Sema: Pass down tentative type witnesses in substCurrentTypeWitnesses()

Author: slavapestov

lib/Sema/TypeCheckProtocolInference.cpp

@@ -1126,6 +1126,8 @@ void AssociatedTypeInference::collectAbstractTypeWitnesses(
 }
 
 Type AssociatedTypeInference::substCurrentTypeWitnesses(Type type) {
+  auto substOptions = getSubstOptionsWithCurrentTypeWitnesses();
+
   // Local function that folds dependent member types with non-dependent
   // bases into actual member references.
   std::function<Type(Type)> foldDependentMemberTypes;
@@ -1148,7 +1150,8 @@ Type AssociatedTypeInference::substCurrentTypeWitnesses(Type type) {
       auto *module = dc->getParentModule();
 
       // Try to substitute into the base type.
-      Type result = depMemTy->substBaseType(module, baseTy);
+      Type result = depMemTy->substBaseType(
+          baseTy, LookUpConformanceInModule(module), substOptions);
       if (!result->hasError())
         return result;
 

test/decl/protocol/req/associated_type_inference_stdlib.swift

@@ -0,0 +1,34 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-associated-type-inference
+// RUN: %target-typecheck-verify-swift
+
+// We would fail here when Indices was explicitly specified, as in X2.
+
+public struct X1: RandomAccessCollection {
+  public var startIndex: Int {
+    return 0
+  }
+
+  public var endIndex: Int {
+    return 0
+  }
+
+  public subscript(position: Int) -> String {
+    return ""
+  }
+}
+
+public struct X2: RandomAccessCollection {
+  public typealias Indices = Range<Int>
+
+  public var startIndex: Int {
+    return 0
+  }
+
+  public var endIndex: Int {
+    return 0
+  }
+
+  public subscript(position: Int) -> String {
+    return ""
+  }
+}