[torque] Generalize type argument inference for generic calls

With the arrival of generic structs (https://chromium-review.googlesource.com/c/v8/v8/+/1714868) the existing type inference procedure for generic calls became incomplete, since it could not infer types that were only constrained as part of generic types. For instance, given

  struct Box<T: Type> { ... }

  macro unbox<T: type>(box: Box<T>): T

the type argument (Smi) at the following call site

  const box: Box<Smi> = ...;
  unbox(box);

could not be inferred.

This CL re-implements the inference procedure and documents the semantics of type argument inference in Torque a bit more clearly.

R=tebbi@chromium.org

Change-Id: I868f16afbd9864b9c810ac49bc1639b467df939c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1720812
Commit-Queue: Georg Schmid <gsps@google.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63005}

BUILD.gn

@@ -3364,6 +3364,8 @@ v8_source_set("torque_base") {
     "src/torque/torque-compiler.h",
     "src/torque/torque-parser.cc",
     "src/torque/torque-parser.h",
+    "src/torque/type-inference.cc",
+    "src/torque/type-inference.h",
     "src/torque/type-oracle.cc",
     "src/torque/type-oracle.h",
     "src/torque/type-visitor.cc",

src/torque/declarable.cc

@@ -7,6 +7,7 @@
 
 #include "src/torque/declarable.h"
 #include "src/torque/global-context.h"
+#include "src/torque/type-inference.h"
 #include "src/torque/type-visitor.h"
 
 namespace v8 {
@@ -65,56 +66,20 @@ std::ostream& operator<<(std::ostream& os, const Generic& g) {
   return os;
 }
 
-namespace {
-base::Optional<const Type*> InferTypeArgument(const std::string& to_infer,
-                                              TypeExpression* parameter,
-                                              const Type* argument) {
-  BasicTypeExpression* basic = BasicTypeExpression::DynamicCast(parameter);
-  if (basic && basic->namespace_qualification.empty() && !basic->is_constexpr &&
-      basic->name == to_infer) {
-    return argument;
-  }
-  // TODO(gsps): Perform type inference for generic types
-  return base::nullopt;
-}
-
-base::Optional<const Type*> InferTypeArgument(
-    const std::string& to_infer, const std::vector<TypeExpression*>& parameters,
-    const TypeVector& arguments) {
-  for (size_t i = 0; i < arguments.size() && i < parameters.size(); ++i) {
-    if (base::Optional<const Type*> inferred =
-            InferTypeArgument(to_infer, parameters[i], arguments[i])) {
-      return *inferred;
-    }
-  }
-  return base::nullopt;
-}
-
-}  // namespace
-
-base::Optional<TypeVector> Generic::InferSpecializationTypes(
+TypeArgumentInference Generic::InferSpecializationTypes(
     const TypeVector& explicit_specialization_types,
     const TypeVector& arguments) {
-  TypeVector result = explicit_specialization_types;
-  size_t type_parameter_count = declaration()->generic_parameters.size();
-  if (explicit_specialization_types.size() > type_parameter_count) {
-    return base::nullopt;
-  }
-  for (size_t i = explicit_specialization_types.size();
-       i < type_parameter_count; ++i) {
-    const std::string type_name = declaration()->generic_parameters[i]->value;
-    size_t implicit_count =
-        declaration()->callable->signature->parameters.implicit_count;
-    const std::vector<TypeExpression*>& parameters =
-        declaration()->callable->signature->parameters.types;
-    std::vector<TypeExpression*> explicit_parameters(
-        parameters.begin() + implicit_count, parameters.end());
-    base::Optional<const Type*> inferred =
-        InferTypeArgument(type_name, explicit_parameters, arguments);
-    if (!inferred) return base::nullopt;
-    result.push_back(*inferred);
-  }
-  return result;
+  size_t implicit_count =
+      declaration()->callable->signature->parameters.implicit_count;
+  const std::vector<TypeExpression*>& parameters =
+      declaration()->callable->signature->parameters.types;
+  std::vector<TypeExpression*> explicit_parameters(
+      parameters.begin() + implicit_count, parameters.end());
+
+  TypeArgumentInference inference(declaration()->generic_parameters,
+                                  explicit_specialization_types,
+                                  explicit_parameters, arguments);
+  return inference;
 }
 
 bool Namespace::IsDefaultNamespace() const {

src/torque/declarable.h

@@ -21,6 +21,7 @@ namespace torque {
 
 class Scope;
 class Namespace;
+class TypeArgumentInference;
 
