Show why incomplete auto is not diagnosed in a template definition. [GH #27]

Author: mohitmishra786

src/Changes

@@ -13,6 +13,8 @@ is for decltype(auto).  For example:
   auto *p = g();   // Error: cannot deduce "auto" type
 
 Forms that are not a plain "auto", such as "auto *", still fail deduction.
+The diagnostic is not issued while scanning a template definition, because
+the initializer type may be completed before the template is instantiated.
 
 
 10/5/26  [GH #195]

src/overload.c

@@ -31244,9 +31244,11 @@ TRUE and FALSE is returned.
       /* Adjust the argument and parameter types for deduction.  Some types can
          never succeed:  Issue an error and don't attempt deduction any
          further. */
-      /* In a prototype instantiation a deduction failure is often
-         suppressed.  Keep that behavior for an incomplete initializer:
-         the incomplete-type diagnostic is issued at instantiation. */
+      /* Do not deduce an incomplete type while scanning a template
+         definition.  The type may be completed before instantiation, and
+         a deduction failure here is suppressed in GNU and Microsoft modes.
+         Diagnosing an incomplete type instead would reject a valid
+         template.  See GH27. */
       a_boolean  allow_incomplete_arg =
                                  !scope_stack_top().in_prototype_instantiation;
       if (!adjust_deduction_pair(&type, &arg_type, initializer_operand,

tests/tests/changes/.GH27.rto/default.2.1.txt

@@ -0,0 +1,30 @@
+fe_only -DTEST_NUMBER=2 --c++23 --gnu_version=999999 -DSHOW_TEMPLATE_DEFN Test_name.c
+"Test_name.c", line 27: error: incomplete type "void" is not allowed
+    auto v1 = void();
+         ^
+
+"Test_name.c", line 28: error: incomplete type "void" is not allowed
+    decltype(auto) v2 = void();
+                   ^
+
+"Test_name.c", line 29: error: incomplete type "Incomplete" is not allowed
+    auto v3 = getIncomplete();
+         ^
+
+"Test_name.c", line 30: error: incomplete type "void" is not allowed
+    auto opts = Args{}.parse();
+         ^
+
+"Test_name.c", line 32: error: cannot deduce "auto" type
+    auto *v4 = void();
+    ^
+
+"Test_name.c", line 33: error: cannot deduce "auto" type
+    auto *v5 = 1;
+    ^
+
+"Test_name.c", line 34: error: cannot deduce "auto" type
+    auto *p = g();
+    ^
+
+7 errors detected in the compilation of "Test_name.c".

tests/tests/changes/GH27.sft.cpp

@@ -1,5 +1,5 @@
 //type:fn
-//options_all:--c++23
+//options:--c++23:--c++23 --gnu_version=999999 -DSHOW_TEMPLATE_DEFN
 //remark:[GH #27] Clearer diagnostic when auto deduces to void
 // 10/6/26  [GH #27]
 //
@@ -33,3 +33,23 @@ void f()
   auto *v5 = 1;
   auto *p = g();
 }
+
+#if SHOW_TEMPLATE_DEFN
+/* Incomplete in the template definition, complete before instantiation.
+   The definition must not be rejected. */
+struct Later;
+extern Later later_obj;
+
+template<int>
+void use_later()
+{
+  auto x = later_obj;
+}
+
+struct Later { int n; };
+
+void call_later()
+{
+  use_later<0>();
+}
+#endif /* SHOW_TEMPLATE_DEFN */