diff --git a/explorer/data/prelude.carbon b/explorer/data/prelude.carbon
index d957ae6df9d9..5acd6e49fadd 100644
--- a/explorer/data/prelude.carbon
+++ b/explorer/data/prelude.carbon
@@ -712,9 +712,8 @@ class Optional(T:! type) {
       }
     }
     Assert(false, "Attempted to unwrap empty Optional");
-    // TODO: Drop uninitialized variable & return when we can flag unreachable paths.
-    var y: T;
-    return y;
+    // TODO: Drop return when we can flag unreachable paths.
+    return self.Get();
   }
 
   var element: OptionalElement(T);
diff --git a/explorer/interpreter/BUILD b/explorer/interpreter/BUILD
index 6847b363c283..83e52485c586 100644
--- a/explorer/interpreter/BUILD
+++ b/explorer/interpreter/BUILD
@@ -219,6 +219,7 @@ cc_library(
         "resolve_unformed.h",
     ],
     deps = [
+        ":stack_space",
         "//common:check",
         "//explorer/ast",
         "//explorer/ast:static_scope",
diff --git a/explorer/interpreter/heap.cpp b/explorer/interpreter/heap.cpp
index 196694ced619..27df83ba37fd 100644
--- a/explorer/interpreter/heap.cpp
+++ b/explorer/interpreter/heap.cpp
@@ -38,6 +38,11 @@ auto Heap::Write(const Address& a, Nonnull<const Value*> v,
                  SourceLocation source_loc) -> ErrorOr<Success> {
   CARBON_RETURN_IF_ERROR(this->CheckAlive(a.allocation_, source_loc));
   if (states_[a.allocation_.index_] == ValueState::Uninitialized) {
+    if (!a.element_path_.IsEmpty()) {
+      return ProgramError(source_loc)
+             << "undefined behavior: store to subobject of uninitialized value "
+             << *values_[a.allocation_.index_];
+    }
     states_[a.allocation_.index_] = ValueState::Alive;
   }
   CARBON_ASSIGN_OR_RETURN(values_[a.allocation_.index_],
diff --git a/explorer/interpreter/interpreter.cpp b/explorer/interpreter/interpreter.cpp
index 69a0b2d4b12a..20d151460705 100644
--- a/explorer/interpreter/interpreter.cpp
+++ b/explorer/interpreter/interpreter.cpp
@@ -1596,10 +1596,6 @@ auto Interpreter::StepExp() -> ErrorOr<Success> {
               *print_stream_ << llvm::formatv(format_string);
               break;
             case 1: {
-              if ((*args[1]).kind() == Value::Kind::UninitializedValue) {
-                return ProgramError(exp.source_loc())
-                       << "Printing uninitialized value";
-              }
               *print_stream_ << llvm::formatv(format_string,
                                               cast<IntValue>(*args[1]).value());
               break;
@@ -2134,18 +2130,6 @@ auto Interpreter::StepStmt() -> ErrorOr<Success> {
         if (definition.has_init()) {
           CARBON_ASSIGN_OR_RETURN(
               v, Convert(act.results()[0], dest_type, stmt.source_loc()));
-        } else if (dest_type->kind() == Value::Kind::StaticArrayType) {
-          const auto& array = cast<StaticArrayType>(dest_type);
-          CARBON_CHECK(array->has_size());
-          const auto& element_type = array->element_type();
-          const auto size = array->size();
-
-          std::vector<Nonnull<const Value*>> elements;
-          elements.reserve(size);
-          for (size_t i = 0; i < size; i++) {
-            elements.push_back(arena_->New<UninitializedValue>(&element_type));
-          }
-          v = arena_->New<TupleValueBase>(Value::Kind::TupleValue, elements);
         } else {
           v = arena_->New<UninitializedValue>(p);
         }
@@ -2290,7 +2274,6 @@ auto Interpreter::StepDeclaration() -> ErrorOr<Success> {
   switch (decl.kind()) {
     case DeclarationKind::VariableDeclaration: {
       const auto& var_decl = cast<VariableDeclaration>(decl);
-      const auto* var_type = &var_decl.binding().static_type();
       if (var_decl.has_initializer()) {
         if (act.pos() == 0) {
           return todo_.Spawn(
@@ -2303,22 +2286,6 @@ auto Interpreter::StepDeclaration() -> ErrorOr<Success> {
           todo_.Initialize(&var_decl.binding(), v);
           return todo_.FinishAction();
         }
-      } else if (var_type->kind() == Value::Kind::StaticArrayType) {
-        const auto& array = cast<StaticArrayType>(var_type);
-        CARBON_CHECK(array->has_size());
-        const auto& element_type = array->element_type();
-        const auto size = array->size();
-
-        std::vector<Nonnull<const Value*>> elements;
-        elements.reserve(size);
-        for (size_t i = 0; i < size; i++) {
-          elements.push_back(arena_->New<UninitializedValue>(&element_type));
-        }
-
-        Nonnull<const Value*> v =
-            arena_->New<TupleValueBase>(Value::Kind::TupleValue, elements);
-        todo_.Initialize(&var_decl.binding(), v);
-        return todo_.FinishAction();
       } else {
         Nonnull<const Value*> v =
             arena_->New<UninitializedValue>(&var_decl.binding().value());
diff --git a/explorer/interpreter/resolve_names.cpp b/explorer/interpreter/resolve_names.cpp
index e6945ab1f028..c62fce1fbd8a 100644
--- a/explorer/interpreter/resolve_names.cpp
+++ b/explorer/interpreter/resolve_names.cpp
@@ -276,7 +276,7 @@ auto NameResolver::AddExposedNames(const Declaration& declaration,
 auto NameResolver::ResolveNames(Expression& expression,
                                 const StaticScope& enclosing_scope)
     -> ErrorOr<std::optional<ValueNodeView>> {
-  return RunWithExtraStack<ErrorOr<std::optional<ValueNodeView>>>(
+  return RunWithExtraStack(
       [&]() { return ResolveNamesImpl(expression, enclosing_scope); });
 }
 
