The pathname underneath gcm.cache/ is determined from the effective name used for the main input file of a particular module. When modules are built, no canonicalization occurs for the main input file. Hence the module file wouldn't be found if a different (the canonicalized) file name was used when importing that same module. (This is an effect of importing happening in the preprocessor, just like #include handling.)
Since it doesn't look easy to make module generation use libcpp's maybe_shorter_path() (in fact I'd consider this a layering violation, while cloning the logic would - at least in principle - be prone to both going out of sync), simply suppress system header path canonicalization for the respective tests. gcc/testsuite/ * g++.dg/modules/alias-1_b.C: Add -fno-canonical-system-headers. * g++.dg/modules/alias-1_d.C: Likewise. * g++.dg/modules/alias-1_e.C: Likewise. * g++.dg/modules/alias-1_f.C: Likewise. * g++.dg/modules/cpp-6_c.C: Likewise. * g++.dg/modules/dir-only-2_b.C: Likewise. --- Strictly speaking it could be necessary to also suppress canonicalization when generating the modules, but for now they're self- contained, i.e. don't include any "real" system headers. IOW at the moment the tests aren't susceptible to the issue at generation time. --- a/gcc/testsuite/g++.dg/modules/alias-1_b.C +++ b/gcc/testsuite/g++.dg/modules/alias-1_b.C @@ -1,4 +1,4 @@ -// { dg-additional-options "-fmodules-ts -fdump-lang-module -isystem [srcdir]" } +// { dg-additional-options "-fmodules-ts -fdump-lang-module -isystem [srcdir] -fno-canonical-system-headers" } // Alias at the header file. We have one CMI file import "alias-1_a.H"; --- a/gcc/testsuite/g++.dg/modules/alias-1_d.C +++ b/gcc/testsuite/g++.dg/modules/alias-1_d.C @@ -1,4 +1,4 @@ -// { dg-additional-options "-fmodules-ts -isystem [srcdir]" } +// { dg-additional-options "-fmodules-ts -isystem [srcdir] -fno-canonical-system-headers" } // { dg-module-cmi kevin } export module kevin; --- a/gcc/testsuite/g++.dg/modules/alias-1_e.C +++ b/gcc/testsuite/g++.dg/modules/alias-1_e.C @@ -1,4 +1,4 @@ -// { dg-additional-options "-fmodules-ts -isystem [srcdir]" } +// { dg-additional-options "-fmodules-ts -isystem [srcdir] -fno-canonical-system-headers" } import bob; import kevin; --- a/gcc/testsuite/g++.dg/modules/alias-1_f.C +++ b/gcc/testsuite/g++.dg/modules/alias-1_f.C @@ -1,4 +1,4 @@ -// { dg-additional-options "-fmodules-ts -fdump-lang-module -isystem [srcdir]" } +// { dg-additional-options "-fmodules-ts -fdump-lang-module -isystem [srcdir] -fno-canonical-system-headers" } import kevin; import bob; --- a/gcc/testsuite/g++.dg/modules/cpp-6_c.C +++ b/gcc/testsuite/g++.dg/modules/cpp-6_c.C @@ -1,5 +1,5 @@ // { dg-do preprocess } -// { dg-additional-options "-fmodules-ts -isystem [srcdir]" } +// { dg-additional-options "-fmodules-ts -isystem [srcdir] -fno-canonical-system-headers" } #define empty #define nop(X) X --- a/gcc/testsuite/g++.dg/modules/dir-only-2_b.C +++ b/gcc/testsuite/g++.dg/modules/dir-only-2_b.C @@ -1,5 +1,5 @@ // { dg-do preprocess } -// { dg-additional-options "-fmodules-ts -fdirectives-only -isystem [srcdir]" } +// { dg-additional-options "-fmodules-ts -fdirectives-only -isystem [srcdir] -fno-canonical-system-headers" } // a comment module; // line frob