Fix checking of recursive callback types

src/compiler/checker.ts

@@ -8579,16 +8579,16 @@ namespace ts {
             for (let i = 0; i < checkCount; i++) {
                 const sourceType = i < sourceMax ? getTypeOfParameter(sourceParams[i]) : getRestTypeOfSignature(source);
                 const targetType = i < targetMax ? getTypeOfParameter(targetParams[i]) : getRestTypeOfSignature(target);
-                const sourceSig = getSingleCallSignature(getNonNullableType(sourceType));
-                const targetSig = getSingleCallSignature(getNonNullableType(targetType));
                 // In order to ensure that any generic type Foo<T> is at least co-variant with respect to T no matter
                 // how Foo uses T, we need to relate parameters bi-variantly (given that parameters are input positions,
                 // they naturally relate only contra-variantly). However, if the source and target parameters both have
-                // function types with a single call signature, we known we are relating two callback parameters. In
+                // function types with a single call signature, we know we are relating two callback parameters. In
                 // that case it is sufficient to only relate the parameters of the signatures co-variantly because,
                 // similar to return values, callback parameters are output positions. This means that a Promise<T>,
                 // where T is used only in callback parameter positions, will be co-variant (as opposed to bi-variant)
                 // with respect to T.
+                const sourceSig = callbackCheck ? undefined : getSingleCallSignature(getNonNullableType(sourceType));
+                const targetSig = callbackCheck ? undefined : getSingleCallSignature(getNonNullableType(targetType));
                 const callbacks = sourceSig && targetSig && !sourceSig.typePredicate && !targetSig.typePredicate &&
                     (getFalsyFlags(sourceType) & TypeFlags.Nullable) === (getFalsyFlags(targetType) & TypeFlags.Nullable);
                 const related = callbacks ?

