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Fixes #1990
The underlying problem here is in the `ExtractExistentialType` AST node class.
An "existential" in current Slang is typically a value of interface type. When such a value is used in an operation, the type-checker "opens" the extistential so that subsequent type-checking steps can work with the (statically unknown) specific type of the value stored inside. The `ExtractExistentialType` AST node represents the type of an existential that has been "opened" in this way.
When the front-end performs lookup "into" a value with one of these types, it nees to use a reference to the original interface declaration with a "this-type substitution" that refers to the "opened" type (a this-type substitution tells the compiler the concrete type it should use in place of `This` in signatures within the interface; it allows compiler to "see" the right associated type definitions to use in a context).
Prior to this change, the implementation would store the specialized reference to the original interface declaration in the `ExtractExistentialType` node as part of its state. The catch there is that the specialized interface reference indirectly refers to the `ExtractExistentialType` AST node itself, creating a circularity. As soon as the front-end performs any operation that tries to recurse over that structure, it would go into an infinite loop.
The fix here sounds kind of like a hack, but seems to be pretty nice in practice. Instead of always storing the specialized interface reference, we instead store the few values that are needed to construct it, and then create and cache the actual reference on-demand. The on-demand created fields are not considered part of the state of the AST node for any kind of recursion or serialization, so they avoid the original problem.
A single test case was added that represents the original bug, and confirms the fix.
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The basic bug here was that `enum` types with an explicit tag type:
enum Color : int32_t { ... }
would have an `InheritanceDecl` implying that `Color` inherits from
`int32_t`. The problem is that this is *not* actually an inheritance
relationship, since a `Color` needs to be explicitly cast to/from an
`int32_t`.
Various parts of the compiler currently treat this case like real
inheritance, and as a result the operations taht would apply to an
`int32_t` end up applying to a `Color` as well. This particularly leads
to an ambiguity between applying the `==` operator, because it has
overloads for both the `__EnumType` and `__Builtin{something}`
interfaces.
The fix here is to explicitly exclude the `InheritanceDecl` that
represents an enumeration tag type when considering declared subtype
relationships. A more complete version of this fix would need to go
through all places in the code where `InheritanceDecl`s are used and
make sure that any places using them for true inheritnace relationships
ignore those that represent an enumeration tag type.
(An alternative option would be to use a distinct kind of `Decl` to
represent the tag-type relationship, perhaps even going so far as to
modifying the type of the relevant AST node as part of semantic
checking)
This change includes a regression test for the way this bug surfaced in
user code.
Co-authored-by: jsmall-nvidia <jsmall@nvidia.com>
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The basic feature here is the ability to use the `&` operator to produce the conjunction/intersection of two interfaces. That is, you can have interfaces:
interface IFirst { int getFirst(); }
interface ISecond { int getSecoond(); }
and if you need a generic function where the type parameter `T` must conform to *both* of these interfaces, you express that by constraining the parameter to the intersection of the interfaces:
void someFunction<T : IFirst & ISecond>(T value) { ... }
Without this feature, the main alternative an application would have is to define an intermediate interface, like:
interface IBoth : IFirst, ISecond {}
Forcing users to deal with an intermediate interface creates more work for type authors (they need to remember to inherit from the right combined interface(s)), or for `extension` authors (when you add `ISecond` to a type that used to just support `IFirst`, you had better also add `IBoth`). In the worst case, a family of N related "leaf" interfaces would give rise to an exponential number of intermediate interfaces to represnt the possible combinations.
A conjunction like `IFirst & ISecond` is officially its own type, and can be used to declare a type alias:
typealias IBoth = IFirst & ISecond;
This change only includes the first pass of work on this feature, so there are several caveats to be aware of:
* Using a conjunction as part of an inheritance clause is not yet supported (e.g., `struct X : IFirst & ISecond`). This is true even if the conjunction was introduced by an intermediate `typealias`
* The `&` syntax introduced here is only parsed in places where only a type (not an expression) is possible. This means you cannot do things like cast to a conjunction with `(IFirst & ISecond)(someValue)`.
