ImportC: va_arg fails in working C code (#21186)

Fixes https://github.com/dlang/dmd/issues/20423

Ultimate cause of this issue was that va_arg was being shadowed
by the collected template-like macros. As va_arg is not a normal
function (it takes a type as a parameter), this interfered with
the cparser's rewrite of va_arg to a call to the single argument
template version in core.stdc.stdarg.

compiler/src/dmd/cparse.d

@@ -6085,6 +6085,14 @@ final class CParser(AST) : Parser!AST
             }
         }
 
+        void nextLine()
+        {
+            // scan to end of line
+            while (*p)
+                ++p;
+            ++p; // advance to start of next line
+        }
+
         while (p < endp)
         {
             //printf("|%s|\n", p);
@@ -6096,6 +6104,13 @@ final class CParser(AST) : Parser!AST
                 if (token.value == TOK.identifier)
                 {
                     auto id = token.ident;
+                    // https://github.com/dlang/dmd/issues/20423
+                    // skip macros that could shadow special builtins
+                    if (id == Id.va_arg)
+                    {
+                        nextLine();
+                        continue;
+                    }
                     const params = *p == '(';
                     nextToken();
 
@@ -6341,10 +6356,7 @@ final class CParser(AST) : Parser!AST
                 if (token.value == TOK.identifier)
                     removeSym(token.ident);
             }
-            // scan to end of line
+            nextLine();
-            while (*p)
-                ++p;
-            ++p; // advance to start of next line
         }
 
         if (newSymbols.length)

compiler/test/compilable/test20423.c

@@ -0,0 +1,18 @@
+// https://github.com/dlang/dmd/issues/20423
+#include <stdarg.h>
+#include <stddef.h>
+#include <assert.h>
+
+void foo(double * pm, ...) {
+    va_list ap;
+    double * targ;
+    va_start(ap, pm);
+    for (int i=1; ; i++) {
+        va_arg(ap, int);
+        targ = va_arg(ap, double*);
+        if (targ == NULL) {
+            break;
+        }
+    }
+    va_end(ap);
+}