The support for decorators that were imported via a namespace,
e.g. `import * as core from `@angular/core` was implemented
piecemeal. This meant that it was easy to miss situations where
a decorator identifier needed to be handled as a namepsaced
import rather than a direct import.
One such issue was that UMD processing of decorators was not
correct: the namespace was being omitted from references to
decorators.
Now the types have been modified to make it clear that a
`Decorator.identifier` could hold a namespaced identifier,
and the corresponding code that uses these types has been
fixed.
Fixes#31394
PR Close#31426
There are two places in the ngcc processing where it needs to load the
content of a file given by a general path:
* when determining the format of an entry-point.
To do this ngcc uses the value of the relevant property in package.json.
But in the case of `main` it must parse the contents of the entry-point
file to decide whether the format is UMD or CommonJS.
* when parsing the source files for dependencies to determine the order in
which compilation must occur. The relative imports in each file are parsed
and followed recursively, looking for external imports.
Previously, we naively assumed that the path would match the file name exactly.
But actually we must consider the standard module resolution conventions.
E.g. the extension (.js) may be missing, or the path may refer to a directory
containing an index.js file.
This commit fixes both places.
This commit now requires the `DependencyHost` instances to check
the existence of more files than before (at worst all the different possible
post-fixes). This should not create a significant performance reduction for
ngcc. Since the results of the checks will be cached, and similar work is
done inside the TS compiler, so what we lose in doing it here, is saved later
in the processing. The main performance loss would be where there are lots
of files that need to be parsed for dependencies that do not end up being
processed by TS. But compared to the main ngcc processing this dependency
parsing is a small proportion of the work done and so should not impact
much on the overall performance of ngcc.
// FW-1444
PR Close#31509
When determining if a `main` path points to a UMD or CommonJS
format, the contents of the file need to be loaded and parsed.
Previously, it was assumed that the path referred to the exact filename,
but did not account for normal module resolution semantics, where the
path may be missing an extension or refer to a directory containing an
`index.js` file.
// FW-1444
PR Close#31509
Paths can be mapped directly to files, which was not being taken
into account when computing `basePaths` for the `EntryPointFinder`s.
Now if a `pathMapping` pattern does not exist or is a file, then we try
the containing folder instead.
Fixes#31424
PR Close#30525
Previously, ngcc had to walk the entire `node_modules` tree looking for
entry-points, even if it only needed to process a single target entry-point
and its dependencies.
This added up to a few seconds to each execution of ngcc, which is noticeable
when being run via the CLI integration.
Now, if an entry-point target is provided, only that target and its entry-points
are considered rather than the whole folder tree.
PR Close#30525
When profiling ngcc it is notable that a large amount of time
is spent dealing with an exception that is thrown (and handled
internally by fs) when checking the existence of a file.
We check file existence a lot in both finding entry-points
and when TS is compiling code. This commit adds a simple
cached `FileSystem`, which wraps a real `FileSystem` delegate.
This will reduce the number of calls through to `fs.exists()` and
`fs.readFile()` on the delegate.
Initial benchmarks indicate that the cache is miss to hit ratio
for `exists()` is about 2:1, which means that we save about 1/3
of the calls to `fs.existsSync()`.
Note that this implements a "non-expiring" cache, so it is not suitable
for a long lived `FileSystem`, where files may be modified externally.
The cache will be updated if a file is changed or moved via
calls to `FileSystem` methods but it will not be aware of changes
to the files system from outside the `FileSystem` service.
For ngcc we must create a new `FileSystem` service
for each run of `mainNgcc` and ensure that all file operations
(including TS compilation) use the `FileSystem` service.
This ensures that it is very unlikely that a file will change
externally during `mainNgcc` processing.
PR Close#30525
This message gets called if a format has already been
compiled and we only want the first. So the message itself
is wrong but it is also not very useful anyway.
PR Close#30525
The ngcc tool adds namespaced imports to files when compiling. The ngtsc
tooling was not processing types correctly when they were imported via
such namespaces. For example:
```
export declare class SomeModule {
static withOptions(...): ModuleWithProviders<ɵngcc1.BaseModule>;
```
In this case the `BaseModule` was being incorrectly attributed to coming
from the current module rather than the imported module, represented by
`ɵngcc1`.
Fixes#31342
PR Close#31367
If a package delcares a class internally on an NgModule, ngcc
needs to be able to add a public export to this class's type.
