Re: [PATCH] c++: Fix invalid -Wduplicated-cond warning [PR94265]

[Jason Merrill via Gcc-patches]

On 3/25/20 8:52 AM, Patrick Palka wrote:

This fixes a false-positive warning from -Wduplicate-cond in the presence of an
if-statement with a non-empty init-statement.  Precisely determining whether a

non-empty init-statement has side effects seems tricky and error-prone, so this > patch takes the route of unconditionally invalidating the condition

chain when

it encounters such an if-statement.

Tested on x86_64-pc-linux-gnu, does this look OK?

OK.

gcc/cp/ChangeLog:

        PR c++/94265
        * parser.c (cp_parser_selection_statement) [RID_IF]: Invalidate the
        current condition chain if the if-statement has a non-empty
        init-statement.

gcc/testsuite/ChangeLog:

        PR c++/94265
        * g++.dg/warn/Wduplicated-cond1.C: New test.
---
  gcc/cp/parser.c                               |  7 +++++++
  gcc/testsuite/g++.dg/warn/Wduplicated-cond1.C | 16 ++++++++++++++++
  2 files changed, 23 insertions(+)
  create mode 100644 gcc/testsuite/g++.dg/warn/Wduplicated-cond1.C

diff --git a/gcc/cp/parser.c b/gcc/cp/parser.c
index cbd5510a8fb..05363653691 100644
--- a/gcc/cp/parser.c
+++ b/gcc/cp/parser.c
@@ -11934,6 +11934,13 @@ cp_parser_selection_statement (cp_parser* parser, bool 
*if_p,
              pedwarn (cp_lexer_peek_token (parser->lexer)->location, 0,
                       "init-statement in selection statements only available "
                       "with %<-std=c++17%> or %<-std=gnu++17%>");
+           if (cp_lexer_next_token_is_not (parser->lexer, CPP_SEMICOLON))
+             {
+               /* A non-empty init-statement can have arbitrary side
+                  effects.  */
+               delete chain;
+               chain = NULL;
+             }
            cp_parser_init_statement (parser, &decl);
          }

diff --git a/gcc/testsuite/g++.dg/warn/Wduplicated-cond1.C b/gcc/testsuite/g++.dg/warn/Wduplicated-cond1.C

new file mode 100644
index 00000000000..e85920aaedf
--- /dev/null
+++ b/gcc/testsuite/g++.dg/warn/Wduplicated-cond1.C
@@ -0,0 +1,16 @@
+// PR c++/94265
+// { dg-do compile { target c++17 } }
+// { dg-additional-options "-Wduplicated-cond" }
+
+void
+foo ()
+{
+  if (int a = 0; a)
+  { }
+  else if (a = 5; a) // { dg-message "previously used here" }
+  { }
+  else if (; a) // { dg-warning "duplicated .if. condition" }
+  { }
+  else if (int b = ++a; a)
+  { }
+}