Skip single call signature comparisons in strict variance

src/compiler/checker.ts

@@ -20777,8 +20777,8 @@ export function createTypeChecker(host: TypeCheckerHost): TypeChecker {
                 // 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 = checkMode & SignatureCheckMode.Callback || isInstantiatedGenericParameter(source, i) ? undefined : getSingleCallSignature(getNonNullableType(sourceType));
-                const targetSig = checkMode & SignatureCheckMode.Callback || isInstantiatedGenericParameter(target, i) ? undefined : getSingleCallSignature(getNonNullableType(targetType));
+                const sourceSig = checkMode & SignatureCheckMode.Callback || strictVariance || isInstantiatedGenericParameter(source, i) ? undefined : getSingleCallSignature(getNonNullableType(sourceType));
+                const targetSig = checkMode & SignatureCheckMode.Callback || strictVariance || isInstantiatedGenericParameter(target, i) ? undefined : getSingleCallSignature(getNonNullableType(targetType));
                 const callbacks = sourceSig && targetSig && !getTypePredicateOfSignature(sourceSig) && !getTypePredicateOfSignature(targetSig) &&
                     getTypeFacts(sourceType, TypeFacts.IsUndefinedOrNull) === getTypeFacts(targetType, TypeFacts.IsUndefinedOrNull);
                 let related = callbacks ?

tests/baselines/reference/genericRestTypes.errors.txt

@@ -1,7 +1,8 @@
 genericRestTypes.ts(21,11): error TS2322: Type '(cb: (x: string, ...rest: T) => void) => void' is not assignable to type '(cb: (...args: never) => void) => void'.
   Types of parameters 'cb' and 'cb' are incompatible.
-    Types of parameters 'args' and 'x' are incompatible.
-      Type '[x: string, ...rest: T]' is not assignable to type 'never'.
+    Type '(...args: never) => void' is not assignable to type '(x: string, ...rest: T) => void'.
+      Types of parameters 'args' and 'x' are incompatible.
+        Type '[x: string, ...rest: T]' is not assignable to type 'never'.
 
 
 ==== genericRestTypes.ts (1 errors) ====
@@ -29,8 +30,9 @@ genericRestTypes.ts(21,11): error TS2322: Type '(cb: (x: string, ...rest: T) =>
               ~~~
 !!! error TS2322: Type '(cb: (x: string, ...rest: T) => void) => void' is not assignable to type '(cb: (...args: never) => void) => void'.
 !!! error TS2322:   Types of parameters 'cb' and 'cb' are incompatible.
-!!! error TS2322:     Types of parameters 'args' and 'x' are incompatible.
-!!! error TS2322:       Type '[x: string, ...rest: T]' is not assignable to type 'never'.
+!!! error TS2322:     Type '(...args: never) => void' is not assignable to type '(x: string, ...rest: T) => void'.
+!!! error TS2322:       Types of parameters 'args' and 'x' are incompatible.
+!!! error TS2322:         Type '[x: string, ...rest: T]' is not assignable to type 'never'.
     }
 
     function assignmentWithComplexRest3<T extends any[]>() {

tests/baselines/reference/strictFunctionTypesErrors.errors.txt

@@ -69,23 +69,18 @@ strictFunctionTypesErrors.ts(126,1): error TS2322: Type 'Crate<Dog>' is not assi
 strictFunctionTypesErrors.ts(127,1): error TS2322: Type 'Crate<Animal>' is not assignable to type 'Crate<Dog>'.
   Types of property 'item' are incompatible.
     Type 'Animal' is not assignable to type 'Dog'.
-strictFunctionTypesErrors.ts(133,1): error TS2328: Types of parameters 'f' and 'f' are incompatible.
-  Type 'Animal' is not assignable to type 'Dog'.
-strictFunctionTypesErrors.ts(134,1): error TS2322: Type '(f: (x: Animal) => Animal) => void' is not assignable to type '(f: (x: Dog) => Dog) => void'.
+strictFunctionTypesErrors.ts(133,1): error TS2322: Type '(f: (x: Dog) => Dog) => void' is not assignable to type '(f: (x: Animal) => Animal) => void'.
   Types of parameters 'f' and 'f' are incompatible.
-    Types of parameters 'x' and 'x' are incompatible.
-      Type 'Animal' is not assignable to type 'Dog'.
-strictFunctionTypesErrors.ts(147,5): error TS2322: Type '(cb: (x: Animal) => Animal) => void' is not assignable to type '(cb: (x: Dog) => Animal) => void'.
-  Types of parameters 'cb' and 'cb' are incompatible.
-    Types of parameters 'x' and 'x' are incompatible.
-      Type 'Animal' is not assignable to type 'Dog'.
-strictFunctionTypesErrors.ts(155,5): error TS2322: Type '(cb: (x: Animal) => Animal) => void' is not assignable to type '(cb: (x: Dog) => Animal) => void'.
-  Types of parameters 'cb' and 'cb' are incompatible.
-    Types of parameters 'x' and 'x' are incompatible.
+    Type '(x: Animal) => Animal' is not assignable to type '(x: Dog) => Dog'.
       Type 'Animal' is not assignable to type 'Dog'.
+strictFunctionTypesErrors.ts(134,1): error TS2322: Type '(f: (x: Animal) => Animal) => void' is not assignable to type '(f: (x: Dog) => Dog) => void'.
+  Types of parameters 'f' and 'f' are incompatible.
+    Type '(x: Dog) => Dog' is not assignable to type '(x: Animal) => Animal'.
+      Types of parameters 'x' and 'x' are incompatible.
+        Type 'Animal' is not assignable to type 'Dog'.
 
