c: Implement C2y N3356, if declarations [PR117019]

This patch implements C2y N3356, if declarations as described at
<https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3356.htm>.

This feature is cognate with C++17 Selection statements with initializer
<https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0305r1.html>,
but they are not the same yet.  For example, C++17 allows

  if (lock (); int i = getval ())

whereas C2y does not.

The proposal adds new grammar productions.  selection-header is handled
in c_parser_selection_header which is the gist of the patch.
simple-declaration is handled by c_parser_declaration_or_fndef, which
gets a new parameter.

	PR c/117019

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_declaration_or_fndef): Adjust declaration.
	(c_parser_external_declaration): Adjust a call to
	c_parser_declaration_or_fndef.
	(c_parser_declaration_or_fndef): New bool parameter.  Return a tree
	instead of void.  Adjust for N3356.  Adjust a call to
	c_parser_declaration_or_fndef.
	(c_parser_compound_statement_nostart): Adjust calls to
	c_parser_declaration_or_fndef.
	(c_parser_selection_header): New.
	(c_parser_paren_selection_header): New.
	(c_parser_if_statement): Call c_parser_paren_selection_header
	instead of c_parser_paren_condition.
	(c_parser_switch_statement): Call c_parser_selection_header instead of
	c_parser_expression.
	(c_parser_for_statement): Adjust calls to c_parser_declaration_or_fndef.
	(c_parser_objc_methodprotolist): Likewise.
	(c_parser_oacc_routine): Likewise.
	(c_parser_omp_loop_nest): Likewise.
	(c_parser_omp_declare_simd): Likewise.

gcc/testsuite/ChangeLog:

	* gcc.dg/c23-if-decls-1.c: New test.
	* gcc.dg/c23-if-decls-2.c: New test.
	* gcc.dg/c2y-if-decls-1.c: New test.
	* gcc.dg/c2y-if-decls-2.c: New test.
	* gcc.dg/c2y-if-decls-3.c: New test.
	* gcc.dg/c2y-if-decls-4.c: New test.
	* gcc.dg/c2y-if-decls-5.c: New test.
	* gcc.dg/c2y-if-decls-6.c: New test.
	* gcc.dg/c2y-if-decls-7.c: New test.
	* gcc.dg/c2y-if-decls-8.c: New test.
	* gcc.dg/c2y-if-decls-9.c: New test.
	* gcc.dg/c2y-if-decls-10.c: New test.
	* gcc.dg/c2y-if-decls-11.c: New test.
	* gcc.dg/gnu2y-if-decls-1.c: New test.
	* gcc.dg/gnu99-if-decls-1.c: New test.
	* gcc.dg/gnu99-if-decls-2.c: New test.

gcc/testsuite/gcc.dg/c23-if-decls-1.c

@@ -0,0 +1,15 @@
+/* N3356 - if declarations.  */
+/* PR c/117019 */
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -pedantic-errors" } */
+
+void
+g ()
+{
+  if (int i = 42);		/* { dg-error "ISO C does not support if declarations before C2Y" } */
+  if (int i = 42; i > 10);	/* { dg-error "ISO C does not support if declarations before C2Y" } */
+  if (int i, j; i = 42);	/* { dg-error "ISO C does not support if declarations before C2Y" } */
+  switch (int i = 42);		/* { dg-error "ISO C does not support if declarations before C2Y" } */
+  switch (int i = 42; i);	/* { dg-error "ISO C does not support if declarations before C2Y" } */
+  switch (int i, j; i = 42);	/* { dg-error "ISO C does not support if declarations before C2Y" } */
+}

gcc/testsuite/gcc.dg/c23-if-decls-2.c

@@ -0,0 +1,6 @@
+/* N3356 - if declarations.  */
+/* PR c/117019 */
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -pedantic-errors -Wno-c23-c2y-compat" } */
+
+#include "c23-if-decls-1.c"

