Restore ordering of operations involving type parameters and unions

Author: ahejlsberg

src/compiler/checker.ts

@@ -19383,7 +19383,27 @@ namespace ts {
 
             function structuredTypeRelatedTo(source: Type, target: Type, reportErrors: boolean, intersectionState: IntersectionState): Ternary {
                 const saveErrorInfo = captureErrorCalculationState();
-                const result = structuredTypeRelatedToWorker(source, target, reportErrors, intersectionState, saveErrorInfo);
+                let result = structuredTypeRelatedToWorker(source, target, reportErrors, intersectionState, saveErrorInfo);
+                if (!result && (source.flags & TypeFlags.Intersection || source.flags & TypeFlags.TypeParameter && target.flags & TypeFlags.Union)) {
+                    // The combined constraint of an intersection type is the intersection of the constraints of
+                    // the constituents. When an intersection type contains instantiable types with union type
+                    // constraints, there are situations where we need to examine the combined constraint. One is
+                    // when the target is a union type. Another is when the intersection contains types belonging
+                    // to one of the disjoint domains. For example, given type variables T and U, each with the
+                    // constraint 'string | number', the combined constraint of 'T & U' is 'string | number' and
+                    // we need to check this constraint against a union on the target side. Also, given a type
+                    // variable V constrained to 'string | number', 'V & number' has a combined constraint of
+                    // 'string & number | number & number' which reduces to just 'number'.
+                    // This also handles type parameters, as a type parameter with a union constraint compared against a union
+                    // needs to have its constraint hoisted into an intersection with said type parameter, this way
+                    // the type param can be compared with itself in the target (with the influence of its constraint to match other parts)
+                    // For example, if `T extends 1 | 2` and `U extends 2 | 3` and we compare `T & U` to `T & U & (1 | 2 | 3)`
+                    const constraint = getEffectiveConstraintOfIntersection(source.flags & TypeFlags.Intersection ? (source as IntersectionType).types: [source], !!(target.flags & TypeFlags.Union));
+                    if (constraint && everyType(constraint, c => c !== source)) { // Skip comparison if expansion contains the source itself
+                        // TODO: Stack errors so we get a pyramid for the "normal" comparison above, _and_ a second for this
+                        result = isRelatedTo(constraint, target, RecursionFlags.Source, /*reportErrors*/ false, /*headMessage*/ undefined, intersectionState);
+                    }
+                }
                 if (result) {
                     resetErrorInfo(saveErrorInfo);
                 }
@@ -19442,28 +19462,6 @@ namespace ts {
                     if (result = unionOrIntersectionRelatedTo(source, target, reportErrors, intersectionState)) {
                         return result;
                     }
-                    if (source.flags & TypeFlags.Intersection || source.flags & TypeFlags.TypeParameter && target.flags & TypeFlags.Union) {
-                        // The combined constraint of an intersection type is the intersection of the constraints of
-                        // the constituents. When an intersection type contains instantiable types with union type
-                        // constraints, there are situations where we need to examine the combined constraint. One is
-                        // when the target is a union type. Another is when the intersection contains types belonging
-                        // to one of the disjoint domains. For example, given type variables T and U, each with the
-                        // constraint 'string | number', the combined constraint of 'T & U' is 'string | number' and
-                        // we need to check this constraint against a union on the target side. Also, given a type
-                        // variable V constrained to 'string | number', 'V & number' has a combined constraint of
-                        // 'string & number | number & number' which reduces to just 'number'.
-                        // This also handles type parameters, as a type parameter with a union constraint compared against a union
-                        // needs to have its constraint hoisted into an intersection with said type parameter, this way
-                        // the type param can be compared with itself in the target (with the influence of its constraint to match other parts)
-                        // For example, if `T extends 1 | 2` and `U extends 2 | 3` and we compare `T & U` to `T & U & (1 | 2 | 3)`
-                        const constraint = getEffectiveConstraintOfIntersection(source.flags & TypeFlags.Intersection ? (source as IntersectionType).types: [source], !!(target.flags & TypeFlags.Union));
-                        if (constraint && everyType(constraint, c => c !== source)) { // Skip comparison if expansion contains the source itself
-                            // TODO: Stack errors so we get a pyramid for the "normal" comparison above, _and_ a second for this
-                            if (result = isRelatedTo(constraint, target, RecursionFlags.Source, /*reportErrors*/ false, /*headMessage*/ undefined, intersectionState)) {
-                                return result;
-                            }
-                        }
-                    }
                     // The ordered decomposition above doesn't handle all cases. Specifically, we also need to handle:
                     // Source is instantiable (e.g. source has union or intersection constraint).
                     // Source is an object, target is a union (e.g. { a, b: boolean } <=> { a, b: true } | { a, b: false }).