Don't require named return values be initialized before (other) uninitialized locals

The reason to initialize in declaration order is for things that depend on nested lifetime, which is common for local variables – such as a smart pointer variable followed by another variable whose delayed initialization depends on the smart pointer being constructed and keeping something alive. That doesn’t apply to return values.

Author: hsutter

regression-tests/pure2-initialization-safety-with-else-if.cpp2

@@ -24,6 +24,14 @@ main: (args) = {
     std::cout << p* << std::endl;
 }
 
+ad_test: type = {
+  intermediate_default_no_init: (x: double, y: double) -> (r: double) = {
+    t: double;
+    t = x + y;
+
+    r = t;              // OK, after t but it's a return value
+  }
+}
 
 ok: () = {
     i: int;

regression-tests/test-results/pure2-initialization-safety-with-else-if.cpp

@@ -8,13 +8,29 @@
 
 #line 1 "pure2-initialization-safety-with-else-if.cpp2"
 
+#line 27 "pure2-initialization-safety-with-else-if.cpp2"
+class ad_test;
+  
 
 //=== Cpp2 type definitions and function declarations ===========================
 
 #line 1 "pure2-initialization-safety-with-else-if.cpp2"
 auto main(int const argc_, char** argv_) -> int;
 
+#line 27 "pure2-initialization-safety-with-else-if.cpp2"
+class ad_test {
+using intermediate_default_no_init_ret = double;
+
 #line 28 "pure2-initialization-safety-with-else-if.cpp2"
+  public: [[nodiscard]] static auto intermediate_default_no_init(cpp2::impl::in<double> x, cpp2::impl::in<double> y) -> intermediate_default_no_init_ret;
+  public: ad_test() = default;
+  public: ad_test(ad_test const&) = delete; /* No 'that' constructor, suppress copy */
+  public: auto operator=(ad_test const&) -> void = delete;
+
+
+#line 34 "pure2-initialization-safety-with-else-if.cpp2"
+};
+
 auto ok() -> void;
 
 //=== Cpp2 function definitions =================================================
@@ -49,6 +65,16 @@ auto main(int const argc_, char** argv_) -> int{
 }
 
 #line 28 "pure2-initialization-safety-with-else-if.cpp2"
+  [[nodiscard]] auto ad_test::intermediate_default_no_init(cpp2::impl::in<double> x, cpp2::impl::in<double> y) -> intermediate_default_no_init_ret{
+        cpp2::impl::deferred_init<double> r;
+#line 29 "pure2-initialization-safety-with-else-if.cpp2"
+    cpp2::impl::deferred_init<double> t; 
+    t.construct(x + y);
+
+    r.construct(cpp2::move(t.value()));// OK, after t but it's a return value
+  return std::move(r.value()); }
+
+#line 36 "pure2-initialization-safety-with-else-if.cpp2"
 auto ok() -> void{
     cpp2::impl::deferred_init<int> i; 
     if (true) {

regression-tests/test-results/version

@@ -1,4 +1,4 @@
 
-cppfront compiler v0.8.2 Build A811:1537
+cppfront compiler v0.8.2 Build A812:1040
 SPDX-License-Identifier  Apache-2.0 WITH LLVM-exception
 Copyright (c) 2022-2025  Herb Sutter

source/build.info

@@ -1 +1 @@
-"A811:1537"
+"A812:1040"

source/sema.h

@@ -1160,6 +1160,7 @@ class sema
     ) const
         -> bool
     {
+        assert(decl);
         auto name = decl->identifier->to_string();
 
         struct selection_stack_entry{
@@ -1230,9 +1231,9 @@ class sema
                 auto const& sym = std::get<symbol::active::identifier>(symbols[pos].sym);
                 assert (sym.identifier);
 
-                if (
-                    sym.is_use()
+                if (sym.is_use()
                     && is_definite_initialization(sym.identifier)
+                    && !decl->return_param
                     )
                 {
                     errors.emplace_back(
@@ -2375,7 +2376,7 @@ class sema
         }
 
         if (n.pass != passing_style::out) {
-            push_activation( declaration_sym( true, n.declaration.get(), n.declaration->name(), n.declaration->initializer.get(), &n, inside_returns_list));
+            push_activation( declaration_sym( true, n.declaration.get(), n.declaration->name(), n.declaration->initializer.get(), &n, false, inside_returns_list));
             current_declarations.push_back( &symbols.back().as_declaration() );
         }
     }