Fix an assertion exception when misusing install_data

* Failing test case for trying to install_data a custom_target

* Validate install_data() arguments are either string or file

mesonbuild/interpreter.py

@@ -3421,8 +3421,10 @@ This will become a hard error in the future.''' % kwargs['input'], location=self
         for s in raw_sources:
             if isinstance(s, mesonlib.File):
                 sources.append(s)
-            else:
+            elif isinstance(s, str):
                 source_strings.append(s)
+            else:
+                raise InvalidArguments('Argument {!r} must be string or file.'.format(s))
         sources += self.source_strings_to_files(source_strings)
         install_dir = kwargs.get('install_dir', None)
         if not isinstance(install_dir, (str, type(None))):

test cases/failing/97 custom target install data/Info.plist.cpp

@@ -0,0 +1 @@
+Some data which gets processed before installation

test cases/failing/97 custom target install data/meson.build

@@ -0,0 +1,11 @@
+project('custom target install data')
+
+preproc = find_program('preproc.py')
+
+t = custom_target('Info.plist',
+    command: [preproc, '@INPUT@', '@OUTPUT@'],
+    input: 'Info.plist.cpp',
+    output: 'Info.plist',
+)
+
+install_data(t)

test cases/failing/97 custom target install data/preproc.py

@@ -0,0 +1,13 @@
+#!/usr/bin/env python3
+
+import sys
+
+if len(sys.argv) != 3:
+    print(sys.argv[0], '<input>', '<output>')
+
+inf = sys.argv[1]
+outf = sys.argv[2]
+
+with open(outf, 'wb') as o:
+    with open(inf, 'rb') as i:
+        o.write(i.read())