@@ -463,7 +463,7 @@ auto NameResolver::ResolveNamesImpl(Expression& expression,
 auto NameResolver::ResolveNames(WhereClause& clause,
                                 const StaticScope& enclosing_scope)
     -> ErrorOr<Success> {
-  return RunWithExtraStack<ErrorOr<Success>>(
+  return RunWithExtraStack(
       [&]() { return ResolveNamesImpl(clause, enclosing_scope); });
 }
 
@@ -499,7 +499,7 @@ auto NameResolver::ResolveNamesImpl(WhereClause& clause,
 
 auto NameResolver::ResolveNames(Pattern& pattern, StaticScope& enclosing_scope)
     -> ErrorOr<Success> {
-  return RunWithExtraStack<ErrorOr<Success>>(
+  return RunWithExtraStack(
       [&]() { return ResolveNamesImpl(pattern, enclosing_scope); });
 }
 
@@ -560,7 +560,7 @@ auto NameResolver::ResolveNamesImpl(Pattern& pattern,
 auto NameResolver::ResolveNames(Statement& statement,
                                 StaticScope& enclosing_scope)
     -> ErrorOr<Success> {
-  return RunWithExtraStack<ErrorOr<Success>>(
+  return RunWithExtraStack(
       [&]() { return ResolveNamesImpl(statement, enclosing_scope); });
 }
 
@@ -703,7 +703,7 @@ auto NameResolver::ResolveNames(Declaration& declaration,
                                 StaticScope& enclosing_scope,
                                 ResolveFunctionBodies bodies)
     -> ErrorOr<Success> {
-  return RunWithExtraStack<ErrorOr<Success>>(
+  return RunWithExtraStack(
       [&]() { return ResolveNamesImpl(declaration, enclosing_scope, bodies); });
 }
 
@@ -922,7 +922,7 @@ auto NameResolver::ResolveNamesImpl(Declaration& declaration,
 }
 
 auto ResolveNames(AST& ast) -> ErrorOr<Success> {
-  return RunWithExtraStack<ErrorOr<Success>>([&]() -> ErrorOr<Success> {
+  return RunWithExtraStack([&]() -> ErrorOr<Success> {
     NameResolver resolver;
 
