Fix isProtocolContext condition

Author: Azoy

lib/Sema/TypeCheckDeclPrimary.cpp

@@ -2652,7 +2652,7 @@ class DeclChecker : public DeclVisitor<DeclChecker> {
     // @_staticExclusiveOnly types cannot be put into 'var's, only 'let'.
     if (auto SD = VD->getInterfaceType()->getStructOrBoundGenericStruct()) {
       if (SD->getAttrs().hasAttribute<StaticExclusiveOnlyAttr>()) {
-        auto isProtocolContext = DC->getSelfProtocolDecl() != nullptr;
+        auto isProtocolContext = isa<ProtocolDecl>(DC);
 
         if (isProtocolContext && !VD->supportsMutation()) {
           return;

test/attr/attr_static_exclusive_only.swift

@@ -11,6 +11,7 @@ struct B: ~Copyable { // expected-note {{'B' is a non-mutable type}}
                       // expected-note@-4 {{'B' is a non-mutable type}}
                       // expected-note@-5 {{'B' is a non-mutable type}}
                       // expected-note@-6 {{'B' is a non-mutable type}}
+                      // expected-note@-7 {{'B' is a non-mutable type}}
   mutating func change() { // expected-error {{type 'B' cannot have mutating function 'change()'}}
     print("123")
   }
@@ -92,3 +93,9 @@ extension W: U {
 struct X: ~Copyable, U {
   let v: B // OK
 }
+
+extension U {
+  var y: B { // expected-error {{variable of type 'B' must be declared with a 'let'}}
+    B()
+  }
+}