 
-==== strictFunctionTypesErrors.ts (35 errors) ====
+==== strictFunctionTypesErrors.ts (33 errors) ====
     export {}
 
 
@@ -323,14 +318,17 @@ strictFunctionTypesErrors.ts(155,5): error TS2322: Type '(cb: (x: Animal) => Ani
     declare let fc2: (f: (x: Dog) => Dog) => void;
     fc1 = fc2;  // Error
     ~~~
-!!! error TS2328: Types of parameters 'f' and 'f' are incompatible.
-!!! error TS2328:   Type 'Animal' is not assignable to type 'Dog'.
+!!! error TS2322: Type '(f: (x: Dog) => Dog) => void' is not assignable to type '(f: (x: Animal) => Animal) => void'.
+!!! error TS2322:   Types of parameters 'f' and 'f' are incompatible.
+!!! error TS2322:     Type '(x: Animal) => Animal' is not assignable to type '(x: Dog) => Dog'.
+!!! error TS2322:       Type 'Animal' is not assignable to type 'Dog'.
     fc2 = fc1;  // Error
     ~~~
 !!! error TS2322: Type '(f: (x: Animal) => Animal) => void' is not assignable to type '(f: (x: Dog) => Dog) => void'.
 !!! error TS2322:   Types of parameters 'f' and 'f' are incompatible.
-!!! error TS2322:     Types of parameters 'x' and 'x' are incompatible.
-!!! error TS2322:       Type 'Animal' is not assignable to type 'Dog'.
+!!! error TS2322:     Type '(x: Dog) => Dog' is not assignable to type '(x: Animal) => Animal'.
+!!! error TS2322:       Types of parameters 'x' and 'x' are incompatible.
+!!! error TS2322:         Type 'Animal' is not assignable to type 'Dog'.
 
     // Verify that callback parameters aren't loosely checked when types
     // originate in method declarations
@@ -344,11 +342,6 @@ strictFunctionTypesErrors.ts(155,5): error TS2322: Type '(cb: (x: Animal) => Ani
         declare let f2: (cb: typeof Foo.f2) => void;
         f1 = f2;
         f2 = f1;  // Error
-        ~~
-!!! error TS2322: Type '(cb: (x: Animal) => Animal) => void' is not assignable to type '(cb: (x: Dog) => Animal) => void'.
-!!! error TS2322:   Types of parameters 'cb' and 'cb' are incompatible.
-!!! error TS2322:     Types of parameters 'x' and 'x' are incompatible.
-!!! error TS2322:       Type 'Animal' is not assignable to type 'Dog'.
     }
 
     namespace n2 {
@@ -357,9 +350,4 @@ strictFunctionTypesErrors.ts(155,5): error TS2322: Type '(cb: (x: Animal) => Ani
         declare let f2: (cb: BivariantHack<Dog, Animal>) => void;
         f1 = f2;
         f2 = f1;  // Error
-        ~~
-!!! error TS2322: Type '(cb: (x: Animal) => Animal) => void' is not assignable to type '(cb: (x: Dog) => Animal) => void'.
-!!! error TS2322:   Types of parameters 'cb' and 'cb' are incompatible.
-!!! error TS2322:     Types of parameters 'x' and 'x' are incompatible.
-!!! error TS2322:       Type 'Animal' is not assignable to type 'Dog'.
     }