Sema: Pass substitution options to swift::checkTypeWitness

...to propagate tentative type witnesses when validating a solution to associated type inference

Author: AnthonyLatsis

lib/Sema/TypeCheckProtocol.cpp

@@ -3788,9 +3788,10 @@ ResolveWitnessResult ConformanceChecker::resolveWitnessViaDefault(
 
 # pragma mark Type witness resolution
 
-CheckTypeWitnessResult swift::checkTypeWitness(Type type,
-                                               AssociatedTypeDecl *assocType,
-                                               NormalProtocolConformance *Conf) {
+CheckTypeWitnessResult
+swift::checkTypeWitness(Type type, AssociatedTypeDecl *assocType,
+                        const NormalProtocolConformance *Conf,
+                        SubstOptions options) {
   if (type->hasError())
     return ErrorType::get(assocType->getASTContext());
 
@@ -3804,13 +3805,19 @@ CheckTypeWitnessResult swift::checkTypeWitness(Type type,
                                               : type;
 
   if (auto superclass = genericSig->getSuperclassBound(depTy)) {
-    // If the superclass has a type parameter, substitute in known type
-    // witnesses.
     if (superclass->hasTypeParameter()) {
-      const auto subMap = SubstitutionMap::getProtocolSubstitutions(
-          proto, Conf->getType(), ProtocolConformanceRef(Conf));
-
-      superclass = superclass.subst(subMap);
+      // Replace type parameters with other known or tentative type witnesses.
+      superclass = superclass.subst(
+          [&](SubstitutableType *type) {
+            if (type->isEqual(proto->getSelfInterfaceType()))
+              return Conf->getType();
+
+            return Type();
+          },
+          LookUpConformanceInModule(dc->getParentModule()), options);
+
+      if (superclass->hasTypeParameter())
+        superclass = dc->mapTypeIntoContext(superclass);
     }
     if (!superclass->isExactSuperclassOf(contextType))
       return superclass;

lib/Sema/TypeCheckProtocol.h

@@ -96,7 +96,8 @@ class CheckTypeWitnessResult {
 /// \returns an empty result on success, or a description of the error.
 CheckTypeWitnessResult checkTypeWitness(Type type,
                                         AssociatedTypeDecl *assocType,
-                                        NormalProtocolConformance *Conf);
+                                        const NormalProtocolConformance *Conf,
+                                        SubstOptions options = None);
 
 /// Describes the means of inferring an abstract type witness.
 enum class AbstractTypeWitnessKind : uint8_t {

lib/Sema/TypeCheckProtocolInference.cpp

@@ -1172,6 +1172,7 @@ AssociatedTypeDecl *AssociatedTypeInference::completeSolution(
 
   // Check each abstract type witness we computed against the generic
   // requirements on the corresponding associated type.
+  const auto substOptions = getSubstOptionsWithCurrentTypeWitnesses();
   for (const auto &witness : abstractTypeWitnesses) {
     Type type = witness.getType();
     if (type->hasTypeParameter()) {
@@ -1184,8 +1185,7 @@ AssociatedTypeDecl *AssociatedTypeInference::completeSolution(
 
               return Type();
             },
-            LookUpConformanceInModule(dc->getParentModule()),
-            getSubstOptionsWithCurrentTypeWitnesses());
+            LookUpConformanceInModule(dc->getParentModule()), substOptions);
 
         // If the substitution produced an error, we're done.
         if (type->hasError())
@@ -1194,8 +1194,8 @@ AssociatedTypeDecl *AssociatedTypeInference::completeSolution(
       type = dc->mapTypeIntoContext(type);
     }
 
-    if (const auto &failed =
-            checkTypeWitness(type, witness.getAssocType(), conformance)) {
+    if (const auto &failed = checkTypeWitness(type, witness.getAssocType(),
+                                              conformance, substOptions)) {
       // We failed to satisfy a requirement. If this is a default type
       // witness failure and we haven't seen one already, write it down.
       if (witness.getKind() == AbstractTypeWitnessKind::Default &&

test/decl/protocol/conforms/associated_type.swift

@@ -94,27 +94,8 @@ struct SR_12707_Conform_P2: SR_12707_P2 {
   typealias A = Never
 }
 
