[6.2] [CS] Ensure type variables are eliminated by Solution::simplifyType

lib/Sema/ConstraintSystem.cpp

@@ -1770,7 +1770,7 @@ Type Solution::simplifyType(Type type, bool wantInterfaceType) const {
   if (wantInterfaceType)
     type = type->mapTypeOutOfContext();
 
-  if (!(type->hasTypeVariable() || type->hasPlaceholder()))
+  if (!type->hasTypeVariableOrPlaceholder())
     return type;
 
   // Map type variables to fixed types from bindings.
@@ -1788,16 +1788,28 @@ Type Solution::simplifyType(Type type, bool wantInterfaceType) const {
       });
   ASSERT(!(wantInterfaceType && resolvedType->hasPrimaryArchetype()));
 
-  // Placeholders shouldn't be reachable through a solution, they are only
-  // useful to determine what went wrong exactly.
-  if (resolvedType->hasPlaceholder()) {
-    return resolvedType.transformRec([&](Type type) -> std::optional<Type> {
-      if (type->isPlaceholder())
-        return Type(cs.getASTContext().TheUnresolvedType);
-      return std::nullopt;
-    });
+  // We may have type variables and placeholders left over. These are solver
+  // allocated so cannot escape this function. Turn them into UnresolvedType.
+  // - Type variables may still be present from unresolved pack expansions where
+  //   e.g the count type is a hole, so the pattern may never become a
+  //   concrete type.
+  // - Placeholders may be present for any holes.
+  if (resolvedType->hasTypeVariableOrPlaceholder()) {
+    auto &ctx = cs.getASTContext();
+    resolvedType =
+        resolvedType.transformRec([&](Type type) -> std::optional<Type> {
+          if (!type->hasTypeVariableOrPlaceholder())
+            return type;
+
+          auto *typePtr = type.getPointer();
+          if (isa<TypeVariableType>(typePtr) || isa<PlaceholderType>(typePtr))
+            return Type(ctx.TheUnresolvedType);
+
+          return std::nullopt;
+        });
   }
 
+  CONDITIONAL_ASSERT(!resolvedType->getRecursiveProperties().isSolverAllocated());
   return resolvedType;
 }
 

test/Constraints/pack-expansion-expressions.swift

@@ -798,3 +798,8 @@ do {
     }
   }
 }
+
+func testInvalidDecomposition() {
+  func id<T>(_ x: T) -> T {}
+  let (a, b) = id((repeat each undefined)) // expected-error {{cannot find 'undefined' in scope}}
+}

validation-test/IDE/crashers_fixed/6cef534b98d6b512.swift

@@ -0,0 +1,4 @@
+// {"kind":"complete","signature":"swift::constraints::Solution::simplifyType(swift::Type, bool) const"}
+// RUN: %target-swift-ide-test -code-completion --code-completion-token=COMPLETE -code-completion-diagnostics -source-filename %s
+for (a(b, d)) [
+#^COMPLETE^#, 0, (repeat.c)].enumerated(

validation-test/compiler_crashers_2_fixed/d84cbb5089113ba.swift

@@ -0,0 +1,4 @@
+// {"kind":"typecheck","signature":"swift::constraints::Solution::simplifyType(swift::Type, bool) const"}
+// RUN: not %target-swift-frontend -typecheck %s
+for (a(b, d)) in [(repeat .c)].enumerated(<#expression#>) {
+}

validation-test/compiler_crashers_2_fixed/dc3a3e26162b43be.swift

@@ -0,0 +1,12 @@
+// {"signature":"(anonymous namespace)::SetExprTypes::walkToExprPost(swift::Expr*)"}
+// RUN: not %target-swift-frontend -typecheck %s
+typealias a<b, c> = c
+struct d < each b {
+  typealias e< c > =
+  (repeat a< each b, c >)
+  typealias f< each c > = (repeat e< each c >
+  func 1 {
+    typealias g = h
+    (g<
+    i >
+    typealias g< each c > = f< repeat each c >