[IDE] Inspect all equivalent type variables to replace a type variable by an archetype

Sometimes the type variable itself doesn't have have an originator that can be replaced by an archetype but one of its equivalent type variable does.
Search thorough all equivalent type variables, looking for one that can be replaced by a generic parameter.

Author: ahoppen

lib/Sema/ConstraintSystem.cpp

@@ -3814,7 +3814,7 @@ Type Solution::simplifyTypeForCodeCompletion(Type Ty) const {
 
   // Replace all type variables (which must come from placeholders) by their
   // generic parameters. Because we call into simplifyTypeImpl
-  Ty = CS.simplifyTypeImpl(Ty, [&CS](TypeVariableType *typeVar) -> Type {
+  Ty = CS.simplifyTypeImpl(Ty, [&CS, this](TypeVariableType *typeVar) -> Type {
     // Code completion depends on generic parameter type being represented in
     // terms of `ArchetypeType` since it's easy to extract protocol requirements
     // from it.
@@ -3831,6 +3831,29 @@ Type Solution::simplifyTypeForCodeCompletion(Type Ty) const {
       return archetype;
     }
 
+    // Sometimes the type variable itself doesn't have have an originator that
+    // can be replaced by an archetype but one of its equivalent type variable
+    // does.
+    // Search thorough all equivalent type variables, looking for one that can
+    // be replaced by a generic parameter.
+    std::vector<std::pair<TypeVariableType *, Type>> bindings(
+        typeBindings.begin(), typeBindings.end());
+    // Make sure we iterate the bindings in a deterministic order.
+    llvm::sort(bindings, [](const std::pair<TypeVariableType *, Type> &lhs,
+                            const std::pair<TypeVariableType *, Type> &rhs) {
+      return lhs.first->getID() < rhs.first->getID();
+    });
+    for (auto binding : bindings) {
+      if (auto placeholder = binding.second->getAs<PlaceholderType>()) {
+        if (placeholder->getOriginator().dyn_cast<TypeVariableType *>() ==
+            typeVar) {
+          if (auto archetype = getTypeVarAsArchetype(binding.first)) {
+            return archetype;
+          }
+        }
+      }
+    }
+
     // When applying the logic below to get contextual types inside result
     // builders, the code completion type variable is connected by a one-way
     // constraint to a type variable in the buildBlock call, but that is not the

test/IDE/complete_subscript.swift

@@ -39,7 +39,7 @@ func test1() {
 
   let _ = MyStruct[#^METATYPE_UNRESOLVED_BRACKET^#
 // METATYPE_UNRESOLVED_BRACKET: Begin completions
-// METATYPE_UNRESOLVED_BRACKET-DAG: Decl[Subscript]/CurrNominal/Flair[ArgLabels]:        ['[']{#(x): Int#}, {#static: _#}[']'][#MyStruct<_>#];
+// METATYPE_UNRESOLVED_BRACKET-DAG: Decl[Subscript]/CurrNominal/Flair[ArgLabels]:        ['[']{#(x): Int#}, {#static: T#}[']'][#MyStruct<T>#];
 // METATYPE_UNRESOLVED_BRACKET: End completions
 
   let _ = MyStruct<Int> #^METATYPE_INT^#