Fix definitely assignable relation

src/compiler/checker.ts

@@ -12254,7 +12254,8 @@ namespace ts {
                 else if (target.flags & TypeFlags.IndexedAccess) {
                     // A type S is related to a type T[K], where T and K aren't both type variables, if S is related to C,
                     // where C is the base constraint of T[K]
-                    if (relation !== identityRelation && !(isGenericObjectType((<IndexedAccessType>target).objectType) && isGenericIndexType((<IndexedAccessType>target).indexType))) {
+                    if (relation !== identityRelation && relation !== definitelyAssignableRelation &&
+                        !(isGenericObjectType((<IndexedAccessType>target).objectType) && isGenericIndexType((<IndexedAccessType>target).indexType))) {
                         const constraint = getBaseConstraintOfType(target);
                         if (constraint && constraint !== target) {
                             if (result = isRelatedTo(source, constraint, reportErrors)) {

tests/baselines/reference/conditionalTypes2.errors.txt

@@ -203,4 +203,13 @@ tests/cases/conformance/types/conditional/conditionalTypes2.ts(75,12): error TS2
 
     type C2<T, V, E> =
         T extends object ? { [Q in keyof T]: C2<T[Q], V, E>; } : T;
+
+    // Repro from #28654
+
+    type MaybeTrue<T extends { b: boolean }> = true extends T["b"] ? "yes" : "no";
+
+    type T0 = MaybeTrue<{ b: never }>     // "no"
+    type T1 = MaybeTrue<{ b: false }>;    // "no"
+    type T2 = MaybeTrue<{ b: true }>;     // "yes"
+    type T3 = MaybeTrue<{ b: boolean }>;  // "yes"
 

tests/baselines/reference/conditionalTypes2.js

@@ -145,6 +145,15 @@ type B2<T, V> =
 
 type C2<T, V, E> =
     T extends object ? { [Q in keyof T]: C2<T[Q], V, E>; } : T;
+
+// Repro from #28654
+
+type MaybeTrue<T extends { b: boolean }> = true extends T["b"] ? "yes" : "no";
+
+type T0 = MaybeTrue<{ b: never }>     // "no"
+type T1 = MaybeTrue<{ b: false }>;    // "no"
+type T2 = MaybeTrue<{ b: true }>;     // "yes"
+type T3 = MaybeTrue<{ b: boolean }>;  // "yes"
 
 
 //// [conditionalTypes2.js]
@@ -304,3 +313,18 @@ declare type B2<T, V> = T extends object ? T extends any[] ? T : {
 declare type C2<T, V, E> = T extends object ? {
     [Q in keyof T]: C2<T[Q], V, E>;
 } : T;
+declare type MaybeTrue<T extends {
+    b: boolean;
+}> = true extends T["b"] ? "yes" : "no";
+declare type T0 = MaybeTrue<{
+    b: never;
+}>;
+declare type T1 = MaybeTrue<{
+    b: false;
+}>;
+declare type T2 = MaybeTrue<{
+    b: true;
+}>;
+declare type T3 = MaybeTrue<{
+    b: boolean;
+}>;

tests/baselines/reference/conditionalTypes2.symbols

@@ -551,3 +551,31 @@ type C2<T, V, E> =
 >E : Symbol(E, Decl(conditionalTypes2.ts, 144, 13))
 >T : Symbol(T, Decl(conditionalTypes2.ts, 144, 8))
 
+// Repro from #28654
+
+type MaybeTrue<T extends { b: boolean }> = true extends T["b"] ? "yes" : "no";
+>MaybeTrue : Symbol(MaybeTrue, Decl(conditionalTypes2.ts, 145, 63))
+>T : Symbol(T, Decl(conditionalTypes2.ts, 149, 15))
+>b : Symbol(b, Decl(conditionalTypes2.ts, 149, 26))
+>T : Symbol(T, Decl(conditionalTypes2.ts, 149, 15))
+
+type T0 = MaybeTrue<{ b: never }>     // "no"
+>T0 : Symbol(T0, Decl(conditionalTypes2.ts, 149, 78))
+>MaybeTrue : Symbol(MaybeTrue, Decl(conditionalTypes2.ts, 145, 63))
+>b : Symbol(b, Decl(conditionalTypes2.ts, 151, 21))
+
+type T1 = MaybeTrue<{ b: false }>;    // "no"
+>T1 : Symbol(T1, Decl(conditionalTypes2.ts, 151, 33))
+>MaybeTrue : Symbol(MaybeTrue, Decl(conditionalTypes2.ts, 145, 63))
+>b : Symbol(b, Decl(conditionalTypes2.ts, 152, 21))
+
+type T2 = MaybeTrue<{ b: true }>;     // "yes"
+>T2 : Symbol(T2, Decl(conditionalTypes2.ts, 152, 34))
+>MaybeTrue : Symbol(MaybeTrue, Decl(conditionalTypes2.ts, 145, 63))
+>b : Symbol(b, Decl(conditionalTypes2.ts, 153, 21))
+
+type T3 = MaybeTrue<{ b: boolean }>;  // "yes"
+>T3 : Symbol(T3, Decl(conditionalTypes2.ts, 153, 33))
+>MaybeTrue : Symbol(MaybeTrue, Decl(conditionalTypes2.ts, 145, 63))
+>b : Symbol(b, Decl(conditionalTypes2.ts, 154, 21))
+

tests/baselines/reference/conditionalTypes2.types

@@ -341,3 +341,28 @@ type C2<T, V, E> =
 
     T extends object ? { [Q in keyof T]: C2<T[Q], V, E>; } : T;
 
+// Repro from #28654
+
+type MaybeTrue<T extends { b: boolean }> = true extends T["b"] ? "yes" : "no";
+>MaybeTrue : MaybeTrue<T>
+>b : boolean
+>true : true
+
+type T0 = MaybeTrue<{ b: never }>     // "no"
+>T0 : "no"
+>b : never
+
+type T1 = MaybeTrue<{ b: false }>;    // "no"
+>T1 : "no"
+>b : false
+>false : false
+
+type T2 = MaybeTrue<{ b: true }>;     // "yes"
+>T2 : "yes"
+>b : true
+>true : true
+
+type T3 = MaybeTrue<{ b: boolean }>;  // "yes"
+>T3 : "yes"
+>b : boolean
+

tests/cases/conformance/types/conditional/conditionalTypes2.ts

@@ -147,3 +147,12 @@ type B2<T, V> =
 
 type C2<T, V, E> =
     T extends object ? { [Q in keyof T]: C2<T[Q], V, E>; } : T;
+
+// Repro from #28654
+
+type MaybeTrue<T extends { b: boolean }> = true extends T["b"] ? "yes" : "no";
+
+type T0 = MaybeTrue<{ b: never }>     // "no"
+type T1 = MaybeTrue<{ b: false }>;    // "no"
+type T2 = MaybeTrue<{ b: true }>;     // "yes"
+type T3 = MaybeTrue<{ b: boolean }>;  // "yes"