* This work *should* apply to conjunctions of more than two interfaces (like `IA & IB & IC`) but that has not yet been tested
* In the long run it may be sensible to allow conjunctions that use concrete types, but we really ought to have the semantic checking logic rule that out for now.
* During testing, I encountered compiler crashes when trying to use this feature together with `property` declarations. Further investigation and debugging is called for.
* The handling of conjunction types is currently incomplete, in that there are many equivalences the compiler does not yet understand. For example, it is clear that `IA & IB` is equivalent to `IB & IA`, but the compiler currently does not understand this and will treat them as different types. A deeper implementation approach is called for.
* Conjunctions are currently only supported for generic type parameter constraints, when performing full specialization. Use of conjunctions for existential-type value parameters or with dynamic dispatch is not yet supported.
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* Allow mixing unspecialized and specialized existential parameters.
* Fixes.
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* Clean up the way that lookup "through" a base type is encoded
In order to undestand this change, it is important to undestand how lookup through base interfaces works prior to this change. In order to understand *that* it helps to be reminded of how inheritance relationships get encoded in the AST.
Suppose the user writes:
struct Base { int val; }
struct Derived : Base { ... }
...
Derived d = ...;
int v = d.val;
The question is how an expression like `d.val` gets semantically checked, and how it is encoded into the IR after semantic checking. You might assume it gets checked and encoded so that we end up with:
int v = ((Base) d).val;
and that seems like it should Just Work... so of course that isn't what Slang has been doing. Instead, we relied on the fact that the inheritance relationship `Derived : Base` is represented as an `InheritanceDecl` member of the `Derived` type, and we ended up checking the code into something like:
int v = d.<anonymous>.val;
where `<anonymous>` stands in for the name of the `InheritanceDecl` that represents inheritance from `Base`. This design choice makes a limited amount of sense when you consider how inheritance would typically be lowered to a C-like output language:
// struct Derived : Base { ... }
// =>
struct Derived { Base base; ... }
The problem with that encoding is that it really doesn't make sense for almost any other scenario. In particular, if you have a generic type parameter `T` that was constrianed with `T : ISomething`, then the constraint isn't even technically a *member* of the type parameter `T`, so expressing thing as a member reference in the AST is completely incorrect. Unfortunately, by the time it was clear that we needed something better, a bunch of implementation work was done based on the existing representation.
This change tries to clean things up so that lookup of a super-type member through a value of a sub-type does the obvious thing: cast the value to the super-type and then look up the member (as in `((Base) d).val`).
The core of the change is that in lookup, instead of creating `Constraint` breadcrumbs whenever we are looking up in a super-type (with a reference to the `TypeConstraintDecl` being used) we instead use `SuperType` breadcrumbs (with a reference to a `SubtypeWitness`). Then when we create the expression from a `LookupResultItem`, we translate any `SuperType` breadcrumbs into `CastToSuperTypeExpr`s (an expression type that already existed).
This change also adds support for lookup through the `This` type in the context of an interface, and in order for that to work we need a new kind of subtype witness to represent the knowledge that a `This` type is a subtype of the enclosing interface. Making that work forces us to change the representation of `TransitiveSubtypeWitness` so that it takes a pair of subtype witnesses (and not one subtype witness plus one `TypeConstraintDecl`). For the most part this is a small change, but it raises the possibility that some pieces of the code aren't going to be robust against all possible shapes of subtype witnesses.
The IR lowering logic has relied on the weird `d.<anonymous>` representation in order to ensure that when looking up interface members we weren't always casting to the interface type (which would create a `makeExistential` instruction), and then calling using that. Basically, the IR lowering would ignore the `d.<anonymous>` part and just emit `d`, but we can't do that for `((Base) d)` or `((IThing) d)` because whehter or not we should actually perform the cast depends on context.
