[C11/C2x] Change the behavior of the implicit function declaration warning

C89 had a questionable feature where the compiler would implicitly
declare a function that the user called but was never previously
declared. The resulting function would be globally declared as
extern int func(); -- a function without a prototype which accepts zero
or more arguments.

C99 removed support for this questionable feature due to severe
security concerns. However, there was no deprecation period; C89 had
the feature, C99 didn't. So Clang (and GCC) both supported the
functionality as an extension in C99 and later modes.

C2x no longer supports that function signature as it now requires all
functions to have a prototype, and given the known security issues with
the feature, continuing to support it as an extension is not tenable.

This patch changes the diagnostic behavior for the
-Wimplicit-function-declaration warning group depending on the language
mode in effect. We continue to warn by default in C89 mode (due to the
feature being dangerous to use). However, because this feature will not
be supported in C2x mode, we've diagnosed it as being invalid for so
long, the security concerns with the feature, and the trivial
workaround for users (declare the function), we now default the
extension warning to an error in C99-C17 mode. This still gives users
an easy workaround if they are extensively using the extension in those
modes (they can disable the warning or use -Wno-error to downgrade the
error), but the new diagnostic makes it more clear that this feature is
not supported and should be avoided. In C2x mode, we no longer allow an
implicit function to be defined and treat the situation the same as any
other lookup failure.

Differential Revision: https://reviews.llvm.org/D122983

3 files changed, 10 insertions, 4 deletions

diff --git a/clang/test/Rewriter/finally.m b/clang/test/Rewriter/finally.m
index 33b919e..89ecfe0 100644
--- a/clang/test/Rewriter/finally.m
+++ b/clang/test/Rewriter/finally.m
@@ -1,9 +1,10 @@
 // RUN: %clang_cc1 -rewrite-objc -fobjc-runtime=macosx-fragile-10.5 -fobjc-exceptions -verify %s -o -
 
+extern int printf(const char *, ...);
+
 int main(void) {
   @try {
-    printf("executing try"); // expected-warning{{implicitly declaring library function 'printf' with type 'int (const char *, ...)'}} \
-        // expected-note{{include the header <stdio.h> or explicitly provide a declaration for 'printf'}}
+    printf("executing try");
     return(0); // expected-warning{{rewriter doesn't support user-specified control flow semantics for @try/@finally (code may not execute properly)}}
   } @finally {
     printf("executing finally");
diff --git a/clang/test/Rewriter/rewrite-foreach-2.m b/clang/test/Rewriter/rewrite-foreach-2.m
index a0f7db3..c3636812 100644
--- a/clang/test/Rewriter/rewrite-foreach-2.m
+++ b/clang/test/Rewriter/rewrite-foreach-2.m
@@ -25,7 +25,7 @@ void END_LOOP(void);
         for (el in self) 
 	  { LOOP(); 
             for (id el1 in self) 
-	       INNER_LOOP();
+	       INNERLOOP();
 
 	    END_LOOP();
 	  }
diff --git a/clang/test/Rewriter/rewrite-try-catch.m b/clang/test/Rewriter/rewrite-try-catch.m
index 8ac87f5..06b198c 100644
--- a/clang/test/Rewriter/rewrite-try-catch.m
+++ b/clang/test/Rewriter/rewrite-try-catch.m
@@ -1,8 +1,13 @@
-// RUN: %clang_cc1 -rewrite-objc -fobjc-runtime=macosx-fragile-10.5  %s -o -
+// RUN: %clang_cc1 -rewrite-objc -fobjc-runtime=macosx-fragile-10.5 -std=c99 %s -o -
 
 @interface Foo @end
 @interface GARF @end
 
+void TRY(void);
+void SPLATCH(void);
+void MYTRY(void);
+void MYCATCH(void);
+
 void foo(void) {
   @try  { TRY(); } 
   @catch (...) { SPLATCH(); @throw; }