Canonicalize the conforming type of BuiltinConformances.

It turns out they aren't always canonical by construction (and they don't
really need to be), so we should canonicalize the type and requirements
when we canonical a conformance ref. Fixes rdar://94877954.

Author: jckarter

lib/AST/ProtocolConformance.cpp

@@ -1638,11 +1638,25 @@ ProtocolConformance *ProtocolConformance::getCanonicalConformance() {
 
   switch (getKind()) {
   case ProtocolConformanceKind::Self:
-  case ProtocolConformanceKind::Normal:
-  case ProtocolConformanceKind::Builtin: {
+  case ProtocolConformanceKind::Normal: {
     // Root conformances are always canonical by construction.
     return this;
   }
+  case ProtocolConformanceKind::Builtin: {
+    // Canonicalize the subject type of the builtin conformance.
+    auto &Ctx = getType()->getASTContext();
+    auto builtinConformance = cast<BuiltinProtocolConformance>(this);
+    SmallVector<Requirement, 4> canonicalRequirements;
+    for (auto &reqt : builtinConformance->getConditionalRequirements()) {
+      canonicalRequirements.push_back(reqt.getCanonical());
+    }
+    return Ctx.getBuiltinConformance(
+      builtinConformance->getType()->getCanonicalType(builtinConformance->getGenericSignature()),
+      builtinConformance->getProtocol(),
+      builtinConformance->getGenericSignature().getCanonicalSignature(),
+      canonicalRequirements,
+      builtinConformance->getBuiltinConformanceKind());
+  }
 
   case ProtocolConformanceKind::Inherited: {
     auto &Ctx = getType()->getASTContext();

test/SILGen/opaque_type_sendable_requirement_canonicalization.swift

@@ -0,0 +1,21 @@
+// RUN: %target-swift-emit-silgen -disable-availability-checking -verify %s
+
+// rdar://94877954
+
+// `dynamic` prevents SILGen from lowering away
+// the opaque return type of `foo`
+dynamic func foo<T: Sendable>(f: () -> T) -> some Sendable {
+    if #available(macOS 11.0, *) {
+        return f()
+    } else {
+        return ()
+    }
+}
+
+func bar() {
+    let x: Void = ()
+    let y: () = ()
+    var a = foo { x }
+    a = foo { y }
+    _ = a
+}