[flang] Remove implicit assumption of fixed-form

This change ensures that flang no longer assumes fixed-form when using
`-x f95`. The only case where fixed-form is still assumed is when
`-x f95` is used with `.i` files, unless explicitly overridden by the
user.

Author: inaki-amatria

clang/lib/Driver/ToolChains/Flang.cpp

@@ -817,8 +817,12 @@ void Flang::ConstructJob(Compilation &C, const JobAction &JA,
 
   // 'flang -E' always produces output that is suitable for use as fixed form
   // Fortran. However it is only valid free form source if the original is also
-  // free form.
-  if (InputType == types::TY_PP_Fortran &&
+  // free form. Ensure this logic does not incorrectly assume fixed-form for
+  // cases where it shouldn't, such as `flang -x f95 foo.f90`.
+  bool isAtemporaryPreprocessedFile =
+      llvm::sys::path::extension(Input.getFilename())
+          .ends_with(types::getTypeTempSuffix(InputType, /*CLStyle=*/false));
+  if (InputType == types::TY_PP_Fortran && isAtemporaryPreprocessedFile &&
       !Args.getLastArg(options::OPT_ffixed_form, options::OPT_ffree_form))
     CmdArgs.push_back("-ffixed-form");
 

flang/test/Driver/dash-x-f95-do-not-assume-fixed-form.f90

@@ -0,0 +1,12 @@
+! This test verifies that using `-x f95` does not cause the driver to assume
+! this file is in fixed-form.
+
+program main
+  print *, "Hello, World!"
+end
+
+! RUN: %flang -### -x f95 %s 2>&1 | FileCheck --check-prefix=PRINT-PHASES %s
+! PRINT-PHASES-NOT: -ffixed-form
+
+! RUN: %flang -Werror -x f95 %s 2>&1 | FileCheck --check-prefix=COMPILE --allow-empty %s
+! COMPILE-NOT: error