gcc/testsuite/gcc.dg/c2y-if-decls-1.c

@@ -0,0 +1,168 @@
+/* N3356 - if declarations.  */
+/* PR c/117019 */
+/* { dg-do run } */
+/* { dg-options "-std=c2y -Wc23-c2y-compat" } */
+/* Test C2Y if declarations.  Valid usages.  */
+
+int get () { return 42; }
+int foo (int i) { return i; }
+
+enum E { X = 1, Y };
+
+void
+simple ()
+{
+  if (int i = get ())  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (int i = 0)  /* { dg-warning "if declarations before C2Y" } */
+    __builtin_abort ();
+  else
+    foo (i);
+
+  if (auto i = get ())  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (__typeof__(get ()) i = get ())  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (auto i = 0)  /* { dg-warning "if declarations before C2Y" } */
+    __builtin_abort ();
+  else
+    foo (i);
+
+  if (int (*f)(int) = foo)  /* { dg-warning "if declarations before C2Y" } */
+    f (1);
+  else
+    __builtin_abort ();
+
+  if ([[maybe_unused]] int i = get ())  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (__attribute__((unused)) int i = get ())  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (enum E e = X)  /* { dg-warning "if declarations before C2Y" } */
+    foo (e);
+  else
+    __builtin_abort ();
+
+  if (constexpr int i = 42)  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (int i = 1)  /* { dg-warning "if declarations before C2Y" } */
+    if (int j = 2)  /* { dg-warning "if declarations before C2Y" } */
+      if (int k = 3)  /* { dg-warning "if declarations before C2Y" } */
+	foo (i + j + k);
+
+  if (register int i = 0);  /* { dg-warning "if declarations before C2Y" } */
+  if (static int i = 0);  /* { dg-warning "if declarations before C2Y" } */
+  if (static int arr[3] = {});  /* { dg-warning "if declarations before C2Y" } */
+  if (_Atomic int i = 0);  /* { dg-warning "if declarations before C2Y" } */
+
+  if (int arr[] = { 1 })  /* { dg-warning "if declarations before C2Y" } */
+    foo (arr[0]);
+  else
+    __builtin_abort ();
+
+  double i;
+}
+
+void
+expr ()
+{
+  if (int i = get (); i == 42)	/* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (int i = get (); i != 42)  /* { dg-warning "if declarations before C2Y" } */
+    __builtin_abort ();
+  else
+    foo (i);
+
+  if (auto i = get (); i == 42)  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (__typeof__(get ()) i = get (); i == 42)  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (auto i = get (); i != 42)  /* { dg-warning "if declarations before C2Y" } */
+    __builtin_abort ();
+  else
+    foo (i);
+
+  if (int (*f)(int) = foo; f (42))  /* { dg-warning "if declarations before C2Y" } */
+    f (1);
+  else
+    __builtin_abort ();
+
+  if ([[maybe_unused]] int i = get (); i == 42)  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (__attribute__((unused)) int i = get (); i == 42)  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (enum E e = X; e == X)  /* { dg-warning "if declarations before C2Y" } */
+    foo (e);
+  else
+    __builtin_abort ();
+
+  if (constexpr int i = 42; i == 42)  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (int i = 1; i)  /* { dg-warning "if declarations before C2Y" } */
+    if (int j = 2; j)  /* { dg-warning "if declarations before C2Y" } */
+      if (int k = 3; k)  /* { dg-warning "if declarations before C2Y" } */
+	foo (i + j + k);
+
+  if (int i = 2, j = get (); i + j > 0)  /* { dg-warning "if declarations before C2Y" } */
+    foo (i + j);
+  else
+    __builtin_abort ();
+
+  if (int i; i = 1)  /* { dg-warning "if declarations before C2Y" } */
+    foo (i);
+  else
+    __builtin_abort ();
+
+  if (int arr[] = { 1, 2, 3}; arr[0])  /* { dg-warning "if declarations before C2Y" } */
+    foo (arr[0]);
+  else
+    __builtin_abort ();
+
+  if (register int i = 0; i);  /* { dg-warning "if declarations before C2Y" } */
+  if (static int i = 0; i);  /* { dg-warning "if declarations before C2Y" } */
+  if (_Atomic int i = 0; i);  /* { dg-warning "if declarations before C2Y" } */
+
+  double i;
+}
+
+int
+main ()
+{
+  simple ();
+  expr ();
+}

gcc/testsuite/gcc.dg/c2y-if-decls-10.c

@@ -0,0 +1,38 @@
+/* N3356 - if declarations.  */
+/* PR c/117019 */
+/* { dg-do compile } */
+/* { dg-options "-std=c2y -pedantic-errors" } */
+/* Test C2Y if declarations.  Invalid usages.  */
+
+void
+g (int g)
+{
+  switch (;);	    /* { dg-error "expected" } */
+  switch (int);	    /* { dg-error "expected identifier" } */
+  /* { dg-error "declaration" "" { target *-*-* } .-1 } */
+  switch (auto);    /* { dg-error "expected identifier" } */
+  /* { dg-error "declaration" "" { target *-*-* } .-1 } */
+  switch (int;);    /* { dg-error "declaration" } */
+  switch (auto;);	    /* { dg-error "empty|initializer" } */
+  switch (int i);	    /* { dg-error "initializer" } */
+  switch (int i;);	    /* { dg-error "expected" } */
+  switch (int i = 0;);  /* { dg-error "expected" } */
+
+  switch (extern int i = 0);  /* { dg-error "both .extern. and initializer" } */
+  switch (extern int i);  /* { dg-error "initializer" } */
+  switch (thread_local int i = 0); /* { dg-error "function-scope" } */
+  switch (typedef int i); /* { dg-error "initializer" } */
+  switch (typedef int i = 0); /* { dg-error "initialized" } */
+
+  switch (int i = 2, j = 3);  /* { dg-error "only declare a single object" } */
+
+  switch (void (*fp)(int));  /* { dg-error "initializer" } */
+  switch ([[maybe_unused]] g);  /* { dg-error "expected" } */
+  switch ([[maybe_unused]] 42);  /* { dg-error "expected" } */
+  switch ([[maybe_unused]] int);  /* { dg-error "expected|initializer" } */
+  switch (__attribute__((unused)) g); /* { dg-error "initializer" } */
+  switch (__attribute__((unused)) 42);  /* { dg-error "expected|initializer" } */
+  switch (__attribute__((unused)) int);  /* { dg-error "expected|initializer" } */
+
+  switch (int arr[] = { 1 });  /* { dg-error "switch quantity not an integer" } */
+}