SIL: Lower closure functions' captures in their own expansion context.

lib/SIL/IR/SILFunctionType.cpp

@@ -2212,8 +2212,13 @@ static CanSILFunctionType getSILFunctionType(
 
   // Lower the capture context parameters, if any.
   if (constant && constant->getAnyFunctionRef()) {
+    // Lower in the context of the closure. Since the set of captures is a
+    // private contract between the closure and its enclosing context, we
+    // don't need to keep its capture types opaque.
     auto expansion = TypeExpansionContext::maximal(
-        expansionContext.getContext(), expansionContext.isWholeModuleContext());
+        constant->getAnyFunctionRef()->getAsDeclContext(), false);
+    // ...unless it's inlinable, in which case it might get inlined into
+    // some place we need to keep opaque types opaque.
     if (constant->isSerialized())
       expansion = TypeExpansionContext::minimal();
     lowerCaptureContextParameters(TC, *constant, genericSig, expansion, inputs);

test/SILGen/opaque_result_type_captured.swift

@@ -0,0 +1,26 @@
+// RUN: %target-swift-emit-silgen -disable-availability-checking -verify %s
+// rdar://83378116
+
+public protocol P {}
+public protocol Q {}
+
+struct SP<T: P>: P { init(t: T) {} }
+struct SQ: Q {}
+
+struct Tubb<C: Q, T, F> {
+}
+
+extension Tubb: P where T: P, F: P {
+    init(c: C, t: () -> T, f: () -> F) {}
+}
+
+struct SP2: P {}
+
+func bar() -> some P { return SP2() }
+
+public struct Butt {
+    public func foo() -> some P {
+        let sp = SP(t: bar())
+        return Tubb(c: SQ(), t: { sp }, f: { sp })
+    }
+}