[ConstraintSystem] Allow solving same-type requirements for associated types

where the base does not conform to the associated type's protocol.

We can only reach this case when the solver has already applied a fix for
the conformance failure.

Author: hborla

lib/Sema/CSSimplify.cpp

@@ -3331,10 +3331,13 @@ bool ConstraintSystem::repairFailures(
     if (lhs->hasHole() || rhs->hasHole())
       return true;
 
-    // If dependent members are present here it's because
-    // base doesn't conform to associated type's protocol.
-    if (lhs->hasDependentMember() || rhs->hasDependentMember())
-      break;
+    // If dependent members are present here it's because the base doesn't
+    // conform to the associated type's protocol. We can only get here if we
+    // already applied a fix for the conformance failure.
+    if (lhs->hasDependentMember() || rhs->hasDependentMember()) {
+      increaseScore(SK_Fix);
+      return true;
+    }
 
     // If requirement is something like `T == [Int]` let's let
     // type matcher a chance to match generic parameters before
@@ -3949,12 +3952,14 @@ ConstraintSystem::matchTypes(Type type1, Type type2, ConstraintKind kind,
       llvm_unreachable("type variables should have already been handled by now");
 
     case TypeKind::DependentMember: {
-      // If one of the dependent member types has no type variables,
-      // this comparison is effectively illformed, because dependent
-      // member couldn't be simplified down to the actual type, and
-      // we wouldn't be able to solve this constraint, so let's just fail.
-      if (!desugar1->hasTypeVariable() || !desugar2->hasTypeVariable())
-        return getTypeMatchFailure(locator);
+      // If one of the dependent member types has no type variables, the
+      // dependent member can't be simplified because the base doesn't conform
+      // to the associated type's protocol. We can only get here if we already
+      // applied a fix for the conformance failure.
+      if (!desugar1->hasTypeVariable() || !desugar2->hasTypeVariable()) {
+        increaseScore(SK_Fix);
+        return getTypeMatchSuccess();
+      }
 
       // Nothing we can solve yet, since we need to wait until
       // type variables will get resolved.

test/Constraints/same_types.swift

@@ -333,3 +333,25 @@ class R<T>: P7 where T: P7, T.A == T.Type { // expected-note {{'T' declared as p
 
 R.fn(args: R.self) // expected-error {{generic parameter 'T' could not be inferred}}
 // expected-note@-1 {{explicitly specify the generic arguments to fix this issue}}
+
+// rdar://problem/58607155
+protocol AssocType1 { associatedtype A }
+protocol AssocType2 { associatedtype A }
+
+func rdar58607155() {
+  func f<T1: AssocType1, T2: AssocType2>(t1: T1, t2: T2) where T1.A == T2.A {}
+  // expected-note@-1 2 {{where 'T2' = 'MissingConformance'}}
+  // expected-note@-2 2 {{where 'T1' = 'MissingConformance'}}
+
+  class Conformance: AssocType1, AssocType2 { typealias A = Int }
+  class MissingConformance {}
+
+  // One generic argument has a conformance failure
+  f(t1: MissingConformance(), t2: Conformance()) // expected-error {{local function 'f(t1:t2:)' requires that 'MissingConformance' conform to 'AssocType1'}}
+  f(t1: Conformance(), t2: MissingConformance()) // expected-error {{local function 'f(t1:t2:)' requires that 'MissingConformance' conform to 'AssocType2'}}
+
+  // Both generic arguments have a conformance failure
+  f(t1: MissingConformance(), t2: MissingConformance())
+  // expected-error@-1 {{local function 'f(t1:t2:)' requires that 'MissingConformance' conform to 'AssocType1'}}
+  // expected-error@-2 {{local function 'f(t1:t2:)' requires that 'MissingConformance' conform to 'AssocType2'}}
+}