 DECLARE_CONTEXTUAL_VARIABLE(CurrentScope, Scope*);
 
@@ -489,7 +490,7 @@ class Generic : public Declarable {
   }
   SpecializationMap<Callable>& specializations() { return specializations_; }
 
-  base::Optional<TypeVector> InferSpecializationTypes(
+  TypeArgumentInference InferSpecializationTypes(
       const TypeVector& explicit_specialization_types,
       const TypeVector& arguments);
 

src/torque/implementation-visitor.cc

@@ -10,6 +10,7 @@
 #include "src/torque/implementation-visitor.h"
 #include "src/torque/parameter-difference.h"
 #include "src/torque/server-data.h"
+#include "src/torque/type-inference.h"
 #include "src/torque/type-visitor.h"
 
 namespace v8 {
@@ -1582,7 +1583,9 @@ namespace {
 void FailCallableLookup(const std::string& reason, const QualifiedName& name,
                         const TypeVector& parameter_types,
                         const std::vector<Binding<LocalLabel>*>& labels,
-                        const std::vector<Signature>& candidates) {
+                        const std::vector<Signature>& candidates,
+                        const std::vector<std::tuple<Generic*, const char*>>
+                            inapplicable_generics) {
   std::stringstream stream;
   stream << "\n" << reason << ": \n  " << name << "(" << parameter_types << ")";
   if (labels.size() != 0) {
@@ -1596,6 +1599,16 @@ void FailCallableLookup(const std::string& reason, const QualifiedName& name,
     stream << "\n  " << name;
     PrintSignature(stream, signature, false);
   }
+  if (inapplicable_generics.size() != 0) {
+    stream << "\nfailed to instantiate all of these generic declarations:";
+    for (auto& failure : inapplicable_generics) {
+      Generic* generic;
+      const char* reason;
+      std::tie(generic, reason) = failure;
+      stream << "\n  " << generic->name() << " defined at "
+             << generic->Position() << ":\n    " << reason << "\n";
+    }
+  }
   ReportError(stream.str());
 }
 
@@ -1655,17 +1668,20 @@ Callable* ImplementationVisitor::LookupCallable(
 
   std::vector<Declarable*> overloads;
   std::vector<Signature> overload_signatures;
+  std::vector<std::tuple<Generic*, const char*>> inapplicable_generics;
   for (auto* declarable : declaration_container) {
     if (Generic* generic = Generic::DynamicCast(declarable)) {
-      base::Optional<TypeVector> inferred_specialization_types =
-          generic->InferSpecializationTypes(specialization_types,
-                                            parameter_types);
-      if (!inferred_specialization_types) continue;
+      TypeArgumentInference inference = generic->InferSpecializationTypes(
+          specialization_types, parameter_types);
+      if (inference.HasFailed()) {
+        inapplicable_generics.push_back(
+            std::make_tuple(generic, inference.GetFailureReason()));
+        continue;
+      }
       overloads.push_back(generic);
       overload_signatures.push_back(
           DeclarationVisitor::MakeSpecializedSignature(
-              SpecializationKey<Generic>{generic,
-                                         *inferred_specialization_types}));
+              SpecializationKey<Generic>{generic, inference.GetResult()}));
     } else if (Callable* callable = Callable::DynamicCast(declarable)) {
       overloads.push_back(callable);
       overload_signatures.push_back(callable->signature());
@@ -1684,7 +1700,7 @@ Callable* ImplementationVisitor::LookupCallable(
     }
   }
 
-  if (overloads.empty()) {
+  if (overloads.empty() && inapplicable_generics.empty()) {
     if (silence_errors) return nullptr;
     std::stringstream stream;
     stream << "no matching declaration found for " << name;
@@ -1692,7 +1708,8 @@ Callable* ImplementationVisitor::LookupCallable(
   } else if (candidates.empty()) {
     if (silence_errors) return nullptr;
     FailCallableLookup("cannot find suitable callable with name", name,
-                       parameter_types, labels, overload_signatures);
+                       parameter_types, labels, overload_signatures,
+                       inapplicable_generics);
   }
 
   auto is_better_candidate = [&](size_t a, size_t b) {
@@ -1713,14 +1730,15 @@ Callable* ImplementationVisitor::LookupCallable(
         candidate_signatures.push_back(overload_signatures[i]);
       }
       FailCallableLookup("ambiguous callable ", name, parameter_types, labels,
-                         candidate_signatures);
+                         candidate_signatures, inapplicable_generics);
     }
   }
 
