Only open existentials for generic arguments on the called declaration

Author: DougGregor

lib/Sema/CSSimplify.cpp

@@ -1397,10 +1397,19 @@ shouldOpenExistentialCallArgument(
   if (!genericParam)
     return None;
 
-  // Ensure that the formal parameter is only used in covariant positions,
-  // because it won't match anywhere else.
+  // Only allow opening the innermost generic parameters.
+  auto genericContext = callee->getAsGenericContext();
+  if (!genericContext || !genericContext->isGeneric())
+    return None;
+
   auto genericSig = callee->getInnermostDeclContext()
       ->getGenericSignatureOfContext().getCanonicalSignature();
+  if (genericParam->getDepth() <
+          genericSig.getGenericParams().back()->getDepth())
+    return None;
+
+  // Ensure that the formal parameter is only used in covariant positions,
+  // because it won't match anywhere else.
   auto referenceInfo = findGenericParameterReferences(
       callee, genericSig, genericParam,
       /*treatNonResultCovarianceAsInvariant=*/false,

test/Constraints/opened_existentials.swift

@@ -126,13 +126,20 @@ func arrayOfOne<T: P>(_ value: T) -> [T] {
 }
 
 struct X<T: P> {
+  // expected-note@-1{{required by generic struct 'X' where 'T' = 'P'}}
+  func f(_: T) { }
 }
 
 // expected-note@+1{{required by global function 'createX' where 'T' = 'P'}}
 func createX<T: P>(_ value: T) -> X<T> {
   X<T>()
 }
 
+func doNotOpenOuter(p: any P) {
+  _ = X().f(p) // expected-error{{protocol 'P' as a type cannot conform to the protocol itself}}
+  // expected-note@-1{{only concrete types such as structs, enums and classes can conform to protocols}}
+}
+
 @available(SwiftStdlib 5.1, *)
 func testReturningOpaqueTypes(p: any P) {
   let q = p.getQ()