Improve clarity of the implicit declaration diagnostic

clang/docs/ReleaseNotes.rst

@@ -716,6 +716,10 @@ Improvements to Clang's diagnostics
   Added a new warning in this group for the case where the attribute is missing/implicit on
   an override of a virtual method.
 
+ - Reworded the ``-Wimplicit-function-declaration`` diagnostic to make it more
+   clear that the type selected for the implicit declaration is based on the
+   signature of a standard (C, C++, POSIX) library function. (#GH146924)
+
 Improvements to Clang's time-trace
 ----------------------------------
 

clang/include/clang/Basic/Builtins.h

@@ -376,6 +376,33 @@ class Context {
     return getInfo(ID).Header.getName();
   }
 
+  /// Returns true if a library function is declared within a C or C++ standard
+  /// header (like stdio.h) or POSIX header (like malloc.h), false if the
+  /// function is not declared within a header or is declared in a non-standard
+  /// header (like Microsoft or Objective-C headers).
+  bool isDeclaredInStandardHeader(unsigned ID) const {
+    switch (getInfo(ID).Header.ID) {
+    default:
+      return false;
+    case HeaderDesc::COMPLEX_H: // C99
+    case HeaderDesc::CTYPE_H:   // C89
+    case HeaderDesc::MATH_H:    // C89
+    case HeaderDesc::MALLOC_H:  // POSIX
+    case HeaderDesc::MEMORY:    // C++98
+    case HeaderDesc::PTHREAD_H: // POSIX
+    case HeaderDesc::SETJMP_H:  // C89
+    case HeaderDesc::STDARG_H:  // C89
+    case HeaderDesc::STDIO_H:   // C89
+    case HeaderDesc::STDLIB_H:  // C89
+    case HeaderDesc::STRING_H:  // C89
+    case HeaderDesc::STRINGS_H: // POSIX
+    case HeaderDesc::UNISTD_H:  // POSIX
+    case HeaderDesc::UTILITY:   // C++98
+    case HeaderDesc::WCHAR_H:   // C99
+      return true;
+    }
+  }
+
   /// Determine whether this builtin is like printf in its
   /// formatting rules and, if so, set the index to the format string
   /// argument and whether this function as a va_list argument.

clang/include/clang/Basic/DiagnosticSemaKinds.td

@@ -816,11 +816,12 @@ def warn_unreachable_association : Warning<
 
 /// Built-in functions.
 def ext_implicit_lib_function_decl : ExtWarn<
-  "implicitly declaring library function '%0' with type %1">,
-  InGroup<ImplicitFunctionDeclare>;
+  "implicitly declaring library function '%0' with%select{| standards-mandated}2 "
+  "type %1">, InGroup<ImplicitFunctionDeclare>;
 def ext_implicit_lib_function_decl_c99 : ExtWarn<
-  "call to undeclared library function '%0' with type %1; ISO C99 and later "
-  "do not support implicit function declarations">,
+  "call to undeclared library function '%0', will assume it exists with"
+  "%select{| standards-mandated}2 type %1; ISO C99 and later do not support "
+  "implicit function declarations">,
   InGroup<ImplicitFunctionDeclare>, DefaultError;
 def note_include_header_or_declare : Note<
   "include the header <%0> or explicitly provide a declaration for '%1'">;

clang/lib/Sema/SemaDecl.cpp

@@ -2380,9 +2380,11 @@ NamedDecl *Sema::LazilyCreateBuiltin(IdentifierInfo *II, unsigned ID,
   if (!ForRedeclaration &&
       (Context.BuiltinInfo.isPredefinedLibFunction(ID) ||
        Context.BuiltinInfo.isHeaderDependentFunction(ID))) {
+    bool IsStandardsMandated =
+        Context.BuiltinInfo.isDeclaredInStandardHeader(ID);
     Diag(Loc, LangOpts.C99 ? diag::ext_implicit_lib_function_decl_c99
                            : diag::ext_implicit_lib_function_decl)
-        << Context.BuiltinInfo.getName(ID) << R;
+        << Context.BuiltinInfo.getName(ID) << R << IsStandardsMandated;
     if (const char *Header = Context.BuiltinInfo.getHeaderName(ID))
       Diag(Loc, diag::note_include_header_or_declare)
           << Header << Context.BuiltinInfo.getName(ID);

