Accept empty aggregate initializers when emulating clang versions before 17 [GH #238]

src/Changes

@@ -54,6 +54,20 @@ This is now fixed: When generating code targeting GCC, the C-generating back
 end now preserves the __weak__ attribute and suppresses the __always_inline__
 attribute instead.
 
+10/7/26 [GH #238]
+Clang C compatibility: Empty aggregate initializers
+
+The changes for EDGcpfe/29042 (version 7.0) restricted empty initializers
+in pre-C23 Clang C modes to Clang 17 and later.  However, earlier versions
+of Clang accept empty initializers for aggregate types as an extension.
+This is now fixed.  For example, the following was incorrectly rejected
+when emulating Clang 16 and earlier:
+
+  struct A {
+    int i;
+  };
+
+  struct A a = {};  // Previously an error. Now okay.
 
 10/5/26  [GH #195]
 C++-generating back end: dependent multi-dimensional subscripts

src/decl_inits.c

@@ -2365,7 +2365,7 @@ for this array initialization).  *is describes the initialization as a whole.
       diag_pos = &icp->variant.braced.end_pos;
       /* Unwrap the braced list for the processing that follows. */
       icp = icp->variant.braced.list;
-      if (icp == NULL && C_mode() && !empty_c_initializer_allowed) {
+      if (icp == NULL && C_mode() && !empty_c_aggregate_initializer_allowed) {
         /* Empty initializer lists are permitted only in the C modes that
            offer them. */
         pos_error(ec_exp_primary_expr, diag_pos);
@@ -3778,7 +3778,7 @@ issued if no more specific position is available.
       /* Unwrap the braced list for the processing that follows. */
       icp = icp->variant.braced.list;
       top_icp = icp;
-      if (icp == NULL && C_mode() && !empty_c_initializer_allowed) {
+      if (icp == NULL && C_mode() && !empty_c_aggregate_initializer_allowed) {
         /* Empty initializer lists are permitted only in the C modes that
            offer them. */
         pos_error(ec_exp_primary_expr, diag_pos);

src/lang_feat.h

@@ -1959,6 +1959,15 @@ by the C23 standard or later C standards.
 */
 #define c23_mode (C_mode() && std_version >= 202311)
 
+/*
+Macro that is TRUE when an empty aggregate initializer is accepted in C
+mode.  Such initializers are standard in C23.  They are also an extension in
+the GNU C modes, and in the Clang C modes.
+*/
+#define empty_c_aggregate_initializer_allowed (c23_mode || \
+                                               gcc_version_is(any_version) || \
+                                               clangc_version_is(any_version))
+
 /*
 Macro that is TRUE when an empty initializer ("int i = {};") is accepted in C
 mode.  Such initializers are standard in C23.  They are also an extension in

tests/tests/regressions/.clang-empty-aggregate-initialization.rto/default.1.1.txt

@@ -0,0 +1 @@
+fe_only -DTEST_NUMBER=1 --c11 --clang_version 30800 Test_name.c

tests/tests/regressions/.clang-empty-scalar-initialization.rto/default.1.1.txt

@@ -0,0 +1,6 @@
+fe_only -DTEST_NUMBER=1 --c11 --clang_version 160000 Test_name.c
+"Test_name.c", line 6: error: expected an expression
+  int scalar = {};
+                ^
+
+1 error detected in the compilation of "Test_name.c".

tests/tests/regressions/.clang-empty-universal-initialization.rto/default.1.1.txt

@@ -0,0 +1,5 @@
+fe_only -DTEST_NUMBER=1 --c11 --clang_version 170000 Test_name.c
+"Test_name.c", line 18: warning: variable "vla" was declared but never referenced
+    int vla[n] = {};
+        ^
+

tests/tests/regressions/.clang-empty-vla-initialization.rto/default.1.1.txt

@@ -0,0 +1,10 @@
+fe_only -DTEST_NUMBER=1 --c11 --clang_version 160000 Test_name.c
+"Test_name.c", line 8: error: variable "vla" may not be initialized
+    int vla[n] = {};
+        ^
+
+"Test_name.c", line 8: warning: variable "vla" was declared but never referenced
+    int vla[n] = {};
+        ^
+
+1 error detected in the compilation of "Test_name.c".

tests/tests/regressions/clang-empty-aggregate-initialization.sft.cpp

@@ -0,0 +1,25 @@
+//type:fp
+//options_all:--c11 --clang_version 30800
+//
+// Clang accepts empty initializers for aggregate types as an extension in
+// pre-C23 C modes.
+
+struct A {
+  int i;
+};
+
+union U {
+  int i;
+  double d;
+};
+
+struct B {
+  int i;
+  struct A a;
+};
+
+struct A a = {};
+union U u = {};
+int fixed_array[4] = {};
+int unknown_bound_array[] = {};
+struct B nested = { 1, {} };

tests/tests/regressions/clang-empty-scalar-initialization.sft.cpp

@@ -0,0 +1,7 @@
+//type:fn
+//options_all:--c11 --clang_version 160000
+//
+// Clang versions before 17 do not accept accept empty scalar initializers.
+
+int scalar = {};
+

tests/tests/regressions/clang-empty-universal-initialization.sft.cpp

@@ -0,0 +1,19 @@
+//type:fp
+//options_all:--c11 --clang_version 170000
+//
+// Beginning with Clang 17, empty initializers are accepted as an extension for
+// scalar types and variable-length arrays as well as aggregates.
+
+struct A {
+  int i;
+};
+
+struct A aggregate = {};
+int scalar = {};
+double floating = {};
+void *pointer = {};
+int compound_literal = (int){};
+
+void f(int n) {
+  int vla[n] = {};
+}

tests/tests/regressions/clang-empty-vla-initialization.sft.cpp

@@ -0,0 +1,9 @@
+//type:fn
+//options_all:--c11 --clang_version 160000
+//
+// Clang versions before 17 do not accept empty initializers for variable-length
+// arrays.
+
+void f(int n) {
+  int vla[n] = {};
+}