Relate non-augmenting array subtypes without resorting to structural comparison

Author: weswigham

src/compiler/checker.ts

@@ -18219,6 +18219,22 @@ namespace ts {
                     return result;
                 }
 
+                if (isNonAugmentingArraySubtype(source) && isArrayType(target)) {
+                    let sourceBase = getBaseTypes((source as TypeReference).target as InterfaceType)[0];
+                    if (length((target as TypeReference).resolvedTypeArguments) === 2) {
+                        sourceBase = getTypeWithThisArgument(sourceBase, ((source as TypeReference).target as InterfaceType).thisType);
+                    }
+                    return isRelatedTo(sourceBase, target, reportErrors);
+                }
+
+                if (isArrayType(source) && isNonAugmentingArraySubtype(target)) {
+                    let targetBase = getBaseTypes((target as TypeReference).target as InterfaceType)[0];
+                    if (length((source as TypeReference).resolvedTypeArguments) === 2) {
+                        targetBase = getTypeWithThisArgument(targetBase, ((target as TypeReference).target as InterfaceType).thisType);
+                    }
+                    return isRelatedTo(source, targetBase, reportErrors);
+                }
+
                 if (target.flags & TypeFlags.TypeParameter) {
                     // A source type { [P in Q]: X } is related to a target type T if keyof T is related to Q and X is related to T[Q].
                     if (getObjectFlags(source) & ObjectFlags.Mapped && !(<MappedType>source).declaration.nameType && isRelatedTo(getIndexType(target), getConstraintTypeFromMappedType(<MappedType>source))) {
@@ -19868,6 +19884,24 @@ namespace ts {
             return isArrayType(type) || !(type.flags & TypeFlags.Nullable) && isTypeAssignableTo(type, anyReadonlyArrayType);
         }
 
+        function isNonAugmentingArraySubtype(type: Type) {
+            if (!(getObjectFlags(type) & ObjectFlags.Reference) || !(getObjectFlags((type as TypeReference).target) & ObjectFlags.ClassOrInterface)) {
+                return false;
+            }
+            const target = (type as TypeReference).target as InterfaceType;
+            const bases = getBaseTypes(target);
+            if (bases.length !== 1) {
+                return false;
+            }
+            if (!isArrayType(bases[0])) {
+                return false;
+            }
+            if (getMembersOfSymbol(type.symbol).size) {
+                return false; // If the interface has any members, they may subtype members in the base, so we should do a full structural comparison
+            }
+            return true;
+        }
+
         function isEmptyArrayLiteralType(type: Type): boolean {
             const elementType = getElementTypeOfArrayType(type);
             return strictNullChecks ? elementType === implicitNeverType : elementType === undefinedWideningType;

tests/baselines/reference/arrayLiterals3.errors.txt

@@ -8,7 +8,7 @@ tests/cases/conformance/expressions/arrayLiterals/arrayLiterals3.ts(17,5): error
 tests/cases/conformance/expressions/arrayLiterals/arrayLiterals3.ts(33,5): error TS2322: Type 'number[]' is not assignable to type '[number, number, number]'.
   Target requires 3 element(s) but source may have fewer.
 tests/cases/conformance/expressions/arrayLiterals/arrayLiterals3.ts(34,5): error TS2322: Type '(string | number)[]' is not assignable to type 'myArray'.
-  The types returned by 'pop()' are incompatible between these types.
+  Type '(string | number)[]' is not assignable to type 'Number[]'.
     Type 'string | number' is not assignable to type 'Number'.
       Type 'string' is not assignable to type 'Number'.
 
@@ -65,7 +65,7 @@ tests/cases/conformance/expressions/arrayLiterals/arrayLiterals3.ts(34,5): error
     var c2: myArray = [...temp1, ...temp];            // Error cannot assign (number|string)[] to number[]
         ~~
 !!! error TS2322: Type '(string | number)[]' is not assignable to type 'myArray'.
-!!! error TS2322:   The types returned by 'pop()' are incompatible between these types.
+!!! error TS2322:   Type '(string | number)[]' is not assignable to type 'Number[]'.
 !!! error TS2322:     Type 'string | number' is not assignable to type 'Number'.
 !!! error TS2322:       Type 'string' is not assignable to type 'Number'.