| Issue |
181294
|
| Summary |
Clang-cl incorrectly handles `/external:I`. MSVC does not use this arg to add system include paths.
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
Colengms
|
This is the inverse of another issue which contains an incorrect analysis of how the argument works in MSVC: https://github.com/llvm/llvm-project/issues/61796
With MSVC, `/external:I` can add an include path (if `/external:W#` is also specified) to be considered "external" for the purpose of external header diagnostics only. It does not specify that the include path should be a "system" include path (like gcc and clang's `-isystem` arg). I don't believe MSVC provides an argument that would allow the caller to add a system include path.
The following examples use multiple include paths containing a header of the same name.
```
cl /nologo /external:I path_A /c test_include_paths.cpp
-- cl : Command line warning D9007 : '/external:I' requires '/external:W'; option ignored
-- Header is not found
```
The following imply that an include path is added by `/external:I`, but the first example would have found path B if `/external:I` had made `path_A` a system include path, as user include paths take priority over system include paths (and otherwise equal priority is from left-to-right). If `/external:I` had specified a different priority than `/I`, these command lines would have the same result - path B.
```
cl /nologo /external:W4 /external:I path_A /I path_B /c test_include_paths.cpp
-- Finds path A.
cl /nologo /external:W4 /I path_B /external:I path_A /c test_include_paths.cpp
-- Finds path B.
```
It looks link clang-cl (21.1.0) is currently inconsistent with MSVC:
```
clang-cl /nologo /external:I path_A /c test_include_paths.cpp
-- Finds path A.
```
```
clang-cl /nologo /external:W4 /external:I path_A /I path_B /c test_include_paths.cpp
-- Finds path B.
clang-cl /nologo /external:W4 /I path_B /external:I path_A /c test_include_paths.cpp
-- Finds path B.
```
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