Only add '\\usepackage[utf8]{inputenc}' when using pdflatex

doc/config.rst

@@ -1618,8 +1618,13 @@ These options influence LaTeX output.
    * Keys that don't need be overridden unless in special cases are:
 
      ``'inputenc'``
-        "inputenc" package inclusion, default
-        ``'\\usepackage[utf8]{inputenc}'``.
+        "inputenc" package inclusion, defaults to
+        ``'\\usepackage[utf8]{inputenc}'`` when using pdflatex.
+        Otherwise unset.
+
+        .. versionchanged:: 1.5
+           Previously ``'\\usepackage[utf8]{inputenc}'`` was used for all
+           compilers.
      ``'cmappkg'``
         "cmap" package inclusion, default ``'\\usepackage{cmap}'``.
 

sphinx/writers/latex.py

@@ -34,6 +34,7 @@ from sphinx.util.smartypants import educate_quotes_latex
 HEADER = r'''%% Generated by Sphinx.
 \def\sphinxdocclass{%(docclass)s}
 \documentclass[%(papersize)s,%(pointsize)s%(classoptions)s]{%(wrapperclass)s}
+\usepackage{iftex}
 %(passoptionstopackages)s
 %(inputenc)s
 %(utf8extra)s
@@ -295,7 +296,9 @@ class LaTeXTranslator(nodes.NodeVisitor):
         'classoptions':    '',
         'extraclassoptions': '',
         'passoptionstopackages': '',
-        'inputenc':        '\\usepackage[utf8]{inputenc}',
+        'inputenc':        ('\\ifPDFTeX\n'
+                            '  \\usepackage[utf8]{inputenc}\n'
+                            '\\else\\fi'),
         'utf8extra':       ('\\ifdefined\\DeclareUnicodeCharacter\n'
                             '  \\DeclareUnicodeCharacter{00A0}{\\nobreakspace}\n'
                             '\\else\\fi'),