Previously, if the typing file for the declared is not imported
from the typings entry-point file, then ngcc cannot find it.
Now we try to guess the .d.ts files from the equivalent .js
files.
PR Close#31411
Non-wild-card path-mappings were not being matched correctly.
Further path-mapped secondary entry-points that
were imported from the associated primary entry-point were not
being martched correctly.
Fixes#31274
PR Close#31450
Some formats of CommonJS put the decorator helper calls
outside the class IIFE as statements on the top level of the
source file.
This commit adds support to the `CommonJSReflectionHost`
for this format.
PR Close#31335
If an entry-point has missing dependencies then it cannot be
processed and is marked as invalid. Similarly, if an entry-point
has dependencies that have been marked as invalid then that
entry-point too is invalid. In all these cases, ngcc should quietly
ignore these entry-points and continue processing what it can.
Previously, if an entry-point had more than one entry-point that
was transitively invalid then ngcc was crashing rather than
ignoring the entry-point.
PR Close#31276
Our module resolution prefers `.js` files over `.d.ts` files because
occasionally libraries publish their typings in the same directory
structure as the compiled JS files, i.e. adjacent to each other.
The standard TS module resolution would pick up the typings
file and add that to the `ts.Program` and so they would be
ignored by our analyzers. But we need those JS files, if they
are part of the current package.
But this meant that we also bring in JS files from external
imports from outside the package, which is not desired.
This was happening for the `@fire/storage` enty-point
that was importing the `firebase/storage` path.
In this commit we solve this problem, for the case of imports
coming from a completely different package, by saying that any
file that is outside the package root directory must be an external
import and so we do not analyze those files.
This does not solve the potential problem of imports between
secondary entry-points within a package but so far that does
not appear to be a problem.
PR Close#30591
Rather than passing a number of individual arguments, we can
just pass an `EntryPointBundle`, which already contains them.
This is also a precursor to using more of the properties in the bundle.
PR Close#30591
This will allow users of the `EntryPointBundle` to use some of the `EntryPoint`
properties without us having to pass them around one by one.
PR Close#30591
Previously we expected the constructor parameter `decorators`
property to be an array wrapped in a function. Now we also support
an array not wrapped in a function.
PR Close#30591
Some packages do not actually provide a `typings` field in their
package.json. But TypeScript naturally infers the typings file from
the location of the JavaScript source file.
This commit modifies ngcc to do a similar inference when finding
entry-points to process.
Fixes#28603 (FW-1299)
PR Close#30591
There are scenarios where it is not possible for ngcc to guess the format
or configuration of an entry-point just from the files on disk.
Such scenarios include:
1) Unwanted entry-points: A spurious package.json makes ngcc think
there is an entry-point when there should not be one.
2) Deep-import entry-points: some packages allow deep-imports but do not
provide package.json files to indicate to ngcc that the imported path is
actually an entry-point to be processed.
3) Invalid/missing package.json properties: For example, an entry-point
that does not provide a valid property to a required format.
The configuration is provided by one or more `ngcc.config.js` files:
* If placed at the root of the project, this file can provide configuration
for named packages (and their entry-points) that have been npm installed
into the project.
* If published as part of a package, the file can provide configuration
for entry-points of the package.
The configured of a package at the project level will override any
configuration provided by the package itself.
PR Close#30591
Previously each test relied on large shared mock file-systems, which
makes it difficult to reason about what is actually being tested.
This commit breaks up these big mock file-systems into smaller more
focused chunks.
PR Close#30591
To improve cross platform support, all file access (and path manipulation)
is now done through a well known interface (`FileSystem`).
For testing a number of `MockFileSystem` implementations are provided.
These provide an in-memory file-system which emulates operating systems
like OS/X, Unix and Windows.
The current file system is always available via the static method,
`FileSystem.getFileSystem()`. This is also used by a number of static
methods on `AbsoluteFsPath` and `PathSegment`, to avoid having to pass
`FileSystem` objects around all the time. The result of this is that one
must be careful to ensure that the file-system has been initialized before
using any of these static methods. To prevent this happening accidentally
the current file system always starts out as an instance of `InvalidFileSystem`,
which will throw an error if any of its methods are called.
You can set the current file-system by calling `FileSystem.setFileSystem()`.
