Merge pull request swiftlang#80963 from slavapestov/fix-rdar149438520

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

Author: slavapestov

lib/Sema/TypeCheckProtocol.cpp

@@ -912,8 +912,7 @@ RequirementMatch swift::matchWitness(
 
     case ThrownErrorSubtyping::Subtype:
       // If there were no type parameters, we're done.
-      if (!reqThrownError->hasTypeVariable() &&
-          !reqThrownError->hasTypeParameter())
+      if (!reqThrownError->hasTypeParameter())
         break;
 
       LLVM_FALLTHROUGH;

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
+}