(x: number | undefined) -> void is subtype of (x?: number | undefined) => void

Author: ahejlsberg

src/compiler/checker.ts

@@ -14023,8 +14023,7 @@ namespace ts {
             }
 
             const targetCount = getParameterCount(target);
-            if (!hasEffectiveRestParameter(target) &&
-                (checkMode & SignatureCheckMode.StrictArity ? getParameterCount(source) : getMinArgumentCount(source)) > targetCount) {
+            if (!hasEffectiveRestParameter(target) && (checkMode & SignatureCheckMode.StrictArity ? getParameterCount(source) : getMinArgumentCount(source)) > targetCount) {
                 return Ternary.False;
             }
 
@@ -14084,10 +14083,13 @@ namespace ts {
                 const targetSig = checkMode & SignatureCheckMode.Callback ? undefined : getSingleCallSignature(getNonNullableType(targetType));
                 const callbacks = sourceSig && targetSig && !getTypePredicateOfSignature(sourceSig) && !getTypePredicateOfSignature(targetSig) &&
                     (getFalsyFlags(sourceType) & TypeFlags.Nullable) === (getFalsyFlags(targetType) & TypeFlags.Nullable);
-                const related = callbacks ?
-                    // TODO: GH#18217 It will work if they're both `undefined`, but not if only one is
+                let related = callbacks ?
                     compareSignaturesRelated(targetSig!, sourceSig!, (checkMode & SignatureCheckMode.StrictArity) | (strictVariance ? SignatureCheckMode.StrictCallback : SignatureCheckMode.BivariantCallback), reportErrors, errorReporter, incompatibleErrorReporter, compareTypes, reportUnreliableMarkers) :
                     !(checkMode & SignatureCheckMode.Callback) && !strictVariance && compareTypes(sourceType, targetType, /*reportErrors*/ false) || compareTypes(targetType, sourceType, reportErrors);
+                // With strict arity, (x: number | undefined) => void is a subtype of (x?: number | undefined) => void
+                if (related && checkMode & SignatureCheckMode.StrictArity && i >= getMinArgumentCount(source) && i < getMinArgumentCount(target) && isTypeIdenticalTo(sourceType, targetType)) {
+                    related = Ternary.False;
+                }
                 if (!related) {
                     if (reportErrors) {
                         errorReporter!(Diagnostics.Types_of_parameters_0_and_1_are_incompatible,