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George Karpenkov ec38cf7aed [ast] Do not auto-initialize Objective-C for-loop variables in Objective-C++ in templatized code under ARC 
The AST for the fragment

```
@interface I
@end

template <typename>
void decode(I *p) {
  for (I *k in p) {}
}

void decode(I *p) {
  decode<int>(p);
}
```

differs heavily when templatized and non-templatized:

```
|-FunctionTemplateDecl 0x7fbfe0863940 <line:4:1, line:7:1> line:5:6 decode
| |-TemplateTypeParmDecl 0x7fbfe0863690 <line:4:11> col:11 typename depth 0 index 0
| |-FunctionDecl 0x7fbfe08638a0 <line:5:1, line:7:1> line:5:6 decode 'void (I *__strong)'
| | |-ParmVarDecl 0x7fbfe08637a0 <col:13, col:16> col:16 referenced p 'I *__strong'
| | `-CompoundStmt 0x7fbfe0863b88 <col:19, line:7:1>
| |   `-ObjCForCollectionStmt 0x7fbfe0863b50 <line:6:3, col:20>
| |     |-DeclStmt 0x7fbfe0863a50 <col:8, col:13>
| |     | `-VarDecl 0x7fbfe08639f0 <col:8, col:11> col:11 k 'I *const __strong'
| |     |-ImplicitCastExpr 0x7fbfe0863a90 <col:16> 'I *' <LValueToRValue>
| |     | `-DeclRefExpr 0x7fbfe0863a68 <col:16> 'I *__strong' lvalue ParmVar 0x7fbfe08637a0 'p' 'I *__strong'
| |     `-CompoundStmt 0x7fbfe0863b78 <col:19, col:20>
| `-FunctionDecl 0x7fbfe0863f80 <line:5:1, line:7:1> line:5:6 used decode 'void (I *__strong)'
|   |-TemplateArgument type 'int'
|   |-ParmVarDecl 0x7fbfe0863ef8 <col:13, col:16> col:16 used p 'I *__strong'
|   `-CompoundStmt 0x7fbfe0890cf0 <col:19, line:7:1>
|     `-ObjCForCollectionStmt 0x7fbfe0890cc8 <line:6:3, col:20>
|       |-DeclStmt 0x7fbfe0890c70 <col:8, col:13>
|       | `-VarDecl 0x7fbfe0890c00 <col:8, col:11> col:11 k 'I *__strong' callinit
|       |   `-ImplicitValueInitExpr 0x7fbfe0890c60 <<invalid sloc>> 'I *__strong'
|       |-ImplicitCastExpr 0x7fbfe0890cb0 <col:16> 'I *' <LValueToRValue>
|       | `-DeclRefExpr 0x7fbfe0890c88 <col:16> 'I *__strong' lvalue ParmVar 0x7fbfe0863ef8 'p' 'I *__strong'
|       `-CompoundStmt 0x7fbfe0863b78 <col:19, col:20>
```

Note how in the instantiated version ImplicitValueInitExpr unexpectedly appears.

While objects are auto-initialized under ARC, it does not make sense to
have an initializer for a for-loop variable, and it makes even less
sense to have such a different AST for instantiated and non-instantiated
version.

Digging deeper, I have found that there are two separate Sema* files for
dealing with templates and for dealing with non-templatized code.
In a non-templatized version, an initialization was performed only for
variables which are not loop variables for an Objective-C loop and not
variables for a C++ for-in loop:

```
  if (FRI && (Tok.is(tok::colon) || isTokIdentifier_in())) {
    bool IsForRangeLoop = false;
    if (TryConsumeToken(tok::colon, FRI->ColonLoc)) {
      IsForRangeLoop = true;
      if (Tok.is(tok::l_brace))
        FRI->RangeExpr = ParseBraceInitializer();
      else
        FRI->RangeExpr = ParseExpression();
    }

    Decl *ThisDecl = Actions.ActOnDeclarator(getCurScope(), D);
    if (IsForRangeLoop)
      Actions.ActOnCXXForRangeDecl(ThisDecl);
    Actions.FinalizeDeclaration(ThisDecl);
    D.complete(ThisDecl);
    return Actions.FinalizeDeclaratorGroup(getCurScope(), DS, ThisDecl);
  }

  SmallVector<Decl *, 8> DeclsInGroup;
  Decl *FirstDecl = ParseDeclarationAfterDeclaratorAndAttributes(
      D, ParsedTemplateInfo(), FRI);
```

However the code in SemaTemplateInstantiateDecl was inconsistent,
guarding only against C++ for-in loops.

rdar://38391075

Differential Revision: https://reviews.llvm.org/D44989

llvm-svn: 328749
 		2018-03-29 00:56:24 +00:00
..
ASTCommon.cpp Re-land: "[Support] Replace HashString with djbHash." 2018-02-26 15:16:42 +00:00
ASTCommon.h Generalize "static data member instantiated" notification to cover variable templates too. 2017-12-05 01:31:47 +00:00
ASTReader.cpp [clang] Change std::sort to llvm::sort in response to r327219 2018-03-27 16:50:00 +00:00
ASTReaderDecl.cpp [ast] Do not auto-initialize Objective-C for-loop variables in Objective-C++ in templatized code under ARC 2018-03-29 00:56:24 +00:00
ASTReaderInternals.h [Serialization] Fix some Clang-tidy modernize and Include What You Use warnings; other minor fixes (NFC). 2017-11-11 00:08:50 +00:00
ASTReaderStmt.cpp [CodeGen] Ignore OpaqueValueExprs that are unique references to their 2018-03-20 01:47:58 +00:00
ASTWriter.cpp [ObjC++] Make parameter passing and function return compatible with ObjC 2018-03-28 21:13:14 +00:00
ASTWriterDecl.cpp [ast] Do not auto-initialize Objective-C for-loop variables in Objective-C++ in templatized code under ARC 2018-03-29 00:56:24 +00:00
ASTWriterStmt.cpp [CodeGen] Ignore OpaqueValueExprs that are unique references to their 2018-03-20 01:47:58 +00:00
CMakeLists.txt Introduce module file extensions to piggy-back data onto module files. 2015-11-03 18:33:07 +00:00
GeneratePCH.cpp Apply clang-tidy's performance-unnecessary-value-param to parts of clang. 2017-03-21 21:35:04 +00:00
GlobalModuleIndex.cpp Re-land: "[Support] Replace HashString with djbHash." 2018-02-26 15:16:42 +00:00
Module.cpp Modules: Simplify the ModuleFile constructor; likely NFC 2017-01-29 00:39:09 +00:00
ModuleFileExtension.cpp [NFC] Header cleanup 2016-07-18 19:02:11 +00:00
ModuleManager.cpp [Serialization] Fix some Clang-tidy modernize and Include What You Use warnings; other minor fixes (NFC). 2017-11-11 00:08:50 +00:00
MultiOnDiskHashTable.h [Serialization] Fix some Clang-tidy modernize and Include What You Use warnings; other minor fixes (NFC). 2017-11-02 21:45:30 +00:00