[clang-format] Avoid inserting space after C++ casts.

Fixes https://github.com/llvm/llvm-project/issues/53876.

This is a solution for standard C++ casts: const_cast, dynamic_cast, reinterpret_cast, static_cast.

A general approach handling all possible casts is not possible without semantic information.
Consider the code:
```
static_cast<T>(*function_pointer_variable)(arguments);
```
vs.
```
some_return_type<T> (*function_pointer_variable)(parameters);
// Later used as:
function_pointer_variable = &some_function;
return function_pointer_variable(args);
```
In the latter case, it's not a cast but a variable declaration of a pointer to function.
Without knowing what `some_return_type<T>` is (and clang-format does not know it), it's hard to distinguish between the two cases. Theoretically, one could check whether "parameters" are types (not a cast) and "arguments" are value/expressions (a cast), but that might be inefficient (needs lots of lookahead).

Reviewed By: MyDeveloperDay, HazardyKnusperkeks, owenpan

Differential Revision: https://reviews.llvm.org/D120140

3 files changed, 38 insertions, 1 deletions

diff --git a/clang/lib/Format/FormatToken.h b/clang/lib/Format/FormatToken.h
index bd1d447..d1dc3ff 100644
--- a/clang/lib/Format/FormatToken.h
+++ b/clang/lib/Format/FormatToken.h
@@ -43,6 +43,7 @@ namespace format {
   TYPE(ConflictAlternative)                                                    \
   TYPE(ConflictEnd)                                                            \
   TYPE(ConflictStart)                                                          \
+  TYPE(CppCastLParen)                                                          \
   TYPE(CtorInitializerColon)                                                   \
   TYPE(CtorInitializerComma)                                                   \
   TYPE(DesignatedInitializerLSquare)                                           \
diff --git a/clang/lib/Format/TokenAnnotator.cpp b/clang/lib/Format/TokenAnnotator.cpp
index 01c215f..36ff757 100644
--- a/clang/lib/Format/TokenAnnotator.cpp
+++ b/clang/lib/Format/TokenAnnotator.cpp
@@ -358,7 +358,8 @@ private:
       if (CurrentToken->Previous->is(TT_BinaryOperator))
         Contexts.back().IsExpression = true;
       if (CurrentToken->is(tok::r_paren)) {
-        if (MightBeFunctionType && ProbablyFunctionType && CurrentToken->Next &&
+        if (Left->isNot(TT_CppCastLParen) && MightBeFunctionType &&
+            ProbablyFunctionType && CurrentToken->Next &&
             (CurrentToken->Next->is(tok::l_paren) ||
              (CurrentToken->Next->is(tok::l_square) && Line.MustBeDeclaration)))
           Left->setType(Left->Next->is(tok::caret) ? TT_ObjCBlockLParen
@@ -1733,6 +1734,9 @@ private:
           Current.Tok.setKind(tok::unknown);
       else
         Current.setType(TT_LineComment);
+    } else if (Current.is(tok::l_paren)) {
+      if (lParenStartsCppCast(Current))
+        Current.setType(TT_CppCastLParen);
     } else if (Current.is(tok::r_paren)) {
       if (rParenEndsCast(Current))
         Current.setType(TT_CastRParen);
@@ -1880,6 +1884,25 @@ private:
     return Style.isJavaScript() && PreviousNotConst->is(tok::kw_const);
   }
 
+  /// Determine whether '(' is starting a C++ cast.
+  bool lParenStartsCppCast(const FormatToken &Tok) {
+    // C-style casts are only used in C++.
+    if (!Style.isCpp())
+      return false;
+
+    FormatToken *LeftOfParens = Tok.getPreviousNonComment();
+    if (LeftOfParens && LeftOfParens->is(TT_TemplateCloser) &&
+        LeftOfParens->MatchingParen) {
+      auto *Prev = LeftOfParens->MatchingParen->getPreviousNonComment();
+      if (Prev && Prev->isOneOf(tok::kw_const_cast, tok::kw_dynamic_cast,
+                                tok::kw_reinterpret_cast, tok::kw_static_cast))
+        // FIXME: Maybe we should handle identifiers ending with "_cast",
+        // e.g. any_cast?
+        return true;
+    }
+    return false;
+  }
+
   /// Determine whether ')' is ending a cast.
   bool rParenEndsCast(const FormatToken &Tok) {
     // C-style casts are only used in C++, C# and Java.
diff --git a/clang/unittests/Format/FormatTest.cpp b/clang/unittests/Format/FormatTest.cpp
index 6cd6c4a..624f3a7 100644
--- a/clang/unittests/Format/FormatTest.cpp
+++ b/clang/unittests/Format/FormatTest.cpp
@@ -10570,6 +10570,19 @@ TEST_F(FormatTest, FormatsBinaryOperatorsPrecedingEquals) {
 
 TEST_F(FormatTest, FormatsCasts) {
   verifyFormat("Type *A = static_cast<Type *>(P);");
+  verifyFormat("static_cast<Type *>(P);");
+  verifyFormat("static_cast<Type &>(Fun)(Args);");
+  verifyFormat("static_cast<Type &>(*Fun)(Args);");
+  verifyFormat("if (static_cast<int>(A) + B >= 0)\n  ;");
+  // Check that static_cast<...>(...) does not require the next token to be on
+  // the same line.
+  verifyFormat("some_loooong_output << something_something__ << "
+               "static_cast<const void *>(R)\n"
+               "                    << something;");
+  verifyFormat("a = static_cast<Type &>(*Fun)(Args);");
+  verifyFormat("const_cast<Type &>(*Fun)(Args);");
+  verifyFormat("dynamic_cast<Type &>(*Fun)(Args);");
+  verifyFormat("reinterpret_cast<Type &>(*Fun)(Args);");
   verifyFormat("Type *A = (Type *)P;");
   verifyFormat("Type *A = (vector<Type *, int *>)P;");
   verifyFormat("int a = (int)(2.0f);");