During testing you can call the helper function `initMockFileSystem(os)`
which takes a string name of the OS to emulate, and will also monkey-patch
aspects of the TypeScript library to ensure that TS is also using the
current file-system.
Finally there is the `NgtscCompilerHost` to be used for any TypeScript
compilation, which uses a given file-system.
All tests that interact with the file-system should be tested against each
of the mock file-systems. A series of helpers have been provided to support
such tests:
* `runInEachFileSystem()` - wrap your tests in this helper to run all the
wrapped tests in each of the mock file-systems.
* `addTestFilesToFileSystem()` - use this to add files and their contents
to the mock file system for testing.
* `loadTestFilesFromDisk()` - use this to load a mirror image of files on
disk into the in-memory mock file-system.
* `loadFakeCore()` - use this to load a fake version of `@angular/core`
into the mock file-system.
All ngcc and ngtsc source and tests now use this virtual file-system setup.
PR Close#30921
Packages that have been compiled using an older version of TypeScript
can have their decorators at the top-level of the ES5 bundles, instead
of inside the IIFE that is emitted for the class. Before this change,
ngcc only took static property assignments inside the IIFE into account,
therefore missing the decorators that were assigned at the top-level.
This commit extends the ES5 host to look for static properties in two
places. Testcases for all bundle formats that contain ES5 have been added
to ensure that this works in the various flavours.
A patch is included to support UMD bundles. The UMD factory affects how
TypeScripts binds the static properties to symbols, see the docblock of
the patch function for more details.
PR Close#30795
The `EntryPointFinder` computes the base paths to consider
when searching for entry-points. When there are `pathMappings`
provided it works out the best top level base-paths that cover all
the potential mappings.
If this computed basePath happens to coincide with an entry-point
path itself then we were missing it.
Now we check for an entry-point even at the base-path itself.
Related to https://github.com/angular/angular-cli/pull/14755
PR Close#31027
9d9c9e43e5 has been created a few days ago
and wasn't rebased on top of recent changes that introduces a commonjs host.
This means that tests for the commonjs host haven't been updated to work with
the changes from from #30492 and now fail in `master`.
PR Close#30967
Currently undecorated classes are intentionally not processed
with ngcc. This is causing unexpected behavior because decorator
handlers such as `base_def.ts` are specifically interested in class
definitions without top-level decorators, so that the base definition
can be generated if there are Angular-specific class members.
In order to ensure that undecorated base-classes work as expected
with Ivy, we need to run the decorator handlers for all top-level
class declarations (not only for those with decorators). This is similar
to when `ngtsc` runs decorator handlers when analyzing source-files.
Resolves FW-1355. Fixes https://github.com/angular/components/issues/16178
PR Close#30821
When determining the module type of a bundle pointed to by the "main"
property, ngcc needs to read the bundle to figure out if it is CommonJS
or UMD format. However, when the "main" property does not exist ngcc
would crash while determining the path to the main bundle file.
This commit fixes the crash by checking if the "main" property is present
at all, before attempting to derive a full path to the bundle file.
Fixes#30916
Fixes FW-1369
PR Close#30950
The usage of array spread syntax in source code may be downleveled to a
call to TypeScript's `__spread` helper function from `tslib`, depending
on the options `downlevelIteration` and `emitHelpers`. This proves
problematic for ngcc when it is processing ES5 formats, as the static
evaluator won't be able to interpret those calls.
A custom foreign function resolver is not sufficient in this case, as
`tslib` may be emitted into the library code itself. In that case, a
helper function can be resolved to an actual function with body, such
that it won't be considered as foreign function. Instead, a reflection
host can now indicate that the definition of a function corresponds with
a certain TypeScript helper, such that it becomes statically evaluable
in ngtsc.
Resolves#30299
PR Close#30492
- Refactors compiler to stop generating `ɵɵselect(0)` instructions
- Alters template execution to always call the equivalent of `ɵɵselect(0)` before running a template in update mode
- Updates tests to not check for or call `ɵɵselect(0)`.
The goal here is to reduce the size of generated templates
PR Close#30830
When ngcc processes an entrypoint, it updates `package.json` with
metadata about the processed format. Previously, it overwrote the
`package.json` with the stringified JSON object without spaces. This
made the file difficult to read (for example when looking at the file
while debugging an ngcc failure).
This commit fixes it by using spaces in the new `package.json` content.
PR Close#30831
The AbsoluteModuleStrategy in ngtsc assumed that the source code is
formatted as TypeScript with regards to module exports.
