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

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.

gcc/cp/ChangeLog:

	PR c++/94265
	* parser.c (cp_parser_selection_statement) <case RID_IF>: Invalidate the
	current condition chain when 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/ChangeLog

+2020-03-25  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94265
+	* parser.c (cp_parser_selection_statement) <case RID_IF>: Invalidate the
+	current condition chain when the if-statement has a non-empty
+	init-statement.
+
 2020-03-25  Iain Sandoe  <iain@sandoe.co.uk>
 
 	PR c++/94319

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);
 	  }
 

gcc/testsuite/ChangeLog

+2020-03-25  Patrick Palka  <ppalka@redhat.com>
+
+	PR c++/94265
+	* g++.dg/warn/Wduplicated-cond1.C: New test.
+
 2020-03-25  Martin Sebor  <msebor@redhat.com>
 
 	PR tree-optimization/94131

gcc/testsuite/g++.dg/warn/Wduplicated-cond1.C

+// 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)
+  { }
+}