[PATCH v3] c++: Allow module name to be a single letter on Windows

["Torbjörn SVENSSON" <torbjorn.svensson@foss.st.com>]

v1 -> v2:
Paths without "C:" part can still be absolute if they start with / or
\ on Windows.

v2 -> v3:
Use alternative approach by having platform specific code in module.cc.

Truth table for the new expression:
c:\foo     -> true
c:/foo     -> true
/foo       -> true
\foo       -> true
c:foo      -> false
foo        -> false
./foo      -> true
.\foo      -> true


Ok for trunk?

---

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.

Tested on Windows with arm-none-eabi for Cortex-M3 in gcc-11 tree.

Co-Authored-By: Yvan ROUX <yvan.roux@foss.st.com>
Signed-off-by: Torbjörn SVENSSON <torbjorn.svensson@foss.st.com>
---
 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 0e9af318ba4..fa41a86213f 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -13960,7 +13960,15 @@ 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));
 
-- 
2.25.1