From 65cfef655542184996b6d8bbec44941b63a48546 Mon Sep 17 00:00:00 2001
From: Pavel Yaskevich <pyaskevich@apple.com>
Date: Thu, 23 Mar 2023 17:23:34 -0700
Subject: [PATCH] [CSSimplify] Account for the fact that variadic generic
 parameters could be specialized

If generic parameter comes from a variadic type declaration it's
possible that it got specialized early and is no longer represented
by a pack expansion type. For example, consider expression -
`Test<Int>(42)` where `Test<each T>` and the initializer
is declared as `init(_: repeat each T)`. Although declaration
based information reports parameter at index 0 as variadic generic
the call site specializes it to `Int`.

Resolves: rdar://107151854
---
 lib/Sema/CSSimplify.cpp                       | 29 ++++++++++++-------
 .../pack-expansion-expressions.swift          | 12 ++++++++
 2 files changed, 30 insertions(+), 11 deletions(-)

diff --git a/lib/Sema/CSSimplify.cpp b/lib/Sema/CSSimplify.cpp
index d9f1d46a238df..62c8cea74d9a4 100644
--- a/lib/Sema/CSSimplify.cpp
+++ b/lib/Sema/CSSimplify.cpp
@@ -1742,21 +1742,28 @@ static ConstraintSystem::TypeMatchResult matchCallArguments(
     // inout and @autoclosure.
     if (cs.getASTContext().LangOpts.hasFeature(Feature::VariadicGenerics) &&
         paramInfo.isVariadicGenericParameter(paramIdx)) {
-      auto *paramPackExpansion = paramTy->castTo<PackExpansionType>();
-
-      SmallVector<Type, 2> argTypes;
-      for (auto argIdx : parameterBindings[paramIdx]) {
-        auto argType = argsWithLabels[argIdx].getPlainType();
-        argTypes.push_back(argType);
-      }
+      // If generic parameter comes from a variadic type declaration it's
+      // possible that it got specialized early and is no longer represented
+      // by a pack expansion type. For example, consider expression -
+      // `Test<Int>(42)` where `Test<each T>` and the initializer
+      // is declared as `init(_: repeat each T)`. Although declaration
+      // based information reports parameter at index 0 as variadic generic
+      // the call site specializes it to `Int`.
+      if (auto *paramPackExpansion = paramTy->getAs<PackExpansionType>()) {
+        SmallVector<Type, 2> argTypes;
+        for (auto argIdx : parameterBindings[paramIdx]) {
+          auto argType = argsWithLabels[argIdx].getPlainType();
+          argTypes.push_back(argType);
+        }
 
-      auto *argPack = PackType::get(cs.getASTContext(), argTypes);
-      auto *argPackExpansion = PackExpansionType::get(argPack, argPack);
+        auto *argPack = PackType::get(cs.getASTContext(), argTypes);
+        auto *argPackExpansion = PackExpansionType::get(argPack, argPack);
 
-      cs.addConstraint(
+        cs.addConstraint(
           subKind, argPackExpansion, paramPackExpansion,
           loc, /*isFavored=*/false);
-      continue;
+        continue;
+      }
     }
 
     // If type inference from default arguments is enabled, let's
diff --git a/test/Constraints/pack-expansion-expressions.swift b/test/Constraints/pack-expansion-expressions.swift
index af71c93c095db..7f82b2e1968be 100644
--- a/test/Constraints/pack-expansion-expressions.swift
+++ b/test/Constraints/pack-expansion-expressions.swift
@@ -271,3 +271,15 @@ func test_pack_expansions_with_closures() {
     takesVariadicFunction { y, z in fn(y, z) } // Ok
   }
 }
+
+// rdar://107151854 - crash on invalid due to specialized pack expansion
+func test_pack_expansion_specialization() {
+  struct Data<each T> {
+    init(_: repeat each T) {} // expected-note {{'init(_:)' declared here}}
+  }
+
+  _ = Data<Int>() // expected-error {{missing argument for parameter #1 in call}}
+  _ = Data<Int>(0) // Ok
+  _ = Data<Int, String>(42, "") // Ok
+  _ = Data<Int>(42, "") // expected-error {{extra argument in call}}
+}