In ngcc this is not always the case, so this commit changes
`AbsoluteModuleStrategy` so that it relies upon a `ReflectionHost` to
compute the exports of a module.
PR Close#30200
There is an encoding issue with using delta `Δ`, where the browser will attempt to detect the file encoding if the character set is not explicitly declared on a `<script/>` tag, and Chrome will find the `Δ` character and decide it is window-1252 encoding, which misinterprets the `Δ` character to be some other character that is not a valid JS identifier character
So back to the frog eyes we go.
```
__
/ɵɵ\
( -- ) - I am ineffable. I am forever.
_/ \_
/ \ / \
== == ==
```
PR Close#30546
Previously we were relying upon the `.get()` method to return `undefined`
but it is clearer and safer to always check with `.has()` first.
PR Close#25445
Previously the same `Renderer` was used to render typings (.d.ts)
files. But the new `UmdRenderer` is not able to render typings files
correctly.
This commit splits out the typings rendering from the src rendering.
To achieve this the previous renderers have been refactored from
sub-classes of the abstract `Renderer` class to classes that implement
the `RenderingFormatter` interface, which are then passed to the
`Renderer` and `DtsRenderer` to modify its rendering behaviour.
Along the way a few utility interfaces and classes have been moved
around and renamed for clarity.
PR Close#25445
Previously these fake files were full TypeScript source
files (`.ts`) but this is not necessary as we only need the
typings not the implementation.
PR Close#25445
Previously we were using an anonymous type `{specifier: string; qualifier: string;}`
throughout the code base. This commit gives this type a name and ensures it
is only defined in one place.
PR Close#25445
Preserve compatibility with rollup_bundle rule.
Add missing npm dependencies, which are now enforced by the strict_deps plugin in tsc_wrapped
PR Close#30370
At the moment the module resolver will end up in an infinite loop in Windows because we are assuming that the root directory is always `/` however in windows this can be any drive letter example `c:/` or `d:/` etc...
With this change we also resolve the drive letter in windows, when using `AbsoluteFsPath.from` for consistence so under `/foo` will be converted to `c:/foo` this is also needed because of relative paths with different drive letters.
PR Close#30297
If an entry-point has a missing dependency then all the entry-points
that would have pointed to that dependency are also removed from
the dependency graph.
Previously we were still processing the dependencies of an entry-point
even if it had already been removed from the graph because it depended
upon a missing dependency that had previously been removed due to another
entry-point depending upon it.
This caused the dependency processing to crash rather than gracefully
logging and handling the missing invalid entry-point.
Fixes#29624
PR Close#30270
Packages that do not follow APF may have the declaration files in the same
directory as one source format, typically ES5. This is problematic for ngcc,
as it needs to create a TypeScript program with all JavaScript sources of
an entry-point, whereas TypeScript's module resolution mechanism would have
resolved an internal module import to the external facing .d.ts declaration
file, instead of the JavaScript source file. This behavior results in the
program to be analysed being incomplete.
This commit introduces a custom compiler host that recognizes the above
scenario and rewires the resolution of a .d.ts declaration file to its
JavaScript counterpart, if applicable.
Fixes#29939
PR Close#30017
This commit introduces a new interface, which abstracts access
to the underlying `FileSystem`. There is initially one concrete
implementation, `NodeJsFileSystem`, which is simply wrapping the
`fs` library of NodeJs.
Going forward, we can provide a `MockFileSystem` for test, which
should allow us to stop using `mock-fs` for most of the unit tests.
We could also implement a `CachedFileSystem` that may improve the
performance of ngcc.
PR Close#29643
By passing a `pathMappings` configuration (a subset of the
`ts.CompilerOptions` interface), we can instuct ngcc to process
additional paths outside the `node_modules` folder.
PR Close#29643
When working out the dependencies between entry-points
ngcc must parse the import statements and then resolve the
import path to the actual file. This is complicated because module
resolution is not trivial.
Previously ngcc used the node.js `require.resolve`, with some
hacking to resolve modules. This change refactors the `DependencyHost`
to use a new custom `ModuleResolver`, which is optimized for this use
case.
Moreover, because we are in full control of the resolution,
we can support TS `paths` aliases, where not all imports come from
`node_modules`. This is the case in some CLI projects where there are
compiled libraries that are stored locally in a `dist` folder.