clang/test/Analysis/exercise-ps.c

@@ -22,7 +22,7 @@ void_typedef f2_helper(void);
 static void f2(void *buf) {
   F12_typedef* x;
   x = f2_helper();
-  memcpy((&x[1]), (buf), 1); // expected-warning{{call to undeclared library function 'memcpy' with type 'void *(void *, const void *}} \
+  memcpy((&x[1]), (buf), 1); // expected-warning{{call to undeclared library function 'memcpy', will assume it exists with standards-mandated type 'void *(void *, const void *,}} \
   // expected-note{{include the header <string.h> or explicitly provide a declaration for 'memcpy'}}
 }
 

clang/test/Driver/implicit-function-as-error.c

@@ -6,6 +6,6 @@
 // to an error.
 
 void radar_10894044(void) {
-  printf("Hi\n"); // expected-error {{call to undeclared library function 'printf' with type 'int (const char *, ...)'}} expected-note {{include the header <stdio.h> or explicitly provide a declaration for 'printf'}}
+  printf("Hi\n"); // expected-error {{call to undeclared library function 'printf', will assume it exists with standards-mandated type 'int (const char *, ...)'; ISO C99 and later do not support implicit function declarations}} expected-note {{include the header <stdio.h> or explicitly provide a declaration for 'printf'}}
   radar_10894044_not_declared(); // expected-error {{call to undeclared function 'radar_10894044_not_declared'; ISO C99 and later do not support implicit function declarations}}
 }

clang/test/Sema/builtin-setjmp.c

@@ -38,7 +38,7 @@ void use(void) {
   // c-error@-3 {{call to undeclared function 'setjmp'; ISO C99 and later do not support implicit function declarations}}
   #elif ONLY_JMP_BUF
   // cxx-error@-5 {{undeclared identifier 'setjmp'}}
-  // c-error@-6 {{call to undeclared library function 'setjmp' with type 'int (jmp_buf)' (aka 'int (int *)'); ISO C99 and later do not support implicit function declarations}}
+  // c-error@-6 {{call to undeclared library function 'setjmp', will assume it exists with standards-mandated type 'int (jmp_buf)' (aka 'int (int *)'); ISO C99 and later do not support implicit function declarations}}
   // c-note@-7 {{include the header <setjmp.h> or explicitly provide a declaration for 'setjmp'}}
   #else
   // cxx-no-diagnostics

clang/test/Sema/implicit-builtin-decl.c

@@ -1,7 +1,7 @@
 // RUN: %clang_cc1 -fsyntax-only -Wno-strict-prototypes -verify %s
 
 void f() {
-  int *ptr = malloc(sizeof(int) * 10); // expected-error{{call to undeclared library function 'malloc' with type}} \
+  int *ptr = malloc(sizeof(int) * 10); // expected-error{{call to undeclared library function 'malloc'}} \
   // expected-note{{include the header <stdlib.h> or explicitly provide a declaration for 'malloc'}} \
   // expected-note{{'malloc' is a builtin with type 'void *}}
 }

clang/test/SemaObjC/builtin_objc_lib_functions.m

@@ -1,27 +1,27 @@
 // RUN: %clang_cc1 -x objective-c %s -fsyntax-only -verify
-Class f0(void) { return objc_getClass("a"); } // expected-error {{call to undeclared library function 'objc_getClass' with type 'id (const char *)'}} \
+Class f0(void) { return objc_getClass("a"); } // expected-error {{call to undeclared library function 'objc_getClass', will assume it exists with type 'id (const char *)'; ISO C99 and later do not support implicit function declarations}} \
 					  // expected-note {{include the header <objc/runtime.h> or explicitly provide a declaration for 'objc_getClass'}}
 
-Class f1(void) { return objc_getMetaClass("a"); } // expected-error {{call to undeclared library function 'objc_getMetaClass' with type 'id (const char *)'}} \
+Class f1(void) { return objc_getMetaClass("a"); } // expected-error {{call to undeclared library function 'objc_getMetaClass', will assume it exists with type 'id (const char *)'; ISO C99 and later do not support implicit function declarations}} \
 					  // expected-note {{include the header <objc/runtime.h> or explicitly provide a declaration for 'objc_getMetaClass'}}
 