     StaticScope file_scope;
diff --git a/explorer/interpreter/resolve_unformed.cpp b/explorer/interpreter/resolve_unformed.cpp
index 0b048f10d742..58610fd21001 100644
--- a/explorer/interpreter/resolve_unformed.cpp
+++ b/explorer/interpreter/resolve_unformed.cpp
@@ -11,6 +11,7 @@
 #include "explorer/ast/expression.h"
 #include "explorer/ast/pattern.h"
 #include "explorer/common/nonnull.h"
+#include "explorer/interpreter/stack_space.h"
 
 using llvm::cast;
 
@@ -54,22 +55,35 @@ auto FlowFacts::TakeAction(Nonnull<const AstNode*> node, ActionType action,
   return Success();
 }
 
-// Traverses the sub-AST rooted at the given node, resolving the formed/unformed
-// states of local variables within it and updating the flow facts.
-static auto ResolveUnformed(Nonnull<const Expression*> expression,
-                            FlowFacts& flow_facts, FlowFacts::ActionType action)
+static auto ResolveUnformedImpl(Nonnull<const Expression*> expression,
+                                FlowFacts& flow_facts,
+                                FlowFacts::ActionType action)
     -> ErrorOr<Success>;
-static auto ResolveUnformed(Nonnull<const Pattern*> pattern,
-                            FlowFacts& flow_facts, FlowFacts::ActionType action)
+static auto ResolveUnformedImpl(Nonnull<const Pattern*> pattern,
+                                FlowFacts& flow_facts,
+                                FlowFacts::ActionType action)
     -> ErrorOr<Success>;
-static auto ResolveUnformed(Nonnull<const Statement*> statement,
-                            FlowFacts& flow_facts, FlowFacts::ActionType action)
+static auto ResolveUnformedImpl(Nonnull<const Statement*> statement,
+                                FlowFacts& flow_facts,
+                                FlowFacts::ActionType action)
     -> ErrorOr<Success>;
-static auto ResolveUnformed(Nonnull<const Declaration*> declaration)
+static auto ResolveUnformedImpl(Nonnull<const Expression*> expression,
+                                FlowFacts& flow_facts,
+                                FlowFacts::ActionType action)
     -> ErrorOr<Success>;
 
