AST: Try harder to preserve type sugar in AbstractGenericSignatureRequest

AbstractGenericSignatureRequest tries to minimize the number of GSBs that we
spin up by only creating a GSB if the generic parameter and requirement types
are canonical. If they're not canonical, it first canonicalizes them, then
kicks off a request to compute the canonical signature, and finally, re-applies
type sugar.

We would do this by building a mapping for re-sugaring generic parameters,
however this mapping was only populated for the newly-added generic parameters.

If some of the newly-added generic requirements mention the base signature's
generic parameters, they would remain canonicalized.

Fixes <rdar://problem/67579220>.

Author: slavapestov

lib/AST/GenericSignatureBuilder.cpp

@@ -7430,13 +7430,11 @@ AbstractGenericSignatureRequest::evaluate(
     if (baseSignature)
       canBaseSignature = baseSignature->getCanonicalSignature();
 
-    llvm::SmallDenseMap<GenericTypeParamType *, Type> mappedTypeParameters;
     SmallVector<GenericTypeParamType *, 2> canAddedParameters;
     canAddedParameters.reserve(addedParameters.size());
     for (auto gp : addedParameters) {
       auto canGP = gp->getCanonicalType()->castTo<GenericTypeParamType>();
       canAddedParameters.push_back(canGP);
-      mappedTypeParameters[canGP] = Type(gp);
     }
 
     SmallVector<Requirement, 2> canAddedRequirements;
@@ -7453,10 +7451,8 @@ AbstractGenericSignatureRequest::evaluate(
     if (!canSignatureResult || !*canSignatureResult)
       return GenericSignature();
 
-    // Substitute in the original generic parameters to form a more-sugared
-    // result closer to what the original request wanted. Note that this
-    // loses sugar on concrete types, but for abstract signatures that
-    // shouldn't matter.
+    // Substitute in the original generic parameters to form the sugared
+    // result the original request wanted.
     auto canSignature = *canSignatureResult;
     SmallVector<GenericTypeParamType *, 2> resugaredParameters;
     resugaredParameters.reserve(canSignature->getGenericParams().size());
@@ -7474,9 +7470,8 @@ AbstractGenericSignatureRequest::evaluate(
       auto resugaredReq = req.subst(
           [&](SubstitutableType *type) {
             if (auto gp = dyn_cast<GenericTypeParamType>(type)) {
-              auto knownGP = mappedTypeParameters.find(gp);
-              if (knownGP != mappedTypeParameters.end())
-                return knownGP->second;
+              unsigned ordinal = canSignature->getGenericParamOrdinal(gp);
+              return Type(resugaredParameters[ordinal]);
             }
             return Type(type);
           },

test/ModuleInterface/inherited-generic-parameters.swift

@@ -19,6 +19,8 @@ public class Base<In, Out> {
 // CHECK-NEXT: public init<A>(_: A, _: A)
   public init<A>(_: A, _: A) {}
 
+// CHECK-NEXT: public init<C>(_: C) where C : main.Base<In, Out>
+  public init<C>(_: C) where C : Base<In, Out> {}
 // CHECK: }
 }
 
@@ -27,6 +29,7 @@ public class Derived<T> : Base<T, T> {
 // CHECK-NEXT: {{(@objc )?}}deinit
 // CHECK-NEXT: override public init(x: @escaping (T) -> T)
 // CHECK-NEXT: override public init<A>(_ argument: A, _ argument: A)
+// CHECK-NEXT: override public init<C>(_ argument: C) where C : main.Base<T, T>
 // CHECK-NEXT: }
 }
 

test/attr/attr_override.swift

@@ -590,7 +590,7 @@ class SR_4206_Base_7<T> {
 }
 
 class SR_4206_Derived_7<T>: SR_4206_Base_7<T> {
-  override func foo1() where T: SR_4206_Protocol_2 {} // expected-error {{overridden method 'foo1' has generic signature <T where T : SR_4206_Protocol_2> which is incompatible with base method's generic signature <T where T : SR_4206_Protocol_1>; expected generic signature to be <T where τ_0_0 : SR_4206_Protocol_1>}}
+  override func foo1() where T: SR_4206_Protocol_2 {} // expected-error {{overridden method 'foo1' has generic signature <T where T : SR_4206_Protocol_2> which is incompatible with base method's generic signature <T where T : SR_4206_Protocol_1>; expected generic signature to be <T where T : SR_4206_Protocol_1>}}
 
   override func foo2() {} // OK
 }
@@ -627,7 +627,7 @@ class SR_4206_Base_10<T> {
   func foo() where T: SR_4206_Protocol_1 {} // expected-note {{overridden declaration is here}}
 }
 class SR_4206_Derived_10<T, U>: SR_4206_Base_10<T> {
-  override func foo() where U: SR_4206_Protocol_1 {} // expected-error {{overridden method 'foo' has generic signature <T, U where U : SR_4206_Protocol_1> which is incompatible with base method's generic signature <T where T : SR_4206_Protocol_1>; expected generic signature to be <T, U where τ_0_0 : SR_4206_Protocol_1>}}
+  override func foo() where U: SR_4206_Protocol_1 {} // expected-error {{overridden method 'foo' has generic signature <T, U where U : SR_4206_Protocol_1> which is incompatible with base method's generic signature <T where T : SR_4206_Protocol_1>; expected generic signature to be <T, U where T : SR_4206_Protocol_1>}}
 }
 
 // Override with return type specialization