Sema: Workaround for issues with openType() and generic parameter packs

Author: slavapestov

lib/Sema/ConstraintSystem.cpp

@@ -1009,8 +1009,11 @@ Type ConstraintSystem::openType(Type type, OpenedTypeMap &replacements,
       // that gets introduced by the interface type, see
       // \c openUnboundGenericType for more details.
       if (auto *packTy = type->getAs<PackType>()) {
-        if (auto expansion = packTy->unwrapSingletonPackExpansion())
-          type = expansion->getPatternType();
+        if (auto expansion = packTy->unwrapSingletonPackExpansion()) {
+          auto patternTy = expansion->getPatternType();
+          if (patternTy->is<GenericTypeParamType>())
+            type = patternTy;
+        }
       }
 
       if (auto *expansion = type->getAs<PackExpansionType>()) {

validation-test/compiler_crashers_2_fixed/rdar112108253.swift

@@ -6,7 +6,12 @@ public protocol P {
 
 public struct G<each T> {}
 
-public struct S<T: P, each U: P> where repeat T.A == G<repeat each U> {
-    public init(predicate: repeat each U) {}
+public protocol Q {
+  init()
+}
+
+public struct S<T: P, each U: P>: Q where repeat T.A == G<repeat each U.A> {
+  public init() {}
+  public init(predicate: repeat each U) {}
 }