Sema: @cdecl functions C name defaults to the Swift name

Author: xymus

lib/AST/Decl.cpp

@@ -4543,7 +4543,10 @@ StringRef ValueDecl::getCDeclName() const {
 
   // Handle explicit cdecl attributes.
   if (auto cdeclAttr = getAttrs().getAttribute<CDeclAttr>()) {
-    return cdeclAttr->Name;
+    if (!cdeclAttr->Name.empty())
+      return cdeclAttr->Name;
+    else
+      return getBaseIdentifier().str();
   }
 
   return "";

lib/SIL/IR/SILDeclRef.cpp

@@ -1307,7 +1307,7 @@ std::string SILDeclRef::mangle(ManglingKind MKind) const {
           if (!clangMangling.empty())
             return clangMangling;
         }
-        return CDeclA->Name.str();
+        return getDecl()->getCDeclName().str();
       }
 
     if (SKind == ASTMangler::SymbolKind::DistributedThunk) {

lib/Sema/TypeCheckAttr.cpp

@@ -2387,8 +2387,8 @@ void AttributeChecker::visitCDeclAttr(CDeclAttr *attr) {
     diagnose(attr->getLocation(), diag::cdecl_not_at_top_level,
              attr);
 
-  // The name must not be empty.
-  if (attr->Name.empty())
+  // @_cdecl name must not be empty.
+  if (attr->Name.empty() && attr->Underscored)
     diagnose(attr->getLocation(), diag::cdecl_empty_name,
              attr);
 

test/PrintAsObjC/cdecl-official.swift

@@ -33,6 +33,10 @@ func a0_simple(x: Int, bar y: Int) -> Int { return x }
 // CHECK-LABEL: // My documentation
 // CHECK-LABEL: SWIFT_EXTERN ptrdiff_t simple(ptrdiff_t x, ptrdiff_t y) SWIFT_NOEXCEPT SWIFT_WARN_UNUSED_RESULT;
 
+@cdecl
+func a1_defaultName(x: Int) -> Int { return x }
+// CHECK-LABEL: SWIFT_EXTERN ptrdiff_t a1_defaultName(ptrdiff_t x) SWIFT_NOEXCEPT SWIFT_WARN_UNUSED_RESULT;
+
 @cdecl("primitiveTypes")
 public func b_primitiveTypes(i: Int, ci: CInt, l: CLong, c: CChar, f: Float, d: Double, b: Bool) {}
 // CHECK-LABEL: SWIFT_EXTERN void primitiveTypes(ptrdiff_t i, int ci, long l, char c, float f, double d, bool b) SWIFT_NOEXCEPT;

test/attr/attr_cdecl_official.swift

@@ -24,8 +24,7 @@
 // expected-error @-1 {{expected ')' in 'cdecl' attribute}}
 // expected-error @-2 {{expected declaration}}
 
-@cdecl("") // expected-error{{@cdecl symbol name cannot be empty}}
-func emptyName(x: Int) -> Int { return x }
+@cdecl func defaultName() {}
 
 @cdecl("noBody")
 func noBody(x: Int) -> Int // expected-error{{expected '{' in body of function}}