See //FW-1210.
PR Close#29643
Previously we completely ignored entry-points that had not been
compiled with Angular, since we do not need to compile them
with ngcc. But this makes it difficult to reason about dependencies
between entry-points that were compiled with Angular and those that
were not.
Now we do track these non-Angular compiled entry-points but they
are marked as `compiledByAngular: false`.
PR Close#29643
The test now attempts to compile an entry-point (@angular/common/http/testing)
that has a transient "private" dependency. A private dependency is one that is
only visible by looking at the compiled JS code, rather than the generated TS
typings files.
This proves that we can't rely on typings files alone for computing the
dependencies between entry-points.
PR Close#29643
The `Transformer` and `Renderer` classes do not
actually need a `sourcePath` value as by the time
they are doing their work we are only working directly
with full absolute paths.
PR Close#29643
Only the JS files that are actually part of the entry-point
should be copied to the new entry-point folder in the
`NewEntryPointFileWriter`.
Previously some typings and external JS files were
being copied which was messing up the node_modules
structure.
Fixes https://github.com/angular/angular-cli/issues/14193
PR Close#30085
Previously, ngcc would insert new imports at the beginning of the file, for
convenience. This is problematic for imports that have side-effects, as the
side-effects imposed by such imports may affect the behavior of subsequent
imports.
This commit teaches ngcc to insert imports after any existing imports. Special
care has been taken to ensure inserted constants will still follow after the
inserted imports.
Resolves FW-1271
PR Close#30029
Previously, all of a program's files would be copied into the __ivy_ngcc__
folder where ngcc then writes its modifications into. The set of source files
in a program however is much larger than the source files contained within
the entry-point of interest, so many more files were copied than necessary.
Even worse, it may occur that an unrelated file in the program would collide
with an already existing source file, resulting in incorrectly overwriting
a file with unrelated content. This behavior has actually been observed
with @angular/animations and @angular/platform-browser/animations, where
the former package would overwrite declaration files of the latter package.
This commit fixes the issue by only copying relevant source files when cloning
a bundle's content into __ivy_ngcc__.
Fixes#29960
PR Close#30020
Previously, Template.templateAttrs was introduced to capture attribute
bindings which originated from microsyntax (e.g. bindings in *ngFor="...").
This means that a Template node can have two different structures, depending
on whether it originated from microsyntax or from a literal <ng-template>.
In the literal case, the node behaves much like an Element node, it has
attributes, inputs, and outputs which determine which directives apply.
In the microsyntax case, though, only the templateAttrs should be used
to determine which directives apply.
Previously, both the t2_binder and the TemplateDefinitionBuilder were using
the wrong set of attributes to match directives - combining the attributes,
inputs, outputs, and templateAttrs of the Template node regardless of its
origin. In the TDB's case this wasn't a problem, since the TDB collects a
global Set of directives used in the template, so it didn't matter whether
the directive was also recognized on the <ng-template>. t2_binder's API
distinguishes between directives on specific nodes, though, so it's more
sensitive to mismatching.
In particular, this showed up as an assertion failure in template type-
checking in certain cases, when a directive was accidentally matched on
a microsyntax template element and also had a binding which referenced a
variable declared in the microsyntax. This resulted in the type-checker
attempting to generate a reference to a variable that didn't exist in that
scope.
The fix is to distinguish between the two cases and select the appropriate
set of attributes to match on accordingly.
Testing strategy: tested in the t2_binder tests.
PR Close#29698
Previously the template type-checking code only considered the metadata of
directive classes actually referenced in the template. If those directives
had base classes, any inputs/outputs/etc of the base classes were not
tracked when generating the TCB. This resulted in bindings to those inputs
being incorrectly attributed to the host component or element.
This commit uses the new metadata package to follow directive inheritance
chains and use the full metadata for a directive for TCB generation.
Testing strategy: Template type-checking tests included.
PR Close#29698
Previously, metadata registration (the recording of collected metadata
during analysis of directives, pipes, and NgModules) was only used to
produce the `LocalModuleScope`, and thus was handled by the
`LocalModuleScopeRegistry`.
However, the template type-checker also needs information about registered
directives, outside of the NgModule scope determinations. Rather than
reuse the scope registry for an unintended purpose, this commit introduces
new abstractions for metadata registration and lookups in a separate
'metadata' package, which the scope registry implements.
