Default to project_version() in vcs_tag fallback

docs/markdown/Reference-manual.md

@@ -639,7 +639,7 @@ This command detects revision control commit information at build time and place
 
 - `input` file to modify (e.g. `version.c.in`) (required)
 - `output` file to write the results to (e.g. `version.c`) (required)
-- `fallback` version number to use when no revision control information is present, such as when building from a release tarball (required)
+- `fallback` version number to use when no revision control information is present, such as when building from a release tarball (defaults to `meson.project_version()`)
 - `command` string list with the command to execute, see [`custom_target`](#custom_target) for details on how this command must be specified
 - `replace_string` string in the input file to substitute with the commit information (defaults to `@VCS_TAG@`)
 

docs/markdown/Release-notes-for-0.41.0.md

@@ -12,3 +12,8 @@ Add features here as code is merged to master.
 ## Dependency Handler for LLVM
 
 Native support for linking against LLVM using the `dependency` function.
+
+## vcs_tag keyword fallback is is now optional
+
+The `fallback` keyword in `vcs_tag` is now optional. If not given, its value
+defaults to the return value of `meson.project_version()`.

mesonbuild/interpreter.py

@@ -2024,9 +2024,9 @@ class Interpreter(InterpreterBase):
     def func_vcs_tag(self, node, args, kwargs):
         if 'input' not in kwargs or 'output' not in kwargs:
             raise InterpreterException('Keyword arguments input and output must exist')
-        fallback = kwargs.pop('fallback', None)
+        fallback = kwargs.pop('fallback', self.project_version)
         if not isinstance(fallback, str):
-            raise InterpreterException('Keyword argument fallback must exist and be a string.')
+            raise InterpreterException('Keyword argument fallback must be a string.')
         replace_string = kwargs.pop('replace_string', '@VCS_TAG@')
         regex_selector = '(.*)' # default regex selector for custom command: use complete output
         vcs_cmd = kwargs.get('command', None)

test cases/common/73 vcstag/meson.build

@@ -9,6 +9,10 @@ output : 'vcstag-custom.c',
 command : ['git', 'show-ref', '-s', 'refs/heads/master'],
 fallback : '1.0.0')
 
+version_src_fallback = vcs_tag(input : 'vcstag.c.in',
+output : 'vcstag-fallback.c')
+
 executable('tagprog', 'tagprog.c', version_src)
 executable('tagprog-custom', 'tagprog.c', version_src_custom)
+executable('tagprog-fallback', 'tagprog.c', version_src_fallback)