[PATCH] void-cond: allow void in conditional expression

[Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>]

As an extension, allow conditional expressions (?:) with one
side of type 'void' and consider the result to also be void.

The warning can be reinstated with the flag '-Wcond-void'.

Note: I only see a single occurrence of this in the kernel.

Suggested-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@xxxxxxxxx>
---
 evaluate.c                   | 7 +++++++
 lib.c                        | 2 ++
 lib.h                        | 1 +
 sparse.1                     | 7 +++++++
 validation/cond-err-expand.c | 2 +-
 5 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/evaluate.c b/evaluate.c
index 919c944cf..fb1a0548d 100644
--- a/evaluate.c
+++ b/evaluate.c
@@ -1205,6 +1205,13 @@ static struct symbol *evaluate_conditional_expression(struct expression *expr)
 		goto out;
 	}
 
+	// extension: allow the result be void is one side is void
+	if (ltype == &void_ctype || rtype == &void_ctype) {
+		ctype = &void_ctype;
+		if (!Wcond_void)
+			goto out;
+	}
+
 	if ((lclass | rclass) & TYPE_PTR) {
 		int is_null1 = is_null_pointer_constant(*cond);
 		int is_null2 = is_null_pointer_constant(expr->cond_false);
diff --git a/lib.c b/lib.c
index 83e6a1e1b..c16d87778 100644
--- a/lib.c
+++ b/lib.c
@@ -257,6 +257,7 @@ int Wbitwise_pointer = 0;
 int Wcast_from_as = 0;
 int Wcast_to_as = 0;
 int Wcast_truncate = 1;
+int Wcond_void = 0;
 int Wconstant_suffix = 0;
 int Wconstexpr_not_const = 0;
 int Wcontext = 1;
@@ -639,6 +640,7 @@ static const struct flag warnings[] = {
 	{ "cast-from-as", &Wcast_from_as },
 	{ "cast-to-as", &Wcast_to_as },
 	{ "cast-truncate", &Wcast_truncate },
+	{ "cond-void", &Wcond_void },
 	{ "constant-suffix", &Wconstant_suffix },
 	{ "constexpr-not-const", &Wconstexpr_not_const},
 	{ "context", &Wcontext },
diff --git a/lib.h b/lib.h
index 697c977a1..2549c8872 100644
--- a/lib.h
+++ b/lib.h
@@ -145,6 +145,7 @@ extern int Wbitwise_pointer;
 extern int Wcast_from_as;
 extern int Wcast_to_as;
 extern int Wcast_truncate;
+extern int Wcond_void;
 extern int Wconstant_suffix;
 extern int Wconstexpr_not_const;
 extern int Wcontext;
diff --git a/sparse.1 b/sparse.1
index beb484423..71ac0c646 100644
--- a/sparse.1
+++ b/sparse.1
@@ -117,6 +117,13 @@ Sparse issues these warnings by default.  To turn them off, use
 \fB\-Wno\-cast\-truncate\fR.
 .
 .TP
+.B \-Wconditional\-void
+Warn if one side of a conditional expression (\fB? :\fR) is of type
+void and the other one not.
+As an extension, Sparse consider the result of such expressions as also
+having the type void.
+.
+.TP
 .B \-Wconstant\-suffix
 Warn if an integer constant is larger than the maximum representable value
 of the type indicated by its type suffix (if any). For example, on a
diff --git a/validation/cond-err-expand.c b/validation/cond-err-expand.c
index b52624bc9..714cb23eb 100644
--- a/validation/cond-err-expand.c
+++ b/validation/cond-err-expand.c
@@ -15,7 +15,7 @@ void bar(void)
 
 /*
  * check-name: cond-err-expand.c
- * check-command: test-linearize -Wno-decl $file
+ * check-command: test-linearize -Wcond-void -Wno-decl $file
  *
  * check-error-start
 cond-err-expand.c:8:11: error: incompatible types in conditional expression (different base types):
-- 
2.23.0