Fix multiline display equations in LaTeX

This is an identical bugfix fo #1518, which was only applied to the
HTML writer.

docs/src/man/latex.md

@@ -115,3 +115,25 @@ To write a system of equations, use the `aligned` environment:
 ```
 
 These are Maxwell's equations.
+
+## Printing LaTeX from Julia
+
+To pretty-print LaTeX from Julia, overload `Base.show` for the
+`MIME"text/latex"` type. For example:
+```@example
+struct LaTeXEquation
+    content::String
+end
+
+function Base.show(io::IO, ::MIME"text/latex", x::LaTeXEquation)
+    # Wrap in $$ for display math printing
+    return print(io, "\$\$ " * x.content * " \$\$")
+end
+
+LaTeXEquation(raw"""
+    \left[\begin{array}{c}
+        x \\
+        y
+    \end{array}\right]
+""")
+```

src/Writers/HTMLWriter.jl

@@ -2010,6 +2010,8 @@ function mdconvert(d::Dict{MIME,Any}, parent; kwargs...)
         # If the show(io, ::MIME"text/latex", x) output is already wrapped in \[ ... \] or $$ ... $$, we
         # unwrap it first, since when we output Markdown.LaTeX objects we put the correct
         # delimiters around it anyway.
+        #
+        # This bug also occurs in the LaTeXWriter.
         latex = d[MIME"text/latex"()]
         # Make sure to match multiline strings!
         m_bracket = match(r"\s*\\\[(.*)\\\]\s*"s, latex)

src/Writers/LaTeXWriter.jl

@@ -398,7 +398,21 @@ function latex(io::IO, d::Dict{MIME,Any})
         \\end{figure}
         """)
     elseif haskey(d, MIME"text/latex"())
-        latex(io, Utilities.mdparse(d[MIME"text/latex"()]; mode = :single))
+        # If the show(io, ::MIME"text/latex", x) output is already wrapped in
+        # \[ ... \] or $$ ... $$, we unwrap it first, since when we output
+        # Markdown.LaTeX objects we put the correct delimiters around it anyway.
+        #
+        # This bug also occurs in the HTMLWriter.
+        latex_str = d[MIME"text/latex"()]
+        # Make sure to match multiline strings!
+        m_bracket = match(r"\s*\\\[(.*)\\\]\s*"s, latex_str)
+        m_dollars = match(r"\s*\$\$(.*)\$\$\s*"s, latex_str)
+        if m_bracket === nothing && m_dollars === nothing
+            latex(io, Utilities.mdparse(latex_str; mode = :single))
+        else
+            out = m_bracket !== nothing ? m_bracket[1] : m_dollars[1]
+            latex(io, Markdown.LaTeX(out))
+        end
     elseif haskey(d, MIME"text/markdown"())
         latex(io, Markdown.parse(d[MIME"text/markdown"()]))
     elseif haskey(d, MIME"text/plain"())