[gcc r13-4367] c++: Allow module name to be a single letter on Windows

[Torbjorn Svensson <azoff@gcc.gnu.org>]

https://gcc.gnu.org/g:d30e98b54d6a5124bb48b10b593e264f048d38aa

commit r13-4367-gd30e98b54d6a5124bb48b10b593e264f048d38aa
Author: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
Date:   Thu Oct 27 18:03:15 2022 +0200

    c++: Allow module name to be a single letter on Windows
    
    On Windows, the ':' character is special and when the module name is
    a single character, like 'A', then the flatname would be (for
    example) 'A:Foo'. On Windows, 'A:Foo' is treated as an absolute
    path by the module loader and is likely not found.
    
    Without this patch, the test case pr98944_c.C fails with:
    
    In module imported at /src/gcc/testsuite/g++.dg/modules/pr98944_b.C:7:1,
    of module A:Foo, imported at /src/gcc/testsuite/g++.dg/modules/pr98944_c.C:7:
    A:Internals: error: header module expected, module 'A:Internals' found
    A:Internals: error: failed to read compiled module: Bad file data
    A:Internals: note: compiled module file is 'gcm.cache/A-Internals.gcm'
    In module imported at /src/gcc/testsuite/g++.dg/modules/pr98944_c.C:7:8:
    A:Foo: error: failed to read compiled module: Bad import dependency
    A:Foo: note: compiled module file is 'gcm.cache/A-Foo.gcm'
    A:Foo: fatal error: returning to the gate for a mechanical issue
    compilation terminated.
    
    gcc/cp/ChangeLog:
    
            * module.cc: On Windows, 'A:Foo' is supposed to be a module
            and not a path.
    
    Co-Authored-By: Yvan ROUX <yvan.roux@foss.st.com>
    Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>

Diff:
---
 gcc/cp/module.cc | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index a1764354ba5..7133009dba5 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -13987,7 +13987,15 @@ get_module (tree name, module_state *parent, bool partition)
 static module_state *
 get_module (const char *ptr)
 {
-  if (ptr[0] == '.' ? IS_DIR_SEPARATOR (ptr[1]) : IS_ABSOLUTE_PATH (ptr))
+  /* On DOS based file systems, there is an ambiguity with A:B which can be
+     interpreted as a module Module:Partition or Drive:PATH.  Interpret strings
+     which clearly starts as pathnames as header-names and everything else is
+     treated as a (possibly malformed) named moduled.  */
+  if (IS_DIR_SEPARATOR (ptr[ptr[0] == '.']) // ./FOO or /FOO
+#if HAVE_DOS_BASED_FILE_SYSTEM
+      || (HAS_DRIVE_SPEC (ptr) && IS_DIR_SEPARATOR (ptr[2])) // A:/FOO
+#endif
+      || false)
     /* A header name.  */
     return get_module (build_string (strlen (ptr), ptr));