-static auto ResolveUnformed(Nonnull<const Expression*> expression,
-                            FlowFacts& flow_facts, FlowFacts::ActionType action)
+// Traverses the sub-AST rooted at the given node, resolving the formed/unformed
+// states of local variables within it and updating the flow facts.
+template <typename T>
+static auto ResolveUnformed(Nonnull<const T*> expression, FlowFacts& flow_facts,
+                            FlowFacts::ActionType action) -> ErrorOr<Success> {
+  return RunWithExtraStack(
+      [&] { return ResolveUnformedImpl(expression, flow_facts, action); });
+}
+
+static auto ResolveUnformedImpl(Nonnull<const Expression*> expression,
+                                FlowFacts& flow_facts,
+                                FlowFacts::ActionType action)
     -> ErrorOr<Success> {
   switch (expression->kind()) {
     case ExpressionKind::IdentifierExpression: {
@@ -85,6 +99,12 @@ static auto ResolveUnformed(Nonnull<const Expression*> expression,
           ResolveUnformed(&call.argument(), flow_facts, action));
       break;
     }
+    case ExpressionKind::IntrinsicExpression: {
+      const auto& intrin = cast<IntrinsicExpression>(*expression);
+      CARBON_RETURN_IF_ERROR(
+          ResolveUnformed(&intrin.args(), flow_facts, action));
+      break;
+    }
     case ExpressionKind::TupleLiteral:
       for (Nonnull<const Expression*> field :
            cast<TupleLiteral>(*expression).fields()) {
@@ -100,6 +120,9 @@ static auto ResolveUnformed(Nonnull<const Expression*> expression,
             // When a variable is taken address of, defer the unformed check to
             // runtime. A more sound analysis can be implemented when a
             // points-to analysis is available.
+            // TODO: This isn't enough to permit &x.y or &x[i] when x is
+            // uninitialized, because x.y and x[i] both require x to be
+            // initialized.
             ResolveUnformed(opt_exp.arguments().front(), flow_facts,
                             FlowFacts::ActionType::Form));
       } else {
@@ -117,15 +140,35 @@ static auto ResolveUnformed(Nonnull<const Expression*> expression,
       }
       break;
     case ExpressionKind::SimpleMemberAccessExpression:
-      CARBON_RETURN_IF_ERROR(ResolveUnformed(
-          &cast<SimpleMemberAccessExpression>(*expression).object(), flow_facts,
-          FlowFacts::ActionType::Check));
+    case ExpressionKind::CompoundMemberAccessExpression:
+    case ExpressionKind::BaseAccessExpression:
+      CARBON_RETURN_IF_ERROR(
+          ResolveUnformed(&cast<MemberAccessExpression>(*expression).object(),
+                          flow_facts, FlowFacts::ActionType::Check));
       break;
     case ExpressionKind::BuiltinConvertExpression:
       CARBON_RETURN_IF_ERROR(ResolveUnformed(
           cast<BuiltinConvertExpression>(*expression).source_expression(),
           flow_facts, FlowFacts::ActionType::Check));
       break;
+    case ExpressionKind::IndexExpression:
+      CARBON_RETURN_IF_ERROR(
+          ResolveUnformed(&cast<IndexExpression>(*expression).object(),
+                          flow_facts, FlowFacts::ActionType::Check));
+      CARBON_RETURN_IF_ERROR(
+          ResolveUnformed(&cast<IndexExpression>(*expression).offset(),
+                          flow_facts, FlowFacts::ActionType::Check));
+      break;
+    case ExpressionKind::IfExpression: {
+      const auto& if_exp = cast<IfExpression>(*expression);
+      CARBON_RETURN_IF_ERROR(ResolveUnformed(&if_exp.condition(), flow_facts,
+                                             FlowFacts::ActionType::Check));
+      CARBON_RETURN_IF_ERROR(
+          ResolveUnformed(&if_exp.then_expression(), flow_facts, action));
+      CARBON_RETURN_IF_ERROR(
+          ResolveUnformed(&if_exp.else_expression(), flow_facts, action));
+      break;
+    }
     case ExpressionKind::DotSelfExpression:
     case ExpressionKind::IntLiteral:
     case ExpressionKind::BoolLiteral:
@@ -135,13 +178,8 @@ static auto ResolveUnformed(Nonnull<const Expression*> expression,
     case ExpressionKind::StringTypeLiteral:
     case ExpressionKind::TypeTypeLiteral:
     case ExpressionKind::ValueLiteral:
-    case ExpressionKind::IndexExpression:
-    case ExpressionKind::CompoundMemberAccessExpression:
-    case ExpressionKind::BaseAccessExpression:
-    case ExpressionKind::IfExpression:
     case ExpressionKind::WhereExpression:
     case ExpressionKind::StructTypeLiteral:
-    case ExpressionKind::IntrinsicExpression:
     case ExpressionKind::UnimplementedExpression:
     case ExpressionKind::FunctionTypeLiteral:
     case ExpressionKind::ArrayTypeLiteral:
@@ -150,8 +188,9 @@ static auto ResolveUnformed(Nonnull<const Expression*> expression,
   return Success();
 }
 
-static auto ResolveUnformed(Nonnull<const Pattern*> pattern,
-                            FlowFacts& flow_facts, FlowFacts::ActionType action)
+static auto ResolveUnformedImpl(Nonnull<const Pattern*> pattern,
+                                FlowFacts& flow_facts,
+                                FlowFacts::ActionType action)
     -> ErrorOr<Success> {
   switch (pattern->kind()) {
     case PatternKind::BindingPattern: {
@@ -178,8 +217,9 @@ static auto ResolveUnformed(Nonnull<const Pattern*> pattern,
   return Success();
 }
 
-static auto ResolveUnformed(Nonnull<const Statement*> statement,
-                            FlowFacts& flow_facts, FlowFacts::ActionType action)
+static auto ResolveUnformedImpl(Nonnull<const Statement*> statement,
+                                FlowFacts& flow_facts,
+                                FlowFacts::ActionType action)
     -> ErrorOr<Success> {
   switch (statement->kind()) {
     case StatementKind::Block: {
@@ -280,15 +320,36 @@ static auto ResolveUnformed(Nonnull<const Statement*> statement,
       }
       break;
     }
+    case StatementKind::For: {
+      const auto& for_stmt = cast<For>(*statement);
+      CARBON_RETURN_IF_ERROR(ResolveUnformed(
+          &for_stmt.loop_target(), flow_facts, FlowFacts::ActionType::Check));
+      CARBON_RETURN_IF_ERROR(
+          ResolveUnformed(&for_stmt.body(), flow_facts, action));
+      break;
+    }
     case StatementKind::Break:
     case StatementKind::Continue:
-    case StatementKind::For:
       // do nothing
       break;
   }
   return Success();
 }
 