tests/baselines/reference/mutuallyRecursiveCallbacks.errors.txt

@@ -0,0 +1,24 @@
+tests/cases/compiler/mutuallyRecursiveCallbacks.ts(7,1): error TS2322: Type '<T>(bar: Bar<T>) => void' is not assignable to type 'Bar<{}>'.
+  Types of parameters 'bar' and 'foo' are incompatible.
+    Types of parameters 'bar' and 'foo' are incompatible.
+      Type 'Foo<{}>' is not assignable to type 'Bar<{}>'.
+        Types of parameters 'bar' and 'foo' are incompatible.
+          Type 'void' is not assignable to type 'Foo<{}>'.
+
+
+==== tests/cases/compiler/mutuallyRecursiveCallbacks.ts (1 errors) ====
+    // Repro from #18277
+
+    interface Foo<T> { (bar: Bar<T>): void };
+    type Bar<T> = (foo: Foo<T>) => Foo<T>;
+    declare function foo<T>(bar: Bar<T>): void;
+    declare var bar: Bar<{}>;
+    bar = foo;
+    ~~~
+!!! error TS2322: Type '<T>(bar: Bar<T>) => void' is not assignable to type 'Bar<{}>'.
+!!! error TS2322:   Types of parameters 'bar' and 'foo' are incompatible.
+!!! error TS2322:     Types of parameters 'bar' and 'foo' are incompatible.
+!!! error TS2322:       Type 'Foo<{}>' is not assignable to type 'Bar<{}>'.
+!!! error TS2322:         Types of parameters 'bar' and 'foo' are incompatible.
+!!! error TS2322:           Type 'void' is not assignable to type 'Foo<{}>'.
+

tests/baselines/reference/mutuallyRecursiveCallbacks.js

@@ -0,0 +1,14 @@
+//// [mutuallyRecursiveCallbacks.ts]
+// Repro from #18277
+
+interface Foo<T> { (bar: Bar<T>): void };
+type Bar<T> = (foo: Foo<T>) => Foo<T>;
+declare function foo<T>(bar: Bar<T>): void;
+declare var bar: Bar<{}>;
+bar = foo;
+
+
+//// [mutuallyRecursiveCallbacks.js]
+// Repro from #18277
+;
+bar = foo;

tests/baselines/reference/mutuallyRecursiveCallbacks.symbols

@@ -0,0 +1,34 @@
+=== tests/cases/compiler/mutuallyRecursiveCallbacks.ts ===
+// Repro from #18277
+
+interface Foo<T> { (bar: Bar<T>): void };
+>Foo : Symbol(Foo, Decl(mutuallyRecursiveCallbacks.ts, 0, 0))
+>T : Symbol(T, Decl(mutuallyRecursiveCallbacks.ts, 2, 14))
+>bar : Symbol(bar, Decl(mutuallyRecursiveCallbacks.ts, 2, 20))
+>Bar : Symbol(Bar, Decl(mutuallyRecursiveCallbacks.ts, 2, 41))
+>T : Symbol(T, Decl(mutuallyRecursiveCallbacks.ts, 2, 14))
+
+type Bar<T> = (foo: Foo<T>) => Foo<T>;
+>Bar : Symbol(Bar, Decl(mutuallyRecursiveCallbacks.ts, 2, 41))
+>T : Symbol(T, Decl(mutuallyRecursiveCallbacks.ts, 3, 9))
+>foo : Symbol(foo, Decl(mutuallyRecursiveCallbacks.ts, 3, 15))
+>Foo : Symbol(Foo, Decl(mutuallyRecursiveCallbacks.ts, 0, 0))
+>T : Symbol(T, Decl(mutuallyRecursiveCallbacks.ts, 3, 9))
+>Foo : Symbol(Foo, Decl(mutuallyRecursiveCallbacks.ts, 0, 0))
+>T : Symbol(T, Decl(mutuallyRecursiveCallbacks.ts, 3, 9))
+
+declare function foo<T>(bar: Bar<T>): void;
+>foo : Symbol(foo, Decl(mutuallyRecursiveCallbacks.ts, 3, 38))
+>T : Symbol(T, Decl(mutuallyRecursiveCallbacks.ts, 4, 21))
+>bar : Symbol(bar, Decl(mutuallyRecursiveCallbacks.ts, 4, 24))
+>Bar : Symbol(Bar, Decl(mutuallyRecursiveCallbacks.ts, 2, 41))
+>T : Symbol(T, Decl(mutuallyRecursiveCallbacks.ts, 4, 21))
+
+declare var bar: Bar<{}>;
+>bar : Symbol(bar, Decl(mutuallyRecursiveCallbacks.ts, 5, 11))
+>Bar : Symbol(Bar, Decl(mutuallyRecursiveCallbacks.ts, 2, 41))
+
+bar = foo;
+>bar : Symbol(bar, Decl(mutuallyRecursiveCallbacks.ts, 5, 11))
+>foo : Symbol(foo, Decl(mutuallyRecursiveCallbacks.ts, 3, 38))
+

tests/baselines/reference/mutuallyRecursiveCallbacks.types

@@ -0,0 +1,35 @@
+=== tests/cases/compiler/mutuallyRecursiveCallbacks.ts ===
+// Repro from #18277
+
+interface Foo<T> { (bar: Bar<T>): void };
+>Foo : Foo<T>
+>T : T
+>bar : Bar<T>
+>Bar : Bar<T>
+>T : T
+
+type Bar<T> = (foo: Foo<T>) => Foo<T>;
+>Bar : Bar<T>
+>T : T
+>foo : Foo<T>
+>Foo : Foo<T>
+>T : T
+>Foo : Foo<T>
+>T : T
+
+declare function foo<T>(bar: Bar<T>): void;
+>foo : <T>(bar: Bar<T>) => void
+>T : T
+>bar : Bar<T>
+>Bar : Bar<T>
+>T : T
+
+declare var bar: Bar<{}>;
+>bar : Bar<{}>
+>Bar : Bar<T>
+
+bar = foo;
+>bar = foo : <T>(bar: Bar<T>) => void
+>bar : Bar<{}>
+>foo : <T>(bar: Bar<T>) => void
+

tests/cases/compiler/mutuallyRecursiveCallbacks.ts

@@ -0,0 +1,7 @@
+// Repro from #18277
+
+interface Foo<T> { (bar: Bar<T>): void };
+type Bar<T> = (foo: Foo<T>) => Foo<T>;
+declare function foo<T>(bar: Bar<T>): void;
+declare var bar: Bar<{}>;
+bar = foo;