[Compile Time Constant Extraction] Look through underlying-to-opaque conversions

Resolves rdar://106006689

Author: artemcm

lib/ConstExtract/ConstExtract.cpp

@@ -306,6 +306,11 @@ static std::shared_ptr<CompileTimeValue> extractCompileTimeValue(Expr *expr) {
       return std::make_shared<TypeValue>(dotSelfExpr->getType());
     }
 
+    case ExprKind::UnderlyingToOpaque: {
+      auto underlyingToOpaque = cast<UnderlyingToOpaqueExpr>(expr);
+      return extractCompileTimeValue(underlyingToOpaque->getSubExpr());
+    }
+
     default: {
       break;
     }

test/ConstExtraction/ExtractUnderlyingToOpaque.swift

@@ -0,0 +1,33 @@
+// RUN: %empty-directory(%t)
+// RUN: echo "[MyProto]" > %t/protocols.json
+
+// RUN: %target-swift-frontend -typecheck -emit-const-values-path %t/ExtractLiterals.swiftconstvalues -const-gather-protocols-file %t/protocols.json -primary-file %s
+// RUN: cat %t/ExtractLiterals.swiftconstvalues 2>&1 | %FileCheck %s
+
+protocol MyProto {}
+protocol Bird {}
+struct UnderlyingToOpaquePropertyStruct : MyProto {
+    var warbler: some Bird {
+        Warbler("blue")
+    }
+}
+
+public struct Warbler : Bird {
+    let belly: String = "yellow"
+    init(_ color: String = "red") {
+        self.belly = color
+    }
+}
+
+// CHECK:             "valueKind": "InitCall",
+// CHECK-NEXT:        "value": {
+// CHECK-NEXT:          "type": "ExtractUnderlyingToOpaque.Warbler",
+// CHECK-NEXT:          "arguments": [
+// CHECK-NEXT:            {
+// CHECK-NEXT:              "label": "",
+// CHECK-NEXT:              "type": "Swift.String",
+// CHECK-NEXT:              "valueKind": "RawLiteral",
+// CHECK-NEXT:              "value": "blue"
+// CHECK-NEXT:            }
+// CHECK-NEXT:          ]
+// CHECK-NEXT:        }