+static auto ResolveUnformed(Nonnull<const Declaration*> declaration)
+    -> ErrorOr<Success>;
+
+static auto ResolveUnformed(
+    llvm::ArrayRef<Nonnull<const Declaration*>> declarations)
+    -> ErrorOr<Success> {
+  return RunWithExtraStack([declarations]() -> ErrorOr<Success> {
+    for (Nonnull<const Declaration*> declaration : declarations) {
+      CARBON_RETURN_IF_ERROR(ResolveUnformed(declaration));
+    }
+    return Success();
+  });
+}
+
 static auto ResolveUnformed(Nonnull<const Declaration*> declaration)
     -> ErrorOr<Success> {
   switch (declaration->kind()) {
@@ -306,12 +367,7 @@ static auto ResolveUnformed(Nonnull<const Declaration*> declaration)
       break;
     }
     case DeclarationKind::NamespaceDeclaration:
-    case DeclarationKind::ClassDeclaration:
     case DeclarationKind::MixDeclaration:
-    case DeclarationKind::MixinDeclaration:
-    case DeclarationKind::InterfaceDeclaration:
-    case DeclarationKind::ConstraintDeclaration:
-    case DeclarationKind::ImplDeclaration:
     case DeclarationKind::MatchFirstDeclaration:
     case DeclarationKind::ChoiceDeclaration:
     case DeclarationKind::VariableDeclaration:
@@ -322,6 +378,16 @@ static auto ResolveUnformed(Nonnull<const Declaration*> declaration)
     case DeclarationKind::AliasDeclaration:
       // do nothing
       break;
+    case DeclarationKind::ClassDeclaration:
+      return ResolveUnformed(cast<ClassDeclaration>(declaration)->members());
+    case DeclarationKind::MixinDeclaration:
+      return ResolveUnformed(cast<MixinDeclaration>(declaration)->members());
+    case DeclarationKind::InterfaceDeclaration:
+    case DeclarationKind::ConstraintDeclaration:
+      return ResolveUnformed(
+          cast<ConstraintTypeDeclaration>(declaration)->members());
+    case DeclarationKind::ImplDeclaration:
+      return ResolveUnformed(cast<ImplDeclaration>(declaration)->members());
   }
   return Success();
 }
diff --git a/explorer/interpreter/stack_space.h b/explorer/interpreter/stack_space.h
index 0d936fd76dd0..8dd224d10a07 100644
--- a/explorer/interpreter/stack_space.h
+++ b/explorer/interpreter/stack_space.h
@@ -27,11 +27,13 @@ auto RunWithExtraStackHelper(llvm::function_ref<void()> fn) -> void;
 // create the current thread.
 //
 // Usage:
-//   return RunWithExtraStack<ReturnType>([&]() -> ReturnType {
+//   return RunWithExtraStack([&]() -> ReturnType {
 //         <function body>
 //       });
-template <typename ReturnType>
-auto RunWithExtraStack(llvm::function_ref<ReturnType()> fn) -> ReturnType {
+template <typename Fn>
+auto RunWithExtraStack(Fn fn) -> decltype(fn()) {
+  using ReturnType = decltype(fn());
+  static_assert(!std::is_reference_v<ReturnType>);
   if (Internal::IsStackSpaceNearlyExhausted()) {
     std::optional<ReturnType> result;
     Internal::RunWithExtraStackHelper([&] { result = fn(); });
diff --git a/explorer/interpreter/type_checker.cpp b/explorer/interpreter/type_checker.cpp
index f295d74a0f86..951938f9851c 100644
--- a/explorer/interpreter/type_checker.cpp
+++ b/explorer/interpreter/type_checker.cpp
@@ -2742,8 +2742,7 @@ auto TypeChecker::CheckAddrMeAccess(
 auto TypeChecker::TypeCheckExp(Nonnull<Expression*> e,
                                const ImplScope& impl_scope)
     -> ErrorOr<Success> {
-  return RunWithExtraStack<ErrorOr<Success>>(
-      [&]() { return TypeCheckExpImpl(e, impl_scope); });
+  return RunWithExtraStack([&]() { return TypeCheckExpImpl(e, impl_scope); });
 }
 