For now we solve that problem by adding specific logic to ignore up-casts to interface types when they appear in member expressions or method calls. A more robust solution might be needed down the line, but this seems to work in practice.
All of this work is cleanup that I found was needed in order to make `extension`s of `interface` types workable.
* fixup: disable an incorrect test
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dispatch) (#1508)
* Allow calling a generic function with an existential value (dynamic dispatch).
* Fixes per review comments.
* Clean up implementation by having `openExistential` return `ExtractExistentialType` instead of a DeclRef to the interface with a `ThisTypeSubstitution`.
* More cleanups
Co-authored-by: Tim Foley <tfoleyNV@users.noreply.github.com>
Co-authored-by: Yong He <yhe@nvidia.com>
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During semantic checking, the compiler used to link together `ExtensionDecl`s into a singly-linked list dangling off of the `AggTypeDecl` that they applied to. This approach made lookup relatively easy, because given a `DeclRef` to an `AggTypeDecl` one could easily find and walk the list of candidate extensions.
Unfortunately, the simple approach has two major strikes against it:
* First, as we recently ran into, it creates a lifetime/ownership problem, in cases where the `ExtensionDecl` is outlived by the `AggTypeDecl` it applies to. This creates the one and only place in the compiler today where an "old" AST node might point to a "new" AST node, and it resulted in use-after-free problems in client code.
* Second, the scoping of `extension`s ends up being completely wrong. All of the `extension` methods on a type end up being visible in all cases, instead of just in the context of modules where the `extension` itself is visible. The comparable feature in C# (static extension methods) is careful to not make scoping mistakes like this. The Swift langauge has loose scoping for `extension` more akin to what we have in Slang today, but the maintainers seem to consider it a misfeature.
This change attempts to clean up both issues by changing the way that extension declarations are stored. There are two main pieces:
1. The primary "source of truth" for extension lookup has been moved to the `ModuleDecl`, where a module is responsible for storing a cache of the extensions declared within that module (keyed by the declaration of the type being extended). This cache is updated at the same point where the old code would mutate the AST node being depended on.
2. A secondary aggregated cache is added to the `SharedSemanticsContext` used during semantic checking. This cache includes entries from across multiple modules, and is intended to be invalidated and rebuilt on demand if new modules are added during checking.
Access to the candidate extensions has now been put behind subroutines that require a semantics-checking context to be passed in (there was always one available in contexts that care about extensions).
In addition, the operation for looking up members including those from extensions was refactored heavily to involve internal rather than external iteration and, more importantly, was changed so that it actually tests whether the `ExtensionDecl`s it loops over apply to the type in question, rather than blindly letting extensions members be looked up in ways that don't make sense.
There are three test cases added here to confirm aspects of the fix:
* First, I added a test that reproduces the crash that was being seen, so that we have a regression test for the fix.
* Second, I added a basic semantic-checking test to confirm that an `extension` from an `import`ed module is still visible/usable, to confirm that I didn't break existing valid uses of extensions.
* Third, I added a diagnostic test that ensures that we correctly ignore extensions that should not be visible in a given context as a result of `import` declarations.
Co-authored-by: jsmall-nvidia <jsmall@nvidia.com>
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* Add a ASTBuilder to a Module
Only construct on valid ASTBuilder (was being called on nullptr on occassion)
* Add nodes to ASTBuilder.
* Compiles with RefPtr removed from AST node types.
* Initialize all AST node pointer variables in headers to nullptr;
* Initialize AST node variables as nullptr.
Make ASTBuilder keep a ref on node types.
Make SyntaxParseCallback returns a NodeBase
* Don't release canonicalType on dtor (managed by ASTBuilder).
* Give ASTBuilders a name and id, to help in debugging.
For now destroy the session TypeCache, to stop it holding things released when the compile request destroys ASTBuilders.
