Merge pull request #21498 from DougGregor/revert-reasonable-gsb-change-5.0

[5.0] Revert "[GSB] When adding same-type requirements pick representative based on canonical order

Author: DougGregor

lib/AST/GenericSignatureBuilder.cpp

@@ -4801,8 +4801,12 @@ GenericSignatureBuilder::addSameTypeRequirementBetweenTypeParameters(
   auto T1 = OrigT1->getRepresentative();
   auto T2 = OrigT2->getRepresentative();
 
-  // Pick representative based on the canonical ordering of the type parameters.
-  if (compareDependentTypes(depType2, depType1) < 0) {
+  // Decide which potential archetype is to be considered the representative.
+  // We prefer potential archetypes with lower nesting depths, because it
+  // prevents us from unnecessarily building deeply nested potential archetypes.
+  unsigned nestingDepth1 = T1->getNestingDepth();
+  unsigned nestingDepth2 = T2->getNestingDepth();
+  if (nestingDepth2 < nestingDepth1) {
     std::swap(T1, T2);
     std::swap(OrigT1, OrigT2);
     std::swap(equivClass, equivClass2);

test/Generics/rdar45957015.swift

@@ -353,9 +353,9 @@ func intercomponentSameComponents<T: P10>(_: T)
 func intercomponentMoreThanSpanningTree<T: P10>(_: T)
   where T.A == T.B,
         T.B == T.C,
-        T.D == T.E, // expected-note {{previous same-type constraint 'T.D' == 'T.E' written here}}
+        T.D == T.E, // expected-note{{previous same-type constraint 'T.D' == 'T.E' written here}}
         T.D == T.B,
-        T.E == T.B  // expected-warning {{redundant same-type constraint 'T.E' == 'T.B'}}
+        T.E == T.B  // expected-warning{{redundant same-type constraint 'T.E' == 'T.B'}}
         { }
 
 func trivialRedundancy<T: P10>(_: T) where T.A == T.A { } // expected-warning{{redundant same-type constraint 'T.A' == 'T.A'}}

test/Generics/same_type_constraints.swift

@@ -15,7 +15,7 @@ protocol P1 {
 }
 
 protocol P2 {
-  associatedtype T // expected-note 2 {{found this candidate}}
+  associatedtype T // expected-note {{found this candidate}}
 }
 
 // FIXME: This extension's generic signature is still minimized differently from
@@ -36,7 +36,7 @@ extension P1 where Self : P2 {
 // Same as above, but now we have two visible associated types with the same
 // name.
 protocol P3 {
-  associatedtype T // expected-note {{found this candidate}}
+  associatedtype T
 }
 
 // FIXME: This extension's generic signature is still minimized differently from
@@ -48,7 +48,7 @@ extension P2 where Self : P3, T == Int {
 }
 
 extension P2 where Self : P3 {
-  func takeT1(_: T) {} // expected-error {{'T' is ambiguous for type lookup in this context}}
+  func takeT1(_: T) {}
   func takeT2(_: Self.T) {}
 }