 // NOLINTNEXTLINE(readability-function-size)
diff --git a/explorer/parse_and_execute/parse_and_execute.cpp b/explorer/parse_and_execute/parse_and_execute.cpp
index 6ffdeba7e0d4..06f52d40b9a0 100644
--- a/explorer/parse_and_execute/parse_and_execute.cpp
+++ b/explorer/parse_and_execute/parse_and_execute.cpp
@@ -40,7 +40,7 @@ static auto ParseAndExecuteHelper(std::function<ErrorOr<AST>(Arena*)> parse,
                                   Nonnull<TraceStream*> trace_stream,
                                   Nonnull<llvm::raw_ostream*> print_stream)
     -> ErrorOr<int> {
-  return RunWithExtraStack<ErrorOr<int>>([&]() -> ErrorOr<int> {
+  return RunWithExtraStack([&]() -> ErrorOr<int> {
     Arena arena;
     auto cursor = std::chrono::steady_clock::now();
 
diff --git a/explorer/testdata/array/fail_print_uninitalized_array_element.carbon b/explorer/testdata/array/fail_print_uninitalized_array_element.carbon
index 1150cf488196..da61deb605ae 100644
--- a/explorer/testdata/array/fail_print_uninitalized_array_element.carbon
+++ b/explorer/testdata/array/fail_print_uninitalized_array_element.carbon
@@ -8,7 +8,7 @@ package ExplorerTest impl;
 
 fn Main() -> i32 {
   var my_array : [i32; 1];
-  // CHECK:STDERR: RUNTIME ERROR: fail_print_uninitalized_array_element.carbon:[[@LINE+1]]: Printing uninitialized value
+  // CHECK:STDERR: COMPILATION ERROR: fail_print_uninitalized_array_element.carbon:[[@LINE+1]]: use of uninitialized variable my_array
   Print("{0}", my_array[0]);
   return 0;
 }
diff --git a/explorer/testdata/array/uninitialized_local_array_access.carbon b/explorer/testdata/array/fail_store_to_uninitialized_array.carbon
similarity index 72%
rename from explorer/testdata/array/uninitialized_local_array_access.carbon
rename to explorer/testdata/array/fail_store_to_uninitialized_array.carbon
index ec2f93a4558f..fafcf5d37787 100644
--- a/explorer/testdata/array/uninitialized_local_array_access.carbon
+++ b/explorer/testdata/array/fail_store_to_uninitialized_array.carbon
@@ -3,13 +3,12 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDOUT: 100
-// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
 fn Main() -> i32 {
   var my_array : [i32; 1];
+  // CHECK:STDERR: COMPILATION ERROR: fail_store_to_uninitialized_array.carbon:[[@LINE+1]]: use of uninitialized variable my_array
   my_array[0] = 100;
   Print("{0}", my_array[0]);
   return 0;
diff --git a/explorer/testdata/array/fail_store_to_uninitialized_global_array.carbon b/explorer/testdata/array/fail_store_to_uninitialized_global_array.carbon
new file mode 100644
index 000000000000..a6684f71480b
--- /dev/null
+++ b/explorer/testdata/array/fail_store_to_uninitialized_global_array.carbon
@@ -0,0 +1,16 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+var my_array : [i32; 1];
+
+fn Main() -> i32 {
+  // CHECK:STDERR: RUNTIME ERROR: fail_store_to_uninitialized_global_array.carbon:[[@LINE+1]]: undefined behavior: store to subobject of uninitialized value Uninit<Placeholder<my_array>>
+  my_array[0] = 100;
+  Print("{0}", my_array[0]);
+  return 0;
+}
diff --git a/explorer/testdata/choice/generic_choice_nested_in_template_class.carbon b/explorer/testdata/choice/generic_choice_nested_in_template_class.carbon
index d52d6c054129..031a7f3573a0 100644
--- a/explorer/testdata/choice/generic_choice_nested_in_template_class.carbon
+++ b/explorer/testdata/choice/generic_choice_nested_in_template_class.carbon
@@ -31,14 +31,14 @@ class MyOptional(T:! type){
     }
 
