IRGen: Fix crash when emitting type nested inside constrained extension

getFormalTypeInContext() was using the context archetypes of the
original parent type, and not the extension.

Author: slavapestov

lib/IRGen/GenValueWitness.cpp

@@ -419,17 +419,14 @@ static Address getArgAsBuffer(IRGenFunction &IGF,
   return Address(arg, getFixedBufferAlignment(IGF.IGM));
 }
 
-/// Given an abstract type --- a type possibly expressed in terms of
-/// unbound generic types --- return the formal type within the type's
-/// primary defining context.
-static CanType getFormalTypeInContext(CanType abstractType) {
+static CanType getFormalTypeInContext(CanType abstractType, DeclContext *dc) {
   // Map the parent of any non-generic nominal type.
   if (auto nominalType = dyn_cast<NominalType>(abstractType)) {
     // If it doesn't have a parent, or the parent doesn't need remapping,
     // do nothing.
     auto abstractParentType = nominalType.getParent();
     if (!abstractParentType) return abstractType;
-    auto parentType = getFormalTypeInContext(abstractParentType);
+    auto parentType = getFormalTypeInContext(abstractParentType, dc);
     if (abstractParentType == parentType) return abstractType;
 
     // Otherwise, rebuild the type.
@@ -438,14 +435,24 @@ static CanType getFormalTypeInContext(CanType abstractType) {
 
   // Map unbound types into their defining context.
   } else if (auto ugt = dyn_cast<UnboundGenericType>(abstractType)) {
-    return ugt->getDecl()->getDeclaredTypeInContext()->getCanonicalType();
+    return dc->mapTypeIntoContext(ugt->getDecl()->getDeclaredInterfaceType())
+        ->getCanonicalType();
 
   // Everything else stays the same.
   } else {
     return abstractType;
   }
 }
 
+/// Given an abstract type --- a type possibly expressed in terms of
+/// unbound generic types --- return the formal type within the type's
+/// primary defining context.
+static CanType getFormalTypeInContext(CanType abstractType) {
+  if (auto nominal = abstractType.getAnyNominal())
+    return getFormalTypeInContext(abstractType, nominal);
+  return abstractType;
+}
+
 /// Get the next argument and use it as the 'self' type metadata.
 static void getArgAsLocalSelfTypeMetadata(IRGenFunction &IGF,
                                           llvm::Function::arg_iterator &it,

test/IRGen/nested_generics.swift

@@ -79,3 +79,16 @@ public class OuterGenericClass<T> {
     }
   }
 }
+
+// This used to crash while emitting value witnesses.
+
+public struct Fish<Water> {}
+
+public protocol Wet {}
+
+extension Fish where Water : Wet {
+  public enum Fillet {
+    case grilled
+    case fried
+  }
+}