Add a fastpath for comparing common mapped types like Pick which avoids manufacturing intermediate type identities (microsoft#39696)

Author: weswigham

src/compiler/checker.ts

@@ -16959,12 +16959,23 @@ namespace ts {
                             if (includeOptional
                                 ? !(filteredByApplicability!.flags & TypeFlags.Never)
                                 : isRelatedTo(targetConstraint, sourceKeys)) {
-                                const typeParameter = getTypeParameterFromMappedType(target);
-                                const indexingType = filteredByApplicability ? getIntersectionType([filteredByApplicability, typeParameter]) : typeParameter;
-                                const indexedAccessType = getIndexedAccessType(source, indexingType);
                                 const templateType = getTemplateTypeFromMappedType(target);
-                                if (result = isRelatedTo(indexedAccessType, templateType, reportErrors)) {
-                                    return result;
+                                const typeParameter = getTypeParameterFromMappedType(target);
+
+                                // Fastpath: When the template has the form Obj[P] where P is the mapped type parameter, directly compare `source` with `Obj`
+                                // to avoid creating the (potentially very large) number of new intermediate types made by manufacturing `source[P]`
+                                const nonNullComponent = extractTypesOfKind(templateType, ~TypeFlags.Nullable);
+                                if (nonNullComponent.flags & TypeFlags.IndexedAccess && (nonNullComponent as IndexedAccessType).indexType === typeParameter) {
+                                    if (result = isRelatedTo(source, (nonNullComponent as IndexedAccessType).objectType, reportErrors)) {
+                                        return result;
+                                    }
+                                }
+                                else {
+                                    const indexingType = filteredByApplicability ? getIntersectionType([filteredByApplicability, typeParameter]) : typeParameter;
+                                    const indexedAccessType = getIndexedAccessType(source, indexingType);
+                                    if (result = isRelatedTo(indexedAccessType, templateType, reportErrors)) {
+                                        return result;
+                                    }
                                 }
                             }
                             originalErrorInfo = errorInfo;