace ts {
                         for (let i = 0; i < maxArity; i++) {
                             const targetFlags = i < targetArity ? target.target.elementFlags[i] : targetRestFlag;
                             const sourceFlags = isTupleType(source) && i < sourceArity ? source.target.elementFlags[i] : sourceRestFlag;
+                            let canExcludeDiscriminants = !!excludedProperties;
                             if (sourceFlags && targetFlags) {
                                 if (targetFlags & ElementFlags.Variadic && !(sourceFlags & ElementFlags.Variadic) ||
                                    (sourceFlags & ElementFlags.Variadic && !(targetFlags & ElementFlags.Variable))) {
@@ -17514,6 +17519,15 @@ namespace ts {
                                         return Ternary.False;
                                     }
                                 }
+                                // We can only exclude discriminant properties if we have not yet encountered a variable-length element.
+                                if (canExcludeDiscriminants) {
+                                    if (sourceFlags & ElementFlags.Variable || targetFlags & ElementFlags.Variable) {
+                                        canExcludeDiscriminants = false;
+                                    }
+                                    if (canExcludeDiscriminants && excludedProperties?.has(("" + i) as __String)) {
+                                        continue;
+                                    }
+                                }
                                 const sourceType = getTypeArguments(source)[Math.min(i, sourceArity - 1)];
                                 const targetType = getTypeArguments(target)[Math.min(i, targetArity - 1)];
                                 const targetCheckType = sourceFlags & ElementFlags.Variadic && targetFlags & ElementFlags.Rest ? createArrayType(targetType) : targetType;

tests/baselines/reference/assignmentCompatWithDiscriminatedUnion.errors.txt

@@ -221,4 +221,13 @@ tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignme
                 type: obj1.type
             };
         }
-    }
+    }
+
+    // https://github.com/microsoft/TypeScript/issues/39357
+    namespace GH39357 {
+        type A = ["a", number] | ["b", number] | ["c", string];
+        type B = "a" | "b" | "c";
+        declare const b: B;
+        const a: A = b === "a" || b === "b" ? [b, 1] : ["c", ""];
+    }
+

tests/baselines/reference/assignmentCompatWithDiscriminatedUnion.js

@@ -191,7 +191,16 @@ namespace GH20889 {
             type: obj1.type
         };
     }
-}
+}
+
+// https://github.com/microsoft/TypeScript/issues/39357
+namespace GH39357 {
+    type A = ["a", number] | ["b", number] | ["c", string];
+    type B = "a" | "b" | "c";
+    declare const b: B;
+    const a: A = b === "a" || b === "b" ? [b, 1] : ["c", ""];
+}
+
 
 //// [assignmentCompatWithDiscriminatedUnion.js]
 // see 'typeRelatedToDiscriminatedType' in checker.ts:
@@ -289,3 +298,8 @@ var GH20889;
         };
     }
 })(GH20889 || (GH20889 = {}));
+// https://github.com/microsoft/TypeScript/issues/39357
+var GH39357;
+(function (GH39357) {
+    var a = b === "a" || b === "b" ? [b, 1] : ["c", ""];
+})(GH39357 || (GH39357 = {}));

tests/baselines/reference/assignmentCompatWithDiscriminatedUnion.symbols

@@ -492,3 +492,26 @@ namespace GH20889 {
         };
     }
 }
+
+// https://github.com/microsoft/TypeScript/issues/39357
+namespace GH39357 {
+>GH39357 : Symbol(GH39357, Decl(assignmentCompatWithDiscriminatedUnion.ts, 192, 1))
+
+    type A = ["a", number] | ["b", number] | ["c", string];
+>A : Symbol(A, Decl(assignmentCompatWithDiscriminatedUnion.ts, 195, 19))
+
+    type B = "a" | "b" | "c";
+>B : Symbol(B, Decl(assignmentCompatWithDiscriminatedUnion.ts, 196, 59))
+
+    declare const b: B;
+>b : Symbol(b, Decl(assignmentCompatWithDiscriminatedUnion.ts, 198, 17))
+>B : Symbol(B, Decl(assignmentCompatWithDiscriminatedUnion.ts, 196, 59))
+
+    const a: A = b === "a" || b === "b" ? [b, 1] : ["c", ""];
+>a : Symbol(a, Decl(assignmentCompatWithDiscriminatedUnion.ts, 199, 9))
+>A : Symbol(A, Decl(assignmentCompatWithDiscriminatedUnion.ts, 195, 19))
+>b : Symbol(b, Decl(assignmentCompatWithDiscriminatedUnion.ts, 198, 17))
+>b : Symbol(b, Decl(assignmentCompatWithDiscriminatedUnion.ts, 198, 17))
+>b : Symbol(b, Decl(assignmentCompatWithDiscriminatedUnion.ts, 198, 17))
+}
+

tests/baselines/reference/assignmentCompatWithDiscriminatedUnion.types

@@ -464,3 +464,35 @@ namespace GH20889 {
         };
     }
 }
+
+// https://github.com/microsoft/TypeScript/issues/39357
+namespace GH39357 {
+>GH39357 : typeof GH39357
+
+    type A = ["a", number] | ["b", number] | ["c", string];
+>A : A
+
+    type B = "a" | "b" | "c";
+>B : "a" | "b" | "c"
+
+    declare const b: B;
+>b : "a" | "b" | "c"
+
+    const a: A = b === "a" || b === "b" ? [b, 1] : ["c", ""];
+>a : A
+>b === "a" || b === "b" ? [b, 1] : ["c", ""] : ["a" | "b", number] | ["c", string]
+>b === "a" || b === "b" : boolean
+>b === "a" : boolean
+>b : "a" | "b" | "c"
+>"a" : "a"
+>b === "b" : boolean
+>b : "b" | "c"
+>"b" : "b"
+>[b, 1] : ["a" | "b", number]
+>b : "a" | "b"
+>1 : 1
+>["c", ""] : ["c", string]
+>"c" : "c"
+>"" : ""
+}
+

tests/cases/conformance/types/typeRelationships/assignmentCompatibility/assignmentCompatWithDiscriminatedUnion.ts

@@ -190,4 +190,12 @@ namespace GH20889 {
             type: obj1.type
         };
     }
-}
+}
+
+// https://github.com/microsoft/TypeScript/issues/39357
+namespace GH39357 {
+    type A = ["a", number] | ["b", number] | ["c", string];
+    type B = "a" | "b" | "c";
+    declare const b: B;
+    const a: A = b === "a" || b === "b" ? [b, 1] : ["c", ""];
+}