Merge branch 'ebassi/c11-toolchain-req-docs' into 'main'

docs: Update toolchain requirement to C11

See merge request GNOME/glib!4082

docs/toolchain-requirements.md

@@ -1,14 +1,15 @@
 Toolchain/Compiler requirements
 ===
 
-GLib requires a toolchain that supports C99.
+GLib strongly recommends a toolchain that supports C11, and requires a toolchain
+that supports C99.
 
 GLib contains some fall back code that allows supporting toolchains that are not
-fully C99-compatible.
+fully C11-compatible.
 
 GLib makes some assumptions about features of the C library and C preprocessor,
-compiler and linker that may go beyond what C99 mandates.  We will use features
-beyond C99 if they are substantially useful and if they are supported in a wide
+compiler and linker that may go beyond what C11 mandates.  We will use features
+beyond C11 if they are substantially useful and if they are supported in a wide
 range of compilers.
 
 In general, we are primarily interested in supporting these four compilers:
@@ -116,11 +117,10 @@ _Hard requirement._
 Your compiler must support `alloca()`, defined in `<alloca.h>` (or `<malloc.h>`
 on Windows) and it must accept a non-constant argument.
 
-(C11) support for type redefinition
+C11 support for type redefinition
 ---
 
-**This requirement has been temporarily suspended (on account of OpenBSD
-carrying an old version of gcc) but it will probably return in the future.**
+_Hard requirement._
 
 Your compiler must allow “a typedef name [to] be redefined to denote the same
 type as it currently does”, as per C11 §6.7, item 3.
@@ -193,4 +193,3 @@ GLib [requires a C99 `stdint.h`](https://gitlab.gnome.org/GNOME/glib/-/merge_req
 with all the usual sized integer types (`int8_t`, `uint64_t` and so on),
 believed to be supported by all relevant Unix platforms/compilers, as well as
 Microsoft compilers since MSVC 2013.
-

glib/glib-init.c

@@ -215,6 +215,10 @@ G_STATIC_ASSERT (G_SIGNEDNESS_OF (guint64) == 0);
 G_STATIC_ASSERT (G_SIGNEDNESS_OF (int64_t) == 1);
 G_STATIC_ASSERT (G_SIGNEDNESS_OF (uint64_t) == 0);
 
+/* C11 §6.7, item 3 allows us to rely on this being allowed */
+typedef struct Foo Foo;
+typedef struct Foo Foo;
+
 /**
  * g_mem_gc_friendly:
  *

meson.build

@@ -585,6 +585,10 @@ if cc.get_id() == 'gcc' or cc.get_id() == 'clang'
     '-Werror=missing-prototypes',
     '-Werror=pointer-sign',
     '-Wno-string-plus-int',
+    # We require a compiler that supports C11 even though it's not yet a
+    # strict requirement, so allow typedef redefinition not to break clang and
+    # older gcc versions.
+    '-Wno-typedef-redefinition',
   ]
   warning_cxx_args = warning_common_args
   warning_objc_args = warning_c_args