This paves the way for a future commit to make use of this metadata for the
template type-checking system.
Testing strategy: this commit is a refactoring which introduces no new
functionality, so existing tests are sufficient.
PR Close#29698
The `Δ` caused issue with other infrastructure, and we are temporarily
changing it to `ɵɵ`.
This commit also patches ts_api_guardian_test and AIO to understand `ɵɵ`.
PR Close#29850
Previously we always walked the whole folder tree looking for
entry-points before we tested whether a target package had been
processed already. This could take >10secs!
This commit does a quick check of the target package before doing
the full walk which brings down the execution time for ngcc in this
case dramatically.
```
$ time ./node_modules/.bin/ivy-ngcc -t @angular/common/http/testing
Compiling @angular/core : fesm2015 as esm2015
Compiling @angular/core : fesm5 as esm5
Compiling @angular/core : esm2015 as esm2015
Compiling @angular/core : esm5 as esm5
Compiling @angular/common/http : fesm2015 as esm2015
Compiling @angular/common/http : fesm5 as esm5
Compiling @angular/common/http : esm2015 as esm2015
Compiling @angular/common/http : esm5 as esm5
Compiling @angular/common/http/testing : fesm2015 as esm2015
Compiling @angular/common/http/testing : fesm5 as esm5
Compiling @angular/common/http/testing : esm2015 as esm2015
Compiling @angular/common/http/testing : esm5 as esm5
real 0m19.766s
user 0m28.533s
sys 0m2.262s
```
```
$ time ./node_modules/.bin/ivy-ngcc -t @angular/common/http/testing
The target entry-point has already been processed
real 0m0.666s
user 0m0.605s
sys 0m0.113s
```
PR Close#29740
Prior to this change, all module metadata would be included in the
`defineNgModule` call that is set as the `ngModuleDef` field of module
types. Part of the metadata is scope information like declarations,
imports and exports that is used for computing the transitive module
scope in JIT environments, preventing those references from being
tree-shaken for production builds.
This change moves the metadata for scope computations to a pure function
call that patches the scope references onto the module type. Because the
function is marked pure, it may be tree-shaken out during production builds
such that references to declarations and exports are dropped, which in turn
allows for tree-shaken any declaration that is not otherwise referenced.
Fixes#28077, FW-1035
PR Close#29598
With the new API, where you can choose to only process the first
matching format, it is possible to process an entry-point multiple
times, if you pass in a different format each time.
Previously, ngcc would always try to process the typings files for
the entry-point along with processing the first format of the current
execution of ngcc. But this meant that it would be trying to process
the typings a second time.
Now we only process the typings if they have not already been
processed as part of processing another format in another
even if it was in a different execution of ngcc.
PR Close#29657
In ES2015, classes could have been emitted as a variable declaration
initialized with a class expression. In certain situations, an intermediary
variable suffixed with `_1` is present such that the variable
declaration's initializer becomes a binary expression with its rhs being
the class expression, and its lhs being the identifier of the intermediate
variable. This structure was not recognized, resulting in such classes not
being considered as a class in `Esm2015ReflectionHost`.
As a consequence, the analysis of functions/methods that return a
`ModuleWithProviders` object did not take the methods of such classes into
account.
Another edge-case with such intermediate variable was that static
properties would not be considered as class members. A testcase was added
to prevent regressions.
Fixes#29078
PR Close#29119
This commit adds a `tracePerformance` option for tsconfig.json. When
specified, it causes a JSON file with timing information from the ngtsc
compiler to be emitted at the specified path.
This tracing system is used to instrument the analysis/emit phases of
compilation, and will be useful in debugging future integration work with
@angular/cli.
See ngtsc/perf/README.md for more details.
PR Close#29380
Previously we were using absolute paths, but since at rendering time
we do not know exactly where the file will be written it is more correct
to change to using relative paths. This is actually better all round
since it allows the folders to be portable to different machines, etc.
PR Close#29556
We have already removed this concept from the public API. This just cleans it out altogether.
The `targetPath` was an alternative output path to the original `basePath`.
This is not really a very useful concept, since the actual target path
of each output file is more complex and not consistently relative to the `basePath`.
PR Close#29556
Pete Bacon Darwin
George Kalpakas
Matias Niemelä
JoostK
Paul Gschwendtner
Ben Lesh
Alan
Alex Eagle
Filipe Silva
Alex Rickabaugh