-// Default type witness
-protocol SR_12707_P3 {
-  associatedtype A
-  associatedtype B: SR_12707_C<(A, Self)> = SR_12707_C<(A, Self)>
-}
-struct SR_12707_Conform_P3: SR_12707_P3 {
-  typealias A = Never
-}
-
-// FIXME: Type witness resolution success is order-dependent.
-protocol SR_12707_FIXME_P1 {
-  associatedtype A
-}
-protocol SR_12707_FIXME_P2: SR_12707_FIXME_P1 {
-  associatedtype B: SR_12707_C<(A, Self)> = SR_12707_C<(A, Self)> // expected-note {{default type 'associated_type.SR_12707_C<(associated_type.SR_12707_FIXME_Conform_P2.A, associated_type.SR_12707_FIXME_Conform_P2)>' (aka 'SR_12707_C<(Never, SR_12707_FIXME_Conform_P2)>') for associated type 'B' (from protocol 'SR_12707_FIXME_P2') does not inherit from 'associated_type.SR_12707_C<(associated_type.SR_12707_FIXME_Conform_P2.A, associated_type.SR_12707_FIXME_Conform_P2)>'}}
-}
-struct SR_12707_FIXME_Conform_P2: SR_12707_FIXME_P2 { // expected-error {{type 'SR_12707_FIXME_Conform_P2' does not conform to protocol 'SR_12707_FIXME_P2'}}
-  typealias A = Never
-}
-
-// FIXME: Type witness resolution success is order-dependent.
+// FIXME: resolveTypeWitnessViaLookup must not happen independently in the
+// general case.
 protocol SR_12707_FIXME_P3 {
   associatedtype A: SR_12707_C<B> // expected-note {{protocol requires nested type 'A'; do you want to add it?}}
   associatedtype B
@@ -123,3 +104,27 @@ struct SR_12707_FIXME_Conform_P3: SR_12707_FIXME_P3 { // expected-error {{type '
   typealias A = SR_12707_C<B> // expected-note {{possibly intended match 'SR_12707_FIXME_Conform_P3.A' (aka 'SR_12707_C<Never>') does not inherit from 'SR_12707_C<SR_12707_FIXME_Conform_P3.B>'}}
   typealias B = Never
 }
+
+// FIXME: Associated type inference via value witnesses should consider
+// tentative witnesses when checking a candidate.
+protocol SR_12707_FIXME_P4 {
+  associatedtype X = Never
+
+  associatedtype A: SR_12707_C<X> // expected-note {{unable to infer associated type 'A' for protocol 'SR_12707_FIXME_P4'}}
+  func foo(arg: A)
+}
+struct SR_12707_FIXME_Conform_P4: SR_12707_FIXME_P4 { // expected-error {{type 'SR_12707_FIXME_Conform_P4' does not conform to protocol 'SR_12707_FIXME_P4'}}
+  func foo(arg: SR_12707_C<Never>) {} // expected-note {{candidate would match and infer 'A' = 'SR_12707_C<Never>' if 'SR_12707_C<Never>' inherited from 'SR_12707_C<SR_12707_FIXME_Conform_P4.X>'}}
+}
+
+// Abstract type witnesses.
+protocol SR_12707_P5a {
+  associatedtype X = Never
+
+  associatedtype A: SR_12707_C<X>
+  associatedtype B: SR_12707_C<X>
+}
+protocol SR_12707_P5b: SR_12707_P5a where B == SR_12707_C<X> {
+  associatedtype C: SR_12707_C<Self> = SR_12707_C<Self>
+}
+struct SR_12707_Conform_P5<A: SR_12707_C<Never>>: SR_12707_P5b {}