* Moved the TypeCheckingCache over to Linkage from Session.
* NodeBase no longer derived from RefObject.
* Only add/dtor nodes that need destruction.
First pass compile on linux.
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* First steps toward inheritance for struct types
This change adds the ability for a `struct` type to declare a base type that is another `struct`:
```hlsl
struct Base
{
int baseMember;
}
struct Derived : Base
{
int derivedMember;
}
```
The semantics of the feature are that code like the above desugars into code like:
```hlsl
struct Base
{
int baseMember;
}
struct Derived
{
Base _base;
int derivedMember;
}
```
At points where a member from the base type is being projected out, or the value is being implicitly cast to the base type, the compiler transforms the code to reference the implicitly-generated `_base` member. That means code like this:
```hlsl
void f(Base b);
...
Derived d = ...;
int x = d.baseMember;
f(d);
```
gets transformed into a form like this:
```hlsl
void f(Base b);
...
Derived d = ...;
int x = d._base.baseMember;
f(d._base);
```
Note that as a result of this choice, the behavior when passing a `Derived` value to a function that expects a `Base` (including to inherited member functions) is that of "object shearing" from the C++ world: the called function can only "see" the `Base` part of the argument, and any operations performed on it will behave as if the value was indeed a `Base`. There is no polymorphism going on because Slang doesn't currently have `virtual` methods.
In an attempt to work toward inheritance being a robust feature, this change adds a bunch of more detailed logic for checking the bases of various declarations:
* An `interface` declaration is only allowed to inherit from other `interface`s
* An `extension` declaration can only introduce inheritance from `interface`s
* A `struct` declaration can only inherit from at most one other `struct`, and that `struct` must be the first entry in the list of bases
This change also adds a mechanism to control whether a `struct` or `interface` in one module can inherit from a `struct` or `interface` declared in another module:
* If the base declaration is marked `[open]`, then the inheritance is allowed
* If the base declaration is marked `[sealed]`, then the inheritance is allowed
* If it is not marked otherwise, a `struct` is implicitly `[sealed]`
* If it is not marked otherwise, an `interface` is implicitly `[open]`
These seem like reasonable defaults. In order to safeguard the standard library a bit, the interfaces for builtin types have been marked `[sealed]` to make sure that a user cannot declare a `struct` and then mark it as a `BuiltinFloatingPointType`. This step should bring us a bit closer to being able to document and expose these interfaces for built-in types so that users can write code that is generic over them.
There are some big caveats with this work, such that it really only represents a stepping-stone toward a usable inheritance feature. The most important caveats are:
* If a `Derived` type tries to conform to an interface, such that one or more interface requirements are satisfied with members inherited from the `Base` type, that is likely to cause a crash or incorrect code generation.
* If a `Derived` type tries to inherit from a `Base` type that conforms to one or more interfaces, the witness table generated for the conformance of `Derived` to that interface is likely to lead to a crash or incorrect code generation.
It is clear that solving both of those issues will be necessary before we can really promote `struct` inheritance as a feature for users to try out.
* fixup: trying to appease clang error
* fixups: review feedback
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* Small improvements to documentation and code around DiagnosticSink
* Made methods/functions in slang-syntax.h be lowerCamel
Removed some commented out source (was placed elsewhere in code)
* Making AST related methods and function lowerCamel.
Made IsLeftValue -> isLeftValue.
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* Compiles.
* Small tidy up around session/ASTBuilder.
* Tests are now passing.
* Fix Visual Studio project.
* Fix using new X to use builder when protectedness of Ctor is not enough.
Substitute->substitute
* Add some missing ast nodes created outside of ASTBuilder.
* Compile time check that ASTBuilder is making an AST type.
* Moced findClasInfo and findSyntaxClass (essentially the same thing) to SharedASTBuilder from Session.
