fix #20575, syntax error for juxtaposing a string literal

Author: JeffBezanson

NEWS.md

@@ -17,6 +17,8 @@ This section lists changes that do not have deprecation warnings.
   * `ntuple(f, n::Integer)` throws `ArgumentError` if `n` is negative.
     Previously an empty tuple was returned ([#21697]).
 
+  * Juxtaposing string literals (e.g. `"x"y`) is now a syntax error ([#20575]).
+
   * `@__DIR__` returns the current working directory rather than `nothing` when not run
     from a file ([#21759]).
 

src/julia-parser.scm

@@ -876,9 +876,11 @@
            )
        (not (ts:space? s))
        (not (operator? t))
-       (not (initial-reserved-word? t))
        (not (closing-token? t))
        (not (newline? t))
+       (or (not (string? expr))  ;; issue #20575
+           (error "cannot juxtapose string literal"))
+       (not (initial-reserved-word? t))
        (not (and (pair? expr) (syntactic-unary-op? (car expr))))
        ;; TODO: this would disallow juxtaposition with 0, which is ambiguous
        ;; with e.g. hex literals `0x...`. however this is used for `0im`, which

test/parse.jl

@@ -1149,6 +1149,10 @@ f21586(; @m21586(a), @m21586(b)) = a + b
 end
 @test Test21604.X(1.0) === Test21604.X(1.0)
 
+# issue #20575
+@test_throws ParseError parse("\"a\"x")
+@test_throws ParseError parse("\"a\"begin end")
+
 # comment 298107224 on pull #21607
 module Test21607
     using Base.Test