Treat callback parameters as strictly covariant

Author: ahejlsberg

src/compiler/checker.ts

@@ -8157,7 +8157,8 @@ namespace ts {
         function isSignatureAssignableTo(source: Signature,
             target: Signature,
             ignoreReturnTypes: boolean): boolean {
-            return compareSignaturesRelated(source, target, ignoreReturnTypes, /*reportErrors*/ false, /*errorReporter*/ undefined, compareTypesAssignable) !== Ternary.False;
+            return compareSignaturesRelated(source, target, /*strictVariance*/ false, ignoreReturnTypes, /*reportErrors*/ false,
+                /*errorReporter*/ undefined, compareTypesAssignable) !== Ternary.False;
         }
 
         type ErrorReporter = (message: DiagnosticMessage, arg0?: string, arg1?: string) => void;
@@ -8167,6 +8168,7 @@ namespace ts {
          */
         function compareSignaturesRelated(source: Signature,
             target: Signature,
+            strictVariance: boolean,
             ignoreReturnTypes: boolean,
             reportErrors: boolean,
             errorReporter: ErrorReporter,
@@ -8209,9 +8211,17 @@ namespace ts {
             const sourceParams = source.parameters;
             const targetParams = target.parameters;
             for (let i = 0; i < checkCount; i++) {
-                const s = i < sourceMax ? getTypeOfParameter(sourceParams[i]) : getRestTypeOfSignature(source);
-                const t = i < targetMax ? getTypeOfParameter(targetParams[i]) : getRestTypeOfSignature(target);
-                const related = compareTypes(s, t, /*reportErrors*/ false) || compareTypes(t, s, reportErrors);
+                const sourceType = i < sourceMax ? getTypeOfParameter(sourceParams[i]) : getRestTypeOfSignature(source);
+                const targetType = i < targetMax ? getTypeOfParameter(targetParams[i]) : getRestTypeOfSignature(target);
+                const sourceSig = getSingleCallSignature(sourceType);
+                const targetSig = getSingleCallSignature(targetType);
+                // If the source and target parameters both have function types with a single call signature we are
+                // relating two callback parameters. In that case we compare the callback signatures with strict
+                // variance, meaning we require the callback parameters to be pairwise co-variant (because, similar
+                // to return values, callback parameters are output positions).
+                const related = sourceSig && targetSig ?
+                    compareSignaturesRelated(targetSig, sourceSig, /*strictVariance*/ true, /*ignoreReturnTypes*/ false, reportErrors, errorReporter, compareTypes) :
+                    !strictVariance && compareTypes(sourceType, targetType, /*reportErrors*/ false) || compareTypes(targetType, sourceType, reportErrors);
                 if (!related) {
                     if (reportErrors) {
                         errorReporter(Diagnostics.Types_of_parameters_0_and_1_are_incompatible,
@@ -9210,7 +9220,7 @@ namespace ts {
              * See signatureAssignableTo, compareSignaturesIdentical
              */
             function signatureRelatedTo(source: Signature, target: Signature, reportErrors: boolean): Ternary {
-                return compareSignaturesRelated(source, target, /*ignoreReturnTypes*/ false, reportErrors, reportError, isRelatedTo);
+                return compareSignaturesRelated(source, target, /*strictVariance*/ false, /*ignoreReturnTypes*/ false, reportErrors, reportError, isRelatedTo);
             }
 
             function signaturesIdenticalTo(source: Type, target: Type, kind: SignatureKind): Ternary {