SIL: Use correct generic signature when computing yield types for witness thunks

Yield types are not represented in the AST FunctionType, so when we compute
the lowered type of a witness thunk for a 'modify' or 'read' coroutine, we
have to compute the yield type from scratch.

We do this by applying the witness substitutions computed by Sema to the
storage type, and then canonicalizing the resulting substituted type with
respect to the storage's own generic signature.

However, the right hand sides of the witness substitutions are written with
respect to the conformance context, which might be a subclass of the class
that the storage is originally defined in.

By not using the generic signature of this subclass, we could miss associated
types of generic parameters of the base class which were made concrete in the
subclass using a 'where' clause.

Instead, let's pass down the generic signature of the witness thunk, ensuring
we always compute the correct canonical type.

Fixes rdar://problem/77737914.

Author: slavapestov

lib/SIL/IR/SILFunctionType.cpp

@@ -1988,6 +1988,7 @@ static void destructureYieldsForCoroutine(TypeConverter &TC,
                                           Optional<SILDeclRef> origConstant,
                                           Optional<SILDeclRef> constant,
                                           Optional<SubstitutionMap> reqtSubs,
+                                          Optional<GenericSignature> genericSig,
                                           SmallVectorImpl<SILYieldInfo> &yields,
                                           SILCoroutineKind &coroutineKind,
                                           SubstFunctionTypeCollector &subst) {
@@ -2012,12 +2013,14 @@ static void destructureYieldsForCoroutine(TypeConverter &TC,
 
   auto storage = accessor->getStorage();
   auto valueType = storage->getValueInterfaceType();
+
   if (reqtSubs) {
     valueType = valueType.subst(*reqtSubs);
   }
 
-  auto canValueType = valueType->getCanonicalType(
-    accessor->getGenericSignature());
+  auto canValueType = (genericSig
+                       ? valueType->getCanonicalType(*genericSig)
+                       : valueType->getCanonicalType());
 
   // 'modify' yields an inout of the target type.
   if (accessor->getAccessorKind() == AccessorKind::Modify) {
@@ -2178,7 +2181,8 @@ static CanSILFunctionType getSILFunctionType(
   SILCoroutineKind coroutineKind = SILCoroutineKind::None;
   SmallVector<SILYieldInfo, 8> yields;
   destructureYieldsForCoroutine(TC, expansionContext, origConstant, constant,
-                                reqtSubs, yields, coroutineKind, subst);
+                                reqtSubs, genericSig, yields, coroutineKind,
+                                subst);
 
   // Destructure the result tuple type.
   SmallVector<SILResultInfo, 8> results;

test/SILGen/witness-modify-requirement-with-base-class-modify.swift

@@ -0,0 +1,27 @@
+// RUN: %target-swift-emit-silgen %s | %FileCheck %s
+
+public protocol P {
+  associatedtype A
+}
+
+public class Base<T: P> {
+  public var foo: T.A?
+}
+
+public struct S {}
+
+public protocol Q {
+  var foo: S? {set get}
+}
+
+public class Derived<T: P> : Base<T>, Q where T.A == S {}
+
+// CHECK-LABEL: sil shared [transparent] [serialized] [thunk] [ossa] @$s4main7DerivedCyxGAA1QA2aEP3fooAA1SVSgvgTW : $@convention(witness_method: Q) <τ_0_0 where τ_0_0 : P, τ_0_0.A == S> (@in_guaranteed Derived<τ_0_0>) -> Optional<S> {
+// CHECK-LABEL: sil shared [transparent] [serialized] [thunk] [ossa] @$s4main7DerivedCyxGAA1QA2aEP3fooAA1SVSgvsTW : $@convention(witness_method: Q) <τ_0_0 where τ_0_0 : P, τ_0_0.A == S> (Optional<S>, @inout Derived<τ_0_0>) -> () {
+// CHECK-LABEL: sil shared [transparent] [serialized] [thunk] [ossa] @$s4main7DerivedCyxGAA1QA2aEP3fooAA1SVSgvMTW : $@yield_once @convention(witness_method: Q) <τ_0_0 where τ_0_0 : P, τ_0_0.A == S> @substituted <τ_0_0> (@inout τ_0_0) -> @yields @inout Optional<S> for <Derived<τ_0_0>> {
+
+// CHECK-LABEL: sil_witness_table [serialized] <T where T : P, T.A == S> Derived<T>: Q module main {
+// CHECK-NEXT:    method #Q.foo!getter: <Self where Self : Q> (Self) -> () -> S? : @$s4main7DerivedCyxGAA1QA2aEP3fooAA1SVSgvgTW
+// CHECK-NEXT:    method #Q.foo!setter: <Self where Self : Q> (inout Self) -> (S?) -> () : @$s4main7DerivedCyxGAA1QA2aEP3fooAA1SVSgvsTW
+// CHECK-NEXT:    method #Q.foo!modify: <Self where Self : Q> (inout Self) -> () -> () : @$s4main7DerivedCyxGAA1QA2aEP3fooAA1SVSgvMTW
+// CHECK-NEXT:  }