     fn get[self: Self] () -> T {
-        var y: T;
         var x: MyOptionalElement(T) = self.element;
         match(x){
             case MyOptionalElement(T).Element( var x: T ) =>{
                 return x;
             }
         }
-        return y;
+        // TODO: Mark this as unreachable somehow.
+        return get();
     }
 
    var element: MyOptionalElement(T);
diff --git a/explorer/testdata/class/fail_store_to_uninitialized_class.carbon b/explorer/testdata/class/fail_store_to_uninitialized_class.carbon
new file mode 100644
index 000000000000..e8fee33057c2
--- /dev/null
+++ b/explorer/testdata/class/fail_store_to_uninitialized_class.carbon
@@ -0,0 +1,23 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+class Point {
+  var x: i32;
+  var y: i32;
+}
+
+fn Main() -> i32 {
+  var p: Point;
+  if (1 == 0) {
+    p = {.x = 0, .y = 0};
+  }
+  // CHECK:STDERR: RUNTIME ERROR: fail_store_to_uninitialized_class.carbon:[[@LINE+1]]: undefined behavior: store to subobject of uninitialized value Uninit<Placeholder<p>>
+  p.x = 1;
+  p.y = 2;
+  return p.x;
+}
diff --git a/explorer/testdata/array/uninitialized_global_array_access.carbon b/explorer/testdata/unformed/fail_array.carbon
similarity index 65%
rename from explorer/testdata/array/uninitialized_global_array_access.carbon
rename to explorer/testdata/unformed/fail_array.carbon
index 9ba45db5b2ae..f0efb8b74d9d 100644
--- a/explorer/testdata/array/uninitialized_global_array_access.carbon
+++ b/explorer/testdata/unformed/fail_array.carbon
@@ -3,15 +3,11 @@
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 // AUTOUPDATE
-// CHECK:STDOUT: 100
-// CHECK:STDOUT: result: 0
 
 package ExplorerTest api;
 