   if (Generic* generic = Generic::DynamicCast(overloads[best])) {
-    result = GetOrCreateSpecialization(SpecializationKey<Generic>{
-        generic, *generic->InferSpecializationTypes(specialization_types,
-                                                    parameter_types)});
+    TypeArgumentInference inference = generic->InferSpecializationTypes(
+        specialization_types, parameter_types);
+    result = GetOrCreateSpecialization(
+        SpecializationKey<Generic>{generic, inference.GetResult()});
   } else {
     result = Callable::cast(overloads[best]);
   }

src/torque/type-inference.cc

@@ -0,0 +1,118 @@
+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/torque/type-inference.h"
+
+namespace v8 {
+namespace internal {
+namespace torque {
+
+TypeArgumentInference::TypeArgumentInference(
+    const NameVector& type_parameters,
+    const TypeVector& explicit_type_arguments,
+    const std::vector<TypeExpression*> term_parameters,
+    const TypeVector& term_argument_types)
+    : num_explicit_(explicit_type_arguments.size()),
+      type_parameter_from_name_(type_parameters.size()),
+      inferred_(type_parameters.size()) {
+  if (num_explicit_ > type_parameters.size()) {
+    Fail("more explicit type arguments than expected");
+    return;
+  }
+
+  for (size_t i = 0; i < type_parameters.size(); i++) {
+    type_parameter_from_name_[type_parameters[i]->value] = i;
+  }
+  for (size_t i = 0; i < num_explicit_; i++) {
+    inferred_[i] = {explicit_type_arguments[i]};
+  }
+
+  DCHECK_EQ(term_parameters.size(), term_argument_types.size());
+  for (size_t i = 0; i < term_parameters.size(); i++) {
+    Match(term_parameters[i], term_argument_types[i]);
+    if (HasFailed()) return;
+  }
+
+  for (size_t i = 0; i < type_parameters.size(); i++) {
+    if (!inferred_[i]) {
+      Fail("failed to infer arguments for all type parameters");
+      return;
+    }
+  }
+}
+
+TypeVector TypeArgumentInference::GetResult() const {
+  CHECK(!HasFailed());
+  TypeVector result(inferred_.size());
+  std::transform(
+      inferred_.begin(), inferred_.end(), result.begin(),
+      [](base::Optional<const Type*> maybe_type) { return *maybe_type; });
+  return result;
+}
+
+void TypeArgumentInference::Match(TypeExpression* parameter,
+                                  const Type* argument_type) {
+  if (BasicTypeExpression* basic =
+          BasicTypeExpression::DynamicCast(parameter)) {
+    // If the parameter is referring to one of the type parameters, substitute
+    if (basic->namespace_qualification.empty() && !basic->is_constexpr) {
+      auto result = type_parameter_from_name_.find(basic->name);
+      if (result != type_parameter_from_name_.end()) {
+        size_t type_parameter_index = result->second;
+        if (type_parameter_index < num_explicit_) {
+          return;
+        }
+        base::Optional<const Type*>& maybe_inferred =
+            inferred_[type_parameter_index];
+        if (maybe_inferred && *maybe_inferred != argument_type) {
+          Fail("found conflicting types for generic parameter");
+        } else {
+          inferred_[type_parameter_index] = {argument_type};
+        }
+        return;
+      }
+    }
+    // Try to recurse in case of generic types
+    if (!basic->generic_arguments.empty()) {
+      auto* argument_struct_type = StructType::DynamicCast(argument_type);
+      if (argument_struct_type) {
+        MatchGeneric(basic, argument_struct_type);
+      }
+    }
+    // NOTE: We could also check whether ground parameter types match the
+    // argument types, but we are only interested in inferring type arguments
+    // here
+  } else {
+    Fail("unsupported parameter expression");
+  }
+}
+
+void TypeArgumentInference::MatchGeneric(BasicTypeExpression* parameter,
+                                         const StructType* argument_type) {
+  QualifiedName qualified_name{parameter->namespace_qualification,
+                               parameter->name};
+  GenericStructType* generic_struct =
+      Declarations::LookupUniqueGenericStructType(qualified_name);
+  auto& specialized_from = argument_type->GetSpecializedFrom();
+  if (!specialized_from || specialized_from->generic != generic_struct) {
+    return Fail("found conflicting generic type constructors");
+  }
+  auto& parameters = parameter->generic_arguments;
+  auto& argument_types = specialized_from->specialized_types;
+  if (parameters.size() != argument_types.size()) {
+    Error(
+        "cannot infer types from generic-struct-typed parameter with "
+        "incompatible number of arguments")
+        .Position(parameter->pos)
+        .Throw();
+  }
+  for (size_t i = 0; i < parameters.size(); i++) {
+    Match(parameters[i], argument_types[i]);
+    if (HasFailed()) return;
+  }
+}
+
+}  // namespace torque
+}  // namespace internal
+}  // namespace v8

