[llvm-bugs] [Bug 51395] New: Clang error "file not found" on include files when building with modules

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51395

            Bug ID: 51395
           Summary: Clang error "file not found" on include files when
                    building with modules
           Product: clang
           Version: unspecified
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Modules
          Assignee: unassignedclangbugs at nondot.org
          Reporter: akhuang at google.com
                CC: dgregor at apple.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

We saw this "file not found" error when trying to build with -fmodules. This
only happens when we use relative paths in the header search.

To repro:
$ cat t.cc
#include <cstddef>
$ ./<relative-path-to-clang>/clang -fmodules -nostdinc++ -no-canonical-prefixes
-isystem../../llvm-build/include/c++/v1 -E t.cc
# 1 "t.cc"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 385 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "t.cc" 2
<module-includes>:23:10: fatal error:
'../../llvm-build/lib/clang/14.0.0/include/stdint.h' file not found
#include "../../llvm-build/lib/clang/14.0.0/include/stdint.h"
         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
t.cc:1:10: fatal error: could not build module 'std'
#include <cstddef>
 ~~~~~~~~^


(also, to repro, I had to remove a line `header "range"` from <build
dir>/include/c++/v1/module.modulemap to get the build to work)

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