-void f2(id val) { objc_enumerationMutation(val); } // expected-error {{call to undeclared library function 'objc_enumerationMutation' with type 'void (id)'}} \
+void f2(id val) { objc_enumerationMutation(val); } // expected-error {{call to undeclared library function 'objc_enumerationMutation', will assume it exists with type 'void (id)'; ISO C99 and later do not support implicit function declarations}} \
 						   // expected-note {{include the header <objc/runtime.h> or explicitly provide a declaration for 'objc_enumerationMutation'}}
 
-long double f3(id self, SEL op) { return objc_msgSend_fpret(self, op); } // expected-error {{call to undeclared library function 'objc_msgSend_fpret' with type 'long double (id, SEL, ...)'}} \
+long double f3(id self, SEL op) { return objc_msgSend_fpret(self, op); } // expected-error {{call to undeclared library function 'objc_msgSend_fpret', will assume it exists with type 'long double (id, SEL, ...)'; ISO C99 and later do not support implicit function declarations}} \
     // expected-note {{include the header <objc/message.h> or explicitly provide a declaration for 'objc_msgSend_fpret'}}
 
 id f4(struct objc_super *super, SEL op) { // expected-warning {{declaration of 'struct objc_super' will not be visible outside of this function}}
-  return objc_msgSendSuper(super, op); // expected-error {{call to undeclared library function 'objc_msgSendSuper' with type 'id (struct objc_super *, SEL, ...)'}} \
+  return objc_msgSendSuper(super, op); // expected-error {{call to undeclared library function 'objc_msgSendSuper', will assume it exists with type 'id (struct objc_super *, SEL, ...)'; ISO C99 and later do not support implicit function declarations}} \
 					// expected-note {{include the header <objc/message.h> or explicitly provide a declaration for 'objc_msgSendSuper'}}
 }
 
 id f5(id val, id *dest) {
-  return objc_assign_strongCast(val, dest); // expected-error {{call to undeclared library function 'objc_assign_strongCast' with type 'id (id, id *)'}} \
+  return objc_assign_strongCast(val, dest); // expected-error {{call to undeclared library function 'objc_assign_strongCast', will assume it exists with type 'id (id, id *)'; ISO C99 and later do not support implicit function declarations}} \
 					    // expected-note {{include the header <objc/objc-auto.h> or explicitly provide a declaration for 'objc_assign_strongCast'}}
 }
 
 int f6(Class exceptionClass, id exception) {
-  return objc_exception_match(exceptionClass, exception); // expected-error {{call to undeclared library function 'objc_exception_match' with type 'int (id, id)'}} \
+  return objc_exception_match(exceptionClass, exception); // expected-error {{call to undeclared library function 'objc_exception_match', will assume it exists with type 'int (id, id)'; ISO C99 and later do not support implicit function declarations}} \
   							  // expected-note {{include the header <objc/objc-exception.h> or explicitly provide a declaration for 'objc_exception_match'}}
 }

clang/test/SemaObjC/builtin_objc_nslog.m

@@ -3,11 +3,11 @@
 #include <stdarg.h>
 
 void f1(id arg) {
-  NSLog(@"%@", arg); // expected-error {{call to undeclared library function 'NSLog' with type 'void (id, ...)'}} \
+  NSLog(@"%@", arg); // expected-error {{call to undeclared library function 'NSLog', will assume it exists with type 'void (id, ...)'; ISO C99 and later do not support implicit function declaration}} \
   // expected-note {{include the header <Foundation/NSObjCRuntime.h> or explicitly provide a declaration for 'NSLog'}}
 }
 
 void f2(id str, va_list args) {
-  NSLogv(@"%@", args); // expected-error {{call to undeclared library function 'NSLogv' with type }} \
+  NSLogv(@"%@", args); // expected-error {{call to undeclared library function 'NSLogv', will assume it exists with type 'void (id, __builtin_va_list)'; ISO C99 and later do not support implicit function declarations}} \
   // expected-note {{include the header <Foundation/NSObjCRuntime.h> or explicitly provide a declaration for 'NSLogv'}}
 }