Sema: Fix case where witness thrown error type is a subtype of a type parameter

In b300068, I changed the `if`
condition here to check for the absence of type variables as well
as type parameters. The correct condition is to actually not
check for anything at all.

Fixes rdar://149438520.

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -907,17 +907,10 @@ RequirementMatch swift::matchWitness(
       return RequirementMatch(witness, MatchKind::ThrowsConflict);
 
     case ThrownErrorSubtyping::ExactMatch:
+    case ThrownErrorSubtyping::Subtype:
       // All is well.
       break;
 
-    case ThrownErrorSubtyping::Subtype:
-      // If there were no type parameters, we're done.
-      if (!reqThrownError->hasTypeVariable() &&
-          !reqThrownError->hasTypeParameter())
-        break;
-
-      LLVM_FALLTHROUGH;
-
     case ThrownErrorSubtyping::Dependent:
       // We need to perform type matching
       if (auto result = matchTypes(reqThrownError, witnessThrownError)) {

test/decl/protocol/conforms/typed_throws.swift

@@ -77,3 +77,17 @@ public protocol HasRethrowingMap: Sequence {
 }
 
 extension Array: HasRethrowingMap {}
+
+// rdar://149438520 -- incorrect handling of subtype relation between type parameter and Never
+protocol DependentThrowing {
+  associatedtype E: Error
+  func f() throws(E)
+}
+
+extension DependentThrowing {
+  func f() {}
+}
+
+struct DefaultDependentThrowing: DependentThrowing {
+  typealias E = Error
+}