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.

include/ChangeLog:

        * filenames.h: Added IS_REAL_ABSOLUTE_PATH macro to check if
        path is absolute and not semi-absolute on Windows.

gcc/cp/ChangeLog:

        * module.cc: Use IS_REAL_ABSOLUTE_PATH macro.

Co-Authored-By: Yvan ROUX <yvan.r...@foss.st.com>
Signed-off-by: Torbjörn SVENSSON <torbjorn.svens...@foss.st.com>
---
 gcc/cp/module.cc    | 2 +-
 include/filenames.h | 4 ++++
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/gcc/cp/module.cc b/gcc/cp/module.cc
index 9957df510e6..84680e183b7 100644
--- a/gcc/cp/module.cc
+++ b/gcc/cp/module.cc
@@ -13958,7 +13958,7 @@ 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))
+  if (ptr[0] == '.' ? IS_DIR_SEPARATOR (ptr[1]) : IS_REAL_ABSOLUTE_PATH (ptr))
     /* A header name.  */
     return get_module (build_string (strlen (ptr), ptr));
 
diff --git a/include/filenames.h b/include/filenames.h
index 6c72c422edd..d04fccfed64 100644
--- a/include/filenames.h
+++ b/include/filenames.h
@@ -43,6 +43,7 @@ extern "C" {
 #  define HAS_DRIVE_SPEC(f) HAS_DOS_DRIVE_SPEC (f)
 #  define IS_DIR_SEPARATOR(c) IS_DOS_DIR_SEPARATOR (c)
 #  define IS_ABSOLUTE_PATH(f) IS_DOS_ABSOLUTE_PATH (f)
+#  define IS_REAL_ABSOLUTE_PATH(f) IS_DOS_REAL_ABSOLUTE_PATH (f)
 #else /* not DOSish */
 #  if defined(__APPLE__)
 #    ifndef HAVE_CASE_INSENSITIVE_FILE_SYSTEM
@@ -52,6 +53,7 @@ extern "C" {
 #  define HAS_DRIVE_SPEC(f) (0)
 #  define IS_DIR_SEPARATOR(c) IS_UNIX_DIR_SEPARATOR (c)
 #  define IS_ABSOLUTE_PATH(f) IS_UNIX_ABSOLUTE_PATH (f)
+#  define IS_REAL_ABSOLUTE_PATH(f) IS_ABSOLUTE_PATH (f)
 #endif
 
 #define IS_DIR_SEPARATOR_1(dos_based, c)                               \
@@ -67,6 +69,8 @@ extern "C" {
 
 #define IS_DOS_DIR_SEPARATOR(c) IS_DIR_SEPARATOR_1 (1, c)
 #define IS_DOS_ABSOLUTE_PATH(f) IS_ABSOLUTE_PATH_1 (1, f)
+#define IS_DOS_REAL_ABSOLUTE_PATH(f) \
+  ((f)[0] && (f)[1] == ':' && ((f)[2] == '/' || (f)[2] == '\\'))
 #define HAS_DOS_DRIVE_SPEC(f) HAS_DRIVE_SPEC_1 (1, f)
 
 #define IS_UNIX_DIR_SEPARATOR(c) IS_DIR_SEPARATOR_1 (0, c)
-- 
2.25.1

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