Improve handling of ++ and -- in control flow analysis

Author: ahejlsberg

src/compiler/checker.ts

@@ -8426,8 +8426,11 @@ namespace ts {
                 // Assignments only narrow the computed type if the declared type is a union type. Thus, we
                 // only need to evaluate the assigned type if the declared type is a union type.
                 if (isMatchingReference(reference, node)) {
-                    const isIncrementOrDecrement = node.parent.kind === SyntaxKind.PrefixUnaryExpression || node.parent.kind === SyntaxKind.PostfixUnaryExpression;
-                    return declaredType.flags & TypeFlags.Union && !isIncrementOrDecrement ?
+                    if (node.parent.kind === SyntaxKind.PrefixUnaryExpression || node.parent.kind === SyntaxKind.PostfixUnaryExpression) {
+                        const flowType = getTypeAtFlowNode(flow.antecedent);
+                        return createFlowType(getBaseTypeOfLiteralType(getTypeFromFlowType(flowType)), isIncomplete(flowType));
+                    }
+                    return declaredType.flags & TypeFlags.Union ?
                         getAssignmentReducedType(<UnionType>declaredType, getInitialOrAssignedType(node)) :
                         declaredType;
                 }