[Sema] Allow implicit conversions of &overloaded_fn in C.

Also includes a minor ``enable_if`` docs update.

Currently, our address-of overload machinery will only allow implicit
conversions of overloaded functions to void* in C. For example:

```
void f(int) __attribute__((overloadable));
void f(double) __attribute__((overloadable, enable_if(0, "")));

void *fp = f; // OK. This is C and the target is void*.
void (*fp2)(void) = f; // Error. This is C, but the target isn't void*.
```

This patch makes the assignment of `fp2` select the `f(int)` overload,
rather than emitting an error (N.B. you'll still get a warning about the
`fp2` assignment if you use -Wincompatible-pointer-types).

Differential Revision: http://reviews.llvm.org/D13704

llvm-svn: 264132

clang/include/clang/Basic/AttrDocs.td

@@ -260,6 +260,29 @@ only a single candidate. In a call to g(1, 1), the call is ambiguous even though
 not ODR-equivalent.
 
 Query for this feature with ``__has_attribute(enable_if)``.
+
+Note that functions with one or more ``enable_if`` attributes may not have
+their address taken, unless all of the conditions specified by said
+``enable_if`` are constants that evaluate to ``true``. For example:
+
+.. code-block:: c
+
+  const int TrueConstant = 1;
+  const int FalseConstant = 0;
+  int f(int a) __attribute__((enable_if(a > 0, "")));
+  int g(int a) __attribute__((enable_if(a == 0 || a != 0, "")));
+  int h(int a) __attribute__((enable_if(1, "")));
+  int i(int a) __attribute__((enable_if(TrueConstant, "")));
+  int j(int a) __attribute__((enable_if(FalseConstant, "")));
+
+  void fn() {
+    int (*ptr)(int);
+    ptr = &f; // error: 'a > 0' is not always true
+    ptr = &g; // error: 'a == 0 || a != 0' is not a truthy constant
+    ptr = &h; // OK: 1 is a truthy constant
+    ptr = &i; // OK: 'TrueConstant' is a truthy constant
+    ptr = &j; // error: 'FalseConstant' is a constant, but not truthy
+  }
   }];
 }
 

clang/lib/Sema/SemaOverload.cpp

@@ -8488,7 +8488,7 @@ Sema::AddArgumentDependentLookupCandidates(DeclarationName Name,
 // Cand1's first N enable_if attributes have precisely the same conditions as
 // Cand2's first N enable_if attributes (where N = the number of enable_if
 // attributes on Cand2), and Cand1 has more than N enable_if attributes.
-static bool hasBetterEnableIfAttrs(Sema &S, const FunctionDecl *Cand1,
+static bool hasBetterEnableIfAttrs(const Sema &S, const FunctionDecl *Cand1,
                                    const FunctionDecl *Cand2) {
 
   // FIXME: The next several lines are just
@@ -10299,13 +10299,25 @@ public:
   bool hasComplained() const { return HasComplained; }
 
 private:
-  // Is A considered a better overload candidate for the desired type than B?
-  bool isBetterCandidate(const FunctionDecl *A, const FunctionDecl *B) {
-    return hasBetterEnableIfAttrs(S, A, B);
+  bool candidateHasExactlyCorrectType(const FunctionDecl *FD) {
+    QualType Discard;
+    return Context.hasSameUnqualifiedType(TargetFunctionType, FD->getType()) ||
+           S.IsNoReturnConversion(FD->getType(), TargetFunctionType, Discard);
   }
 
-  // Returns true if we've eliminated any (read: all but one) candidates, false
-  // otherwise.
+  /// \return true if A is considered a better overload candidate for the
+  /// desired type than B.
+  bool isBetterCandidate(const FunctionDecl *A, const FunctionDecl *B) {
+    // If A doesn't have exactly the correct type, we don't want to classify it
+    // as "better" than anything else. This way, the user is required to
+    // disambiguate for us if there are multiple candidates and no exact match.
+    return candidateHasExactlyCorrectType(A) &&
+           (!candidateHasExactlyCorrectType(B) ||
+            hasBetterEnableIfAttrs(S, A, B));
+  }
+
+  /// \return true if we were able to eliminate all but one overload candidate,
+  /// false otherwise.
   bool eliminiateSuboptimalOverloadCandidates() {
     // Same algorithm as overload resolution -- one pass to pick the "best",
     // another pass to be sure that nothing is better than the best.
@@ -10418,12 +10430,9 @@ private:
       if (!S.checkAddressOfFunctionIsAvailable(FunDecl))
         return false;
 
-      QualType ResultTy;
-      if (Context.hasSameUnqualifiedType(TargetFunctionType,
-                                         FunDecl->getType()) ||
-          S.IsNoReturnConversion(FunDecl->getType(), TargetFunctionType,
-                                 ResultTy) ||
-          (!S.getLangOpts().CPlusPlus && TargetType->isVoidPointerType())) {
+      // If we're in C, we need to support types that aren't exactly identical.
+      if (!S.getLangOpts().CPlusPlus ||
+          candidateHasExactlyCorrectType(FunDecl)) {
         Matches.push_back(std::make_pair(
             CurAccessFunPair, cast<FunctionDecl>(FunDecl->getCanonicalDecl())));
         FoundNonTemplateFunction = true;