-var my_array : [i32; 1];
-
 fn Main() -> i32 {
-  my_array[0] = 100;
-  Print("{0}", my_array[0]);
-  return 0;
+  var v: [i32; 2];
+  // CHECK:STDERR: COMPILATION ERROR: fail_array.carbon:[[@LINE+1]]: use of uninitialized variable v
+  return v[0];
 }
diff --git a/explorer/testdata/unformed/fail_array_dynamic.carbon b/explorer/testdata/unformed/fail_array_dynamic.carbon
new file mode 100644
index 000000000000..d12bd732b88f
--- /dev/null
+++ b/explorer/testdata/unformed/fail_array_dynamic.carbon
@@ -0,0 +1,16 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var v: [i32; 2];
+  if (0 == 1) {
+    v = (1, 2);
+  }
+  // CHECK:STDERR: RUNTIME ERROR: fail_array_dynamic.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<v>>
+  return v[0];
+}
diff --git a/explorer/testdata/unformed/fail_base_access.carbon b/explorer/testdata/unformed/fail_base_access.carbon
new file mode 100644
index 000000000000..13e2ca3405f7
--- /dev/null
+++ b/explorer/testdata/unformed/fail_base_access.carbon
@@ -0,0 +1,19 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+base class B {
+  var n: i32;
+}
+
+class D extends B {}
+
+fn Main() -> i32 {
+  var d: D;
+  // CHECK:STDERR: COMPILATION ERROR: fail_base_access.carbon:[[@LINE+1]]: use of uninitialized variable d
+  return d.n;
+}
diff --git a/explorer/testdata/unformed/fail_compound_member_access.carbon b/explorer/testdata/unformed/fail_compound_member_access.carbon
new file mode 100644
index 000000000000..4e6cc123f2e5
--- /dev/null
+++ b/explorer/testdata/unformed/fail_compound_member_access.carbon
@@ -0,0 +1,17 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+class C {
+  var n: i32;
+}
+
+fn Main() -> i32 {
+  var c: C;
+  // CHECK:STDERR: COMPILATION ERROR: fail_compound_member_access.carbon:[[@LINE+1]]: use of uninitialized variable c
+  return c.(C.n);
+}
diff --git a/explorer/testdata/unformed/fail_control_flow_defer_to_dynamic.carbon b/explorer/testdata/unformed/fail_control_flow_defer_to_dynamic.carbon
new file mode 100644
index 000000000000..7f0491e0e00f
--- /dev/null
+++ b/explorer/testdata/unformed/fail_control_flow_defer_to_dynamic.carbon
@@ -0,0 +1,17 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var x: i32;
+  if (0 == 1) {
+    x = 0;
+  }
+  // Static analysis thinks `x` may be formed, defer the check to run-time.
+  // CHECK:STDERR: RUNTIME ERROR: fail_control_flow_defer_to_dynamic.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<x>>
+  return x;
+}
diff --git a/explorer/testdata/unformed/fail_if_cond.carbon b/explorer/testdata/unformed/fail_if_cond.carbon
new file mode 100644
index 000000000000..07d735cbeb78
--- /dev/null
+++ b/explorer/testdata/unformed/fail_if_cond.carbon
@@ -0,0 +1,13 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var v: bool;
+  // CHECK:STDERR: COMPILATION ERROR: fail_if_cond.carbon:[[@LINE+1]]: use of uninitialized variable v
+  return if v then 1 else 2;
+}
diff --git a/explorer/testdata/unformed/fail_if_else.carbon b/explorer/testdata/unformed/fail_if_else.carbon
new file mode 100644
index 000000000000..b3f7e4639375
--- /dev/null
+++ b/explorer/testdata/unformed/fail_if_else.carbon
@@ -0,0 +1,13 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var v: i32;
+  // CHECK:STDERR: COMPILATION ERROR: fail_if_else.carbon:[[@LINE+1]]: use of uninitialized variable v
+  return if false then 1 else v;
+}
diff --git a/explorer/testdata/unformed/fail_if_then.carbon b/explorer/testdata/unformed/fail_if_then.carbon
new file mode 100644
index 000000000000..539a2a74864f
--- /dev/null
+++ b/explorer/testdata/unformed/fail_if_then.carbon
@@ -0,0 +1,13 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var v: i32;
+  // CHECK:STDERR: COMPILATION ERROR: fail_if_then.carbon:[[@LINE+1]]: use of uninitialized variable v
+  return if true then v else 2;
+}
diff --git a/explorer/testdata/unformed/fail_in_class.carbon b/explorer/testdata/unformed/fail_in_class.carbon
new file mode 100644
index 000000000000..642c5f4f98b2
--- /dev/null
+++ b/explorer/testdata/unformed/fail_in_class.carbon
@@ -0,0 +1,19 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+class C {
+  fn F() -> i32 {
+    var n: i32;
+    // CHECK:STDERR: COMPILATION ERROR: fail_in_class.carbon:[[@LINE+1]]: use of uninitialized variable n
+    return n;
+  }
+}
+
+fn Main() -> i32 {
+  return C.F();
+}
diff --git a/explorer/testdata/unformed/fail_indirect_member_access.carbon b/explorer/testdata/unformed/fail_indirect_member_access.carbon
new file mode 100644
index 000000000000..b7b57454bab6
--- /dev/null
+++ b/explorer/testdata/unformed/fail_indirect_member_access.carbon
@@ -0,0 +1,17 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var pt: {.x: i32, .y: i32};
+  // TODO: Without this, we don't allow taking the address of `pt.x`.
+  // That's probably too restrictive.
+  if (1 == 0) { pt = {.x = 1, .y = 2}; }
+  var p: i32* = &pt.x;
+  // CHECK:STDERR: RUNTIME ERROR: fail_indirect_member_access.carbon:[[@LINE+1]]: undefined behavior: access to uninitialized value Uninit<Placeholder<pt>>
+  return *p;
+}
diff --git a/explorer/testdata/unformed/fail_intrinsic.carbon b/explorer/testdata/unformed/fail_intrinsic.carbon
new file mode 100644
index 000000000000..52ef6cfc6b7a
--- /dev/null
+++ b/explorer/testdata/unformed/fail_intrinsic.carbon
@@ -0,0 +1,13 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+fn Main() -> i32 {
+  var v: i32;
+  // CHECK:STDERR: COMPILATION ERROR: fail_intrinsic.carbon:[[@LINE+1]]: use of uninitialized variable v
+  return if __intrinsic_int_eq(v, v) then 1 else 2;
+}