RequirementMachine: Narrow fix for concrete type requirements with opaque archetype

Fixes rdar://problem/90665615.

Author: slavapestov

lib/AST/RequirementMachine/PropertyUnification.cpp

@@ -491,9 +491,9 @@ void PropertyMap::concretizeNestedTypesFromConcreteParent(
       continue;
     }
 
-    // FIXME: Maybe this can happen if the concrete type is an
-    // opaque result type?
-    assert(!conformance.isAbstract());
+    // This can happen if the concrete type is an opaque result type.
+    if (conformance.isAbstract())
+      continue;
 
     auto *concrete = conformance.getConcrete();
 

test/Generics/opaque_archetype_concrete_requirement_recursive.swift

@@ -0,0 +1,30 @@
+// RUN: %target-swift-frontend -emit-silgen %s -disable-availability-checking
+
+protocol P {
+  associatedtype T
+
+  var t: T { get }
+}
+
+protocol RecursiveP {
+  associatedtype T : RecursiveP
+}
+
+struct S_RecursiveP : RecursiveP {
+  typealias T = S_RecursiveP
+}
+
+struct DefinesRecursiveP : P {
+  var t: some RecursiveP {
+    return S_RecursiveP()
+  }
+}
+
+protocol HasRecursiveP {
+  associatedtype T : RecursiveP
+}
+
+extension HasRecursiveP where T == DefinesRecursiveP.T {
+  func checkSameType(_ t: T) -> DefinesRecursiveP.T { return t }
+}
+

test/Generics/sr16040.swift

@@ -0,0 +1,40 @@
+// RUN: %target-swift-frontend -typecheck -verify %s -disable-availability-checking
+
+public protocol View {
+  associatedtype Body : View
+  var body: Body { get }
+}
+
+public struct Text : View {
+  public init(_: String) {}
+  public var body: Self { return self }
+}
+
+public protocol DisplayableValue {}
+
+public protocol SingleValueDisplay: View {
+  associatedtype DisplayedValue
+  init (_ singleValue: DisplayedValue)
+  var displayedValue: DisplayedValue { get }
+}
+
+public protocol RawDisplayableValue: DisplayableValue {
+  associatedtype RawDisplay: SingleValueDisplay
+    where RawDisplay.DisplayedValue == Self
+}
+
+public protocol RawTextDisplayableValue: RawDisplayableValue
+  where Self: CustomStringConvertible,
+        RawDisplay == RawTextDisplay<Self> { }
+
+public struct RawTextDisplay <Value: CustomStringConvertible>: SingleValueDisplay {
+  public var displayedValue: Value
+  
+  public init (_ singleValue: Value) {
+    self.displayedValue = singleValue
+  }
+  
+  public var body: some View {
+    Text(displayedValue.description)
+  }
+}