[code-completion] Don't try to equate/convert GenericTypeParameterTypes computing type relations of completion results

For call argument completion, if the expected type and completion result type are
both GenericTypeParameterTypes, comparing them doesn't make sense we can't account
for their different contexts at this point in the code, and hit assertions in the
constraint solver if we try.

For now, don't attempt to add a type relation for this case.
Resolves rdar://problem/38153332.

Author: Nathan Hawes

lib/IDE/CodeCompletion.cpp

@@ -939,10 +939,15 @@ static CodeCompletionResult::ExpectedTypeRelation calculateTypeRelation(
       Ty->is<ErrorType>() ||
       ExpectedTy->is<ErrorType>())
     return CodeCompletionResult::ExpectedTypeRelation::Unrelated;
-  if (Ty->isEqual(ExpectedTy))
-    return CodeCompletionResult::ExpectedTypeRelation::Identical;
-  if (isConvertibleTo(Ty, ExpectedTy, *DC))
-    return CodeCompletionResult::ExpectedTypeRelation::Convertible;
+
+  // Equality/Conversion of GenericTypeParameterType won't account for
+  // requirements – ignore them
+  if (!Ty->hasTypeParameter() && !ExpectedTy->hasTypeParameter()) {
+    if (Ty->isEqual(ExpectedTy))
+      return CodeCompletionResult::ExpectedTypeRelation::Identical;
+    if (isConvertibleTo(Ty, ExpectedTy, *DC))
+      return CodeCompletionResult::ExpectedTypeRelation::Convertible;
+  }
   if (auto FT = Ty->getAs<AnyFunctionType>()) {
     if (FT->getResult()->isVoid())
       return CodeCompletionResult::ExpectedTypeRelation::Invalid;

test/IDE/complete_call_arg.swift

@@ -45,6 +45,8 @@
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=BOUND_IUO | %FileCheck %s -check-prefix=MEMBEROF_IUO
 // RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=FORCED_IUO | %FileCheck %s -check-prefix=MEMBEROF_IUO
 
+// RUN: %target-swift-ide-test -code-completion -source-filename %s -code-completion-token=GENERIC_TO_GENERIC | %FileCheck %s -check-prefix=GENERIC_TO_GENERIC
+
 var i1 = 1
 var i2 = 2
 var oi1 : Int?
@@ -373,6 +375,14 @@ struct TestBoundGeneric1 {
 // BOUND_GENERIC_1: Decl[InstanceVar]/CurrNominal:      y[#[Int]#];
 }
 
+func whereConvertible<T>(lhs: T, rhs: T) where T: Collection {
+  _ = zip(lhs, #^GENERIC_TO_GENERIC^#)
+}
+// GENERIC_TO_GENERIC: Begin completions
+// GENERIC_TO_GENERIC: Decl[LocalVar]/Local: lhs[#Collection#]; name=lhs
+// GENERIC_TO_GENERIC: Decl[LocalVar]/Local: rhs[#Collection#]; name=rhs
+// GENERIC_TO_GENERIC: End completions
+
 func emptyOverload() {}
 func emptyOverload(foo foo: Int) {}
 emptyOverload(foo: #^EMPTY_OVERLOAD_1^#)