clang/test/CodeGen/overloadable.c

@@ -29,3 +29,33 @@ int main() {
   cdv = f(cdv);
   vv = f(vv);
 }
+
+// Ensuring that we pick the correct function for taking the address of an
+// overload when conversions are involved.
+
+void addrof_many(int *a) __attribute__((overloadable, enable_if(0, "")));
+void addrof_many(void *a) __attribute__((overloadable));
+void addrof_many(char *a) __attribute__((overloadable));
+
+void addrof_single(int *a) __attribute__((overloadable, enable_if(0, "")));
+void addrof_single(char *a) __attribute__((overloadable, enable_if(0, "")));
+void addrof_single(char *a) __attribute__((overloadable));
+
+// CHECK-LABEL: define void @foo
+void foo() {
+  // CHECK: store void (i8*)* @_Z11addrof_manyPc
+  void (*p1)(char *) = &addrof_many;
+  // CHECK: store void (i8*)* @_Z11addrof_manyPv
+  void (*p2)(void *) = &addrof_many;
+  // CHECK: void (i8*)* @_Z11addrof_manyPc
+  void *vp1 = (void (*)(char *)) & addrof_many;
+  // CHECK: void (i8*)* @_Z11addrof_manyPv
+  void *vp2 = (void (*)(void *)) & addrof_many;
+
+  // CHECK: store void (i8*)* @_Z13addrof_singlePc
+  void (*p3)(char *) = &addrof_single;
+  // CHECK: @_Z13addrof_singlePc
+  void (*p4)(int *) = &addrof_single;
+  // CHECK: @_Z13addrof_singlePc
+  void *vp3 = &addrof_single;
+}

clang/test/Sema/overloadable.c

@@ -1,4 +1,4 @@
-// RUN: %clang_cc1 -fsyntax-only -verify %s
+// RUN: %clang_cc1 -fsyntax-only -verify %s -Wincompatible-pointer-types
 
 int var __attribute__((overloadable)); // expected-error{{'overloadable' attribute only applies to functions}}
 void params(void) __attribute__((overloadable(12))); // expected-error {{'overloadable' attribute takes no arguments}}
@@ -99,3 +99,26 @@ void conversions() {
   unsigned char *c;
   multi_type(c);
 }
+
+// Ensure that we allow C-specific type conversions in C
+void fn_type_conversions() {
+  void foo(void *c) __attribute__((overloadable));
+  void foo(char *c) __attribute__((overloadable));
+  void (*ptr1)(void *) = &foo;
+  void (*ptr2)(char *) = &foo;
+  void (*ambiguous)(int *) = &foo; // expected-error{{initializing 'void (*)(int *)' with an expression of incompatible type '<overloaded function type>'}} expected-note@105{{candidate function}} expected-note@106{{candidate function}}
+  void *vp_ambiguous = &foo; // expected-error{{initializing 'void *' with an expression of incompatible type '<overloaded function type>'}} expected-note@105{{candidate function}} expected-note@106{{candidate function}}
+
+  void (*specific1)(int *) = (void (*)(void *))&foo; // expected-warning{{incompatible pointer types initializing 'void (*)(int *)' with an expression of type 'void (*)(void *)'}}
+  void *specific2 = (void (*)(void *))&foo;
+
+  void disabled(void *c) __attribute__((overloadable, enable_if(0, "")));
+  void disabled(int *c) __attribute__((overloadable, enable_if(c, "")));
+  void disabled(char *c) __attribute__((overloadable, enable_if(1, "The function name lies.")));
+  // To be clear, these should all point to the last overload of 'disabled'
+  void (*dptr1)(char *c) = &disabled;
+  void (*dptr2)(void *c) = &disabled; // expected-warning{{incompatible pointer types initializing 'void (*)(void *)' with an expression of type '<overloaded function type>'}} expected-note@115{{candidate function made ineligible by enable_if}} expected-note@116{{candidate function made ineligible by enable_if}} expected-note@117{{candidate function has type mismatch at 1st parameter (expected 'void *' but has 'char *')}}
+  void (*dptr3)(int *c) = &disabled; // expected-warning{{incompatible pointer types initializing 'void (*)(int *)' with an expression of type '<overloaded function type>'}} expected-note@115{{candidate function made ineligible by enable_if}} expected-note@116{{candidate function made ineligible by enable_if}} expected-note@117{{candidate function has type mismatch at 1st parameter (expected 'int *' but has 'char *')}}
+
+  void *specific_disabled = &disabled;
+}

clang/test/Sema/pass-object-size.c

@@ -38,8 +38,8 @@ void FunctionPtrs() {
   void (*p)(void *) = NotOverloaded; //expected-error{{cannot take address of function 'NotOverloaded' because parameter 1 has pass_object_size attribute}}
   void (*p2)(void *) = &NotOverloaded; //expected-error{{cannot take address of function 'NotOverloaded' because parameter 1 has pass_object_size attribute}}
 
-  void (*p3)(void *) = IsOverloaded; //expected-error{{initializing 'void (*)(void *)' with an expression of incompatible type '<overloaded function type>'}} expected-note@-6{{candidate address cannot be taken because parameter 1 has pass_object_size attribute}} expected-note@-5{{type mismatch}}
-  void (*p4)(void *) = &IsOverloaded; //expected-error{{initializing 'void (*)(void *)' with an expression of incompatible type '<overloaded function type>'}} expected-note@-7{{candidate address cannot be taken because parameter 1 has pass_object_size attribute}} expected-note@-6{{type mismatch}}
+  void (*p3)(void *) = IsOverloaded; //expected-warning{{incompatible pointer types initializing 'void (*)(void *)' with an expression of type '<overloaded function type>'}} expected-note@-6{{candidate address cannot be taken because parameter 1 has pass_object_size attribute}} expected-note@-5{{type mismatch}}
+  void (*p4)(void *) = &IsOverloaded; //expected-warning{{incompatible pointer types initializing 'void (*)(void *)' with an expression of type '<overloaded function type>'}} expected-note@-7{{candidate address cannot be taken because parameter 1 has pass_object_size attribute}} expected-note@-6{{type mismatch}}
 
   void (*p5)(char *) = IsOverloaded;
   void (*p6)(char *) = &IsOverloaded;