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* Fields from upper to lower case in slang-ast-decl.h
* Lower camel field names in slang-ast-stmt.h
* Fix fields in slang-ast-expr.h
* slang-ast-type.h make fields lowerCamel.
* slang-ast-base.h members functions lowerCamel.
* Method names in slang-ast-type.h to lowerCamel.
* GetCanonicalType -> getCanonicalType
* Substitute -> substitute
* Equals -> equals
ToString -> toString
* ParentDecl -> parentDecl
Members -> members
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* Split apart `SemanticsVisitor`
The existing `SemanticsVisitor` type was the visitor for expressions, statements, and declarations, and its monolithic nature made it hard to introduce distinct visitors for different phases of checking (despite the fact that we had, de facto, multiple phases of declaration checking).
This change splits up `SemanticsVisitor` as follows:
* There is nosw a `SharedSemanticsContext` type which holds the shared state that all semantics visiting logic needs. This includes state that gets mutated during the course of semantic checking.
* The `SemanticsVisitor` type is now a base class that holds a pointer to a `SharedSemanticsContext`. Most of the non-visitor functions are still defined here, just to keep the code as simple as possible. The `SemanticsVisitor` type is no longer a "visitor" in any meaningful way, but retaining the old name minimizes the diffs to client code.
* There are distinct `Semantics{Expr|Stmt|Decl}Visitor` types that have the actual `visit*` methods for an appropriate subset of the AST hierarchy. These all inherit from `SemanticsVisitor` primarily so that they can have easy access to all the helper methods it defines (which used to be accessible because these were all the same object).
Any client code that was constructing a `SemanticsVisitor` now needs to construct a `SharedSemanticsContext` and then use that to initialize a `SemanticsVisitor`. Similarly, any code that was using `dispatch()` to invoke the visitor on an AST node needs to construct the appropriate sub-class and then invoke `dispatch()` on it instead.
This is a pure refactoring change, so no effort has been made to move state or logic onto the visitor sub-types even when it is logical. Similarly, no attempt has been made to hoist any code out of the common headers to avoid duplication between `.h` and `.cpp` files. Those cleanups will follow.
The one cleanup I allowed myself while doing this was getting rid of the `typeResult` member in `SemanticsVisitor` that appears to be a do-nothing field that got written to in a few places (for unclear reasons) but never read.
* Remove some statefulness around statement checking
Some of the state from the old `SemanticsVisitor` was used in a mutable way during semantic checking:
* The `function` field would be set and the restored when checking the body of a function so that things like `return` statements could find the outer function.
* The `outerStmts` list was used like a stack to track lexically surrounding statements to resolve things like `break` and `continue` targets.
Both of these meant that semantic checking code was doing fine-grained mutations on the shared semantic checking state even though the statefullness wasn't needed.
This change moves the relevant state down to `SemanticsStmtVisitor`, which is a type we create on-the-fly to check each statement, so that we now only need to establish the state once at creation time.
The list of outer statements is handled as a linked list threaded up through the stack (a recurring idiom through the codebase).
There was one place where the `function` field was being used that wasn't strictly inside statement checking: it appears that we were using it to detect whether a variable declaration represents a local, so I added an `_isLocalVar` function to serve the same basic purpose.
With this change, the only stateful part of `SharedSemanticsContext` is the information to track imported modules, which seems like a necessary thing (since deduplication requires statefullness).
* Refactor declaration checking to avoid recursion
The flexiblity of the Slang language makes enforcing ordering on semantic checking difficult. In particular, generics (including some of the built-in standard library types) can take value arguments, so that type expressions can include value expressions. This means that being able to determine the type of a function parameter may require checking expressions, which may in turn require resolving calls to an overloaded function, which in turn requires knowing the types of the parameters of candidate callees.
Up to this point there have been two dueling approaches to handling the ordering problem in the semantic checking logic:
1. There was the `EnsureDecl` operation, supported by the `DeclCheckState` type. Every declaration would track "how checked" it is, and `EnsureDecl(d, s)` would try to perform whatever checks are needed to bring declaration `d` up to state `s`.
