add FeatureInheritanceAccessControl test case

Author: tayloraswift

Sources/SymbolGraphCompiler/SSGC.TypeChecker.swift

@@ -493,31 +493,10 @@ extension SSGC.TypeChecker
             let conformance:Symbol.Decl = feature.scopes.first
             else
             {
-                //  We hit this on an extremely unusual edge case where a feature and an heir
-                //  are public, but the protocol the feature is defined on is not. Here is some
-                //  valid Swift code that demonstrates this:
-                //
-                /*  ```
-                    protocol P
-                    {
-                    }
-                    extension P
-                    {
-                        public
-                        func f()
-                        {
-                        }
-                    }
-
-                    public
-                    struct S:P
-                    {
-                    }
-                    ```
-                */
-                //  Ideally, lib/SymbolGraphGen would not emit the feature in this case, but
-                //  it does anyway.
-                return
+                throw AssertionError.init(message: """
+                    Declaration '\(feature.value.path)' has '\(feature.access)' access but \
+                    has no known lexical parents
+                    """)
             }
 
             guard feature.scopes.count == 1

Sources/SymbolGraphCompilerTests/Main.FeatureInheritanceAccessControl.swift

@@ -0,0 +1,43 @@
+import SymbolGraphCompiler
+@_spi(testable)
+import Symbols
+import Testing_
+
+extension Main
+{
+    enum FeatureInheritanceAccessControl
+    {
+    }
+}
+extension Main.FeatureInheritanceAccessControl:CompilerTestBattery
+{
+    static
+    let inputs:[Symbol.Module] =
+    [
+        "FeatureInheritanceAccessControl",
+    ]
+
+    static
+    func run(tests:TestGroup, module:SSGC.ModuleIndex)
+    {
+        let declsBySymbol:[Symbol.Decl: SSGC.Decl] = module.declarations.reduce(into: [:])
+        {
+            for decl:SSGC.Decl in $1.decls
+            {
+                $0[decl.id] = decl
+            }
+        }
+        let features:[Symbol.Decl: [Symbol.Decl]] = module.extensions.reduce(into: [:])
+        {
+            $0[$1.extendee.id, default: []] += $1.features
+        }
+
+
+        if  let tests:TestGroup = tests / "S",
+            let _:SSGC.Decl = tests.expect(
+                value: declsBySymbol["s31FeatureInheritanceAccessControl1SV"])
+        {
+            tests.expect(nil: features["s31FeatureInheritanceAccessControl1SV"])
+        }
+    }
+}

Sources/SymbolGraphCompilerTests/Main.swift

@@ -9,6 +9,7 @@ enum Main:TestMain
         Determinism.self,
         DefaultImplementations.self,
         FeatureInheritance.self,
+        FeatureInheritanceAccessControl.self,
         ExternalExtensionsWithConformances.self,
         ExternalExtensionsWithConstraints.self,
         InternalExtensionsWithConformances.self,

TestModules/Snippets/Features/FeatureInheritanceAccessControl.swift

@@ -0,0 +1,18 @@
+protocol P
+{
+    func f()
+}
+extension P
+{
+    public
+    func f()
+    {
+    }
+}
+
+/// This type exhibits a known lib/SymbolGraphGen bug, in that it “inherits” the nominally
+/// public ``P.f`` method, even though the protocol and all its members are internal.
+public
+struct S:P
+{
+}