Run C preprocessor in C11 mode

dmd/cparse.d

@@ -3437,6 +3437,11 @@ final class CParser(AST) : Parser!AST
                         cparseParens();
                 }
             }
+            else if (token.value == TOK._Noreturn)
+            {
+                specifier.noreturn = true;
+                nextToken();
+            }
             else if (token.value == TOK.restrict) // ImportC assigns no semantics to `restrict`, so just ignore the keyword.
                 nextToken();
             else

driver/cpreprocessor.cpp

@@ -98,6 +98,8 @@ FileName runCPreprocessor(FileName csrcfile, Loc loc, OutBuffer &defines) {
   std::vector<std::string> args;
   const std::string &cc = getCC(isMSVC, args);
 
+  args.push_back(isMSVC ? "/std:c11" : "-std=c11");
+
   if (!isMSVC)
     appendTargetArgsForGcc(args);
 

runtime/druntime/src/importc.h

@@ -167,14 +167,19 @@ typedef unsigned long long __uint64_t;
 /***************************
  * C11 6.10.8.3 Conditional feature macros
  */
+#if !(defined(_MSC_VER) && defined(__STDC_NO_VLA__)) // pre-defined to 1 by MS when using /std:cXX and causing warning C4117
 #define __STDC_NO_VLA__ 1
+#endif
 
 #define _Float16 float
-#if linux  // Microsoft won't allow the following macro
+#ifdef __linux__  // Microsoft won't allow the following macro
 // Ubuntu's assert.h uses this
 #define __PRETTY_FUNCTION__ __func__
 
-#ifndef __aarch64__
+#ifndef __clang__
+// Glibc with clang gets upset when some _Float* is defined:
+// /usr/include/bits/floatn-common.h(214): Error: illegal combination of type specifiers
+// typedef float float;
 #define _Float32 float
 #define _Float32x double
 #define _Float64 double

runtime/druntime/test/importc_compare/Makefile

@@ -23,8 +23,6 @@ include ../common.mak
 extra_dflags += -d
 
 ifeq ($(OS),windows)
-    extra_dflags += -P=/std:c11
-
     ifdef IN_LDC
         # use Microsoft's cl.exe as preprocessor; clang-cl leads to unsupported `_Float16` etc.
         extra_dflags += -gcc=cl

tests/dmd/compilable/fix24187.c

@@ -2,8 +2,8 @@
 
 // https://issues.dlang.org/show_bug.cgi?id=24187
 
-#ifdef linux
-#ifndef __aarch64__
+#ifdef __linux__
+#ifndef __clang__
 extern _Complex _Float32 cacosf32 (_Complex _Float32 __z) __attribute__ ((__nothrow__ , __leaf__));
 
 extern _Complex _Float32x __cacosf32x (_Complex _Float32x __z) __attribute__ ((__nothrow__ , __leaf__));

tests/dmd/compilable/stdcheaders.c

@@ -32,10 +32,12 @@
 #include <limits.h>
 #include <locale.h>
 
+#if !(defined(__linux__) && defined(__aarch64__)) // /usr/include/bits/math-vector.h(162): Error: undefined identifier `__Float32x4_t`
 #include <math.h>
 #ifndef _MSC_VER // C:\Program Files (x86)\Windows Kits\10\include\10.0.26100.0\ucrt\corecrt_math.h(93): Error: reinterpretation through overlapped field `f` is not allowed in CTFE
 float x = NAN;
 #endif
+#endif
 
 #ifndef _MSC_VER // setjmp.h(51): Error: missing tag `identifier` after `struct
 #include <setjmp.h>
@@ -71,16 +73,15 @@ float x = NAN;
 #include <string.h>
 
 #ifndef _MSC_VER // C:\Program Files (x86)\Windows Kits\10\include\10.0.22621.0\ucrt\tgmath.h(33): Error: no type for declarator before `)`
-#ifndef __APPLE__ // /Applications/Xcode-14.2.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/tgmath.h(39): Error: named parameter required before `...`
-#ifndef __OpenBSD__ // /usr/lib/clang/13.0.0/include/tgmath.h(34): Error: named parameter required before `...`
-#if !(defined(__linux__) && defined(__aarch64__)) // /tmp/clang/lib/clang/15.0.3/include/tgmath.h(34): Error: named parameter required before `...`
-#ifndef __FreeBSD__ // /usr/local/llvm15/lib/clang/15.0.7/include/tgmath.h(34): Error: named parameter required before `...`
+#ifndef __clang__
+// Apple: /Applications/Xcode-14.2.0.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX.sdk/usr/include/tgmath.h(39): Error: named parameter required before `...`
+// OpenBSD: /usr/lib/clang/13.0.0/include/tgmath.h(34): Error: named parameter required before `...`
+// Linux: /tmp/clang/lib/clang/15.0.3/include/tgmath.h(34): Error: named parameter required before `...`
+#if !(defined(__linux__) && defined(__aarch64__)) // /usr/include/bits/math-vector.h(162): Error: undefined identifier `__Float32x4_t`
 #include <tgmath.h>
 #endif
 #endif
 #endif
-#endif
-#endif
 
 #ifndef __linux__
 #ifndef __APPLE__

tests/dmd/compilable/testcstuff1.c

@@ -295,6 +295,8 @@ void test2()
 
     inline int f();
     _Noreturn void g();
+    __declspec(noreturn) void g2();
+    __declspec(_Noreturn) void g3();
 
     _Static_assert(1, "ok");
 }

tests/dmd/runnable/test23889.c

@@ -1,8 +1,8 @@
 // DISABLED: win freebsd openbsd
 
-// https://issues.dlang.org/show_bug.cgi?id=23886
+// https://issues.dlang.org/show_bug.cgi?id=23889
 
-#include <stdlib.h>
+#include <alloca.h>
 
 int main()
 {