2. There was top-down orchestration logic in `visitModuleDecl()` that tried to perform checking of declarations in a set of fixed phases that ensure things like all function declarations being checked before any function bodies.
Each of these options had problems:
1. The `EnsureDecl()` approach wasn't implemented completely or consistently. It only understood two basic levels of checking: the "header" of a declaration was checked, and then the "body," and it relied on a single `visit*()` routine to try and handle both cases. Things ended up being checked twice, or in a circular fashion.
2. Rather than fix the problems with `EnsureDecl()` we layered on the top-down orchestration logic, but doing so ignores the fact that no fixed set of phases can work for our language. The orchestration logic was also done in a relatively ad hoc fashion that relied on using a single visitor to implement all phases of checking, but it added a second metric of "checked-ness" that worked alongside `DeclCheckState`.
This change strives to unify the two worlds and make them consistent. One of the key changes is that instead of doing everything through a single visitor type, we now have distinct visitors for distinct phases of semantic checking, and those phases are one-to-one aligned with the values of the `DeclCheckState` type.
More detailed notes:
* Existing sites that used to call `checkDecl` to directly invoke semantic checking recursively now use `ensureDecl` instead. This makes sure that `ensureDecl` is the one bottleneck that everything passes through, so that it can guarantee that each phase of checking gets applied to each declaration at most once.
* The existing `visitModuleDecl` was revamped into a `checkModule` routine that does the global orchestration, but now it is just a driver routine that makes sure `ensureDecl` gets called on everything in an order that represents an idealized "default schedule" for checking, while not ruling out cases where `ensureDecl()` will change the ordering to handle cases where the global order is insufficient.
* Because `checkModule` handles much of the recursion over the declaration hierarchy, many cases where a declaration `visit*()` would recurse on its members have been eliminated. The only case where a declaration should recursively `ensureDecl()` its members is when its validity for a certain phase depends on those members being checked (e.g., determining the type of a function declaration depends on its parameters having been checked).
* All cases where a `visit*()` routine was manually checking the state/phase of checking have been eliminated. It is now the responsibility of `ensureDecl` to make sure that checking logic doesn't get invoked twice or in an inappropriate order.
* Most cases where a `visit*()` routine was manually *setting* the `DeclCheckState` of a declaration have been eliminated. The common case is now handled by `ensureDecl()` directly, and `visit*()` methods only need to override that logic when special cases arise. E.g., when a variable is declared without a type `(e.g., `let foo = ...;`) then we need to check its initial-value expression to determine its type, so that we must check it further than was initially expected/required.
* This change goes to some lengths to try and keep semantic checking logic at the same location in the `slang-check-decl.cpp` file, so each of the per-phase visitor types is forward declared at the top of the file, and then the actual `visit*()` routines are interleaved throughout the rest of the file. A future change could do pure code movement (no semantic changes) to arrive at a more logical organization, but for now I tried to stick with what would minimize the diffs (although the resulting diffs can still be messy at times).
* One important change to the semantic checking logic was that the test for use of a local variable ahead of its declaration (or as part of its own initial-value expression) was moved around, since its old location in the middle of the `ensureDecl` logic made the overall flow and intention of that function less clear. There is still a need to fix this check to be more robust in the future.
* Add some design documentation on semantic checking
The main thing this tries to lay out is the strategy for declaration checking and the rules/constraints on programmers that follow from it.
* fixup: typos found during review
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The semantic checking logic was all inside `slang-check.cpp` and as a result this was a monster file that was extremely hard to follow. This change splits `slang-check.cpp` into several smaller files, although some of the resulting files are still quite large.
This change attempts to be a copy-paste job as much as possible and does *not* perform any cleanup on naming, structure, duplication, etc. in the code it deal with. No function bodies or signatures have been touched.
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