src/torque/type-inference.h

@@ -0,0 +1,78 @@
+// Copyright 2019 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef V8_TORQUE_TYPE_INFERENCE_H_
+#define V8_TORQUE_TYPE_INFERENCE_H_
+
+#include <string>
+#include <unordered_map>
+
+#include "src/base/optional.h"
+#include "src/torque/ast.h"
+#include "src/torque/declarations.h"
+#include "src/torque/types.h"
+
+namespace v8 {
+namespace internal {
+namespace torque {
+
+// Type argument inference computes a potential instantiation of a generic
+// callable given some concrete argument types. As an example, consider the
+// generic macro
+//
+//   macro Pick<T: type>(x: T, y: T): T
+//
+// along with a given call site, such as
+//
+//   Pick(1, 2);
+//
+// The inference proceeds by matching the term argument types (`constexpr
+// int31`, in case of `1` and `2`) against the formal parameter types (`T` in
+// both cases). During this matching we discover that `T` must equal `constexpr
+// int31`.
+//
+// The inference will not perform any comprehensive type checking of its own,
+// but *does* fail if type parameters cannot be soundly instantiated given the
+// call site. For instance, for the following call site
+//
+//   const aSmi: Smi = ...;
+//   Pick(1, aSmi);  // inference fails
+//
+// inference would fail, since `constexpr int31` is distinct from `Smi`. To
+// allow for implicit conversions to be tried in a separate step after type
+// argument inference, a number of type arguments may be given explicitly:
+//
+//   Pick<Smi>(1, aSmi);  // inference succeeds (doing nothing)
+//
+// In the above case the inference simply ignores inconsistent constraints on
+// `T`.
+class TypeArgumentInference {
+ public:
+  TypeArgumentInference(const NameVector& type_parameters,
+                        const TypeVector& explicit_type_arguments,
+                        const std::vector<TypeExpression*> term_parameters,
+                        const TypeVector& term_argument_types);
+
+  bool HasFailed() const { return failure_reason_.has_value(); }
+  const char* GetFailureReason() { return *failure_reason_; }
+  TypeVector GetResult() const;
+
+ private:
+  void Fail(const char* reason) { failure_reason_ = {reason}; }
+
+  void Match(TypeExpression* parameter, const Type* argument_type);
+  void MatchGeneric(BasicTypeExpression* parameter,
+                    const StructType* argument_type);
+
+  size_t num_explicit_;
+  std::unordered_map<std::string, size_t> type_parameter_from_name_;
+  std::vector<base::Optional<const Type*>> inferred_;
+  base::Optional<const char*> failure_reason_;
+};
+
+}  // namespace torque
+}  // namespace internal
+}  // namespace v8
+
+#endif  // V8_TORQUE_TYPE_INFERENCE_H_

src/torque/types.h

@@ -475,6 +475,9 @@ class StructType final : public AggregateType {
   std::string ToExplicitString() const override;
   std::string GetGeneratedTypeNameImpl() const override;
   std::string MangledName() const override;
+  const MaybeSpecializationKey& GetSpecializedFrom() const {
+    return specialized_from_;
+  }
 
   static base::Optional<const Type*> MatchUnaryGeneric(
       const Type* type, GenericStructType* generic);

test/torque/test-torque.tq

@@ -898,8 +898,7 @@ namespace test {
     const ref:&Smi = & array.a;
     * ref = 3 + * ref;
     -- * ref;
-    // TODO(gsps): Remove explicit type arg once inference works
-    Swap<Smi>(& array.b, array.GetA());
+    Swap(& array.b, array.GetA());
     check(array.a == 2);
     check(array.b == 9);
   }
@@ -1002,7 +1001,7 @@ namespace test {
   @export
   macro TestGenericStruct2(): TestTuple<Smi, intptr> {
     const intptrAndSmi = TestTuple<intptr, Smi>{fst: 1, snd: 2};
-    const smiAndIntptr = TupleSwap<intptr, Smi>(intptrAndSmi);
+    const smiAndIntptr = TupleSwap(intptrAndSmi);
     check(intptrAndSmi.fst == smiAndIntptr.snd);
     check(intptrAndSmi.snd == smiAndIntptr.fst);
     return smiAndIntptr;