Handle infix operators in REPL completion (#51366)

Fix #51194

This PR fixes a regression introduced in
#49294, so I believe it should be
backported to v1.10.
In the current code, completion of `qux(foo, bar.` is detected by
parsing `foo(qux, bar` as an incomplete expression, and then looking for
the sub-expression to complete (here, `bar.`). This approach fails
however for infix calls, since completing `foo + bar.` starts by parsing
`foo + bar`, which is a complete call expression, and so the code
behaves as if completing `(foo + bar).` instead of `bar.`. This leads to
the current problematic behaviour:
```julia
julia> Complex(1, 3) + (4//5).#TAB
im
re
```
which would be correct for `(Complex(1, 3) + (4//5)).#TAB`, but here we
expect
```julia
julia> Complex(1, 3) + (4//5).#TAB
den
num
```

This PR fixes that by trying to detect infix calls. In the long term,
all this ad-hoc and probably somewhat wrong string processing should be
replaced by proper use of `JuliaSyntax` (as mentioned in
#49294 (comment),
#50817 (comment)
and probably other places), but for now at least this fixes the
regression.

stdlib/REPL/src/REPLCompletions.jl

@@ -1038,6 +1038,17 @@ function complete_identifiers!(suggestions::Vector{Completion}, @nospecialize(ff
                 ex = Meta.parse(lookup_name, raise=false, depwarn=false)
             end
             isexpr(ex, :incomplete) && (ex = nothing)
+        elseif isexpr(ex, :call) && length(ex.args) > 1
+            isinfix = s[end] != ')'
+            # A complete call expression that does not finish with ')' is an infix call.
+            if !isinfix
+                # Handle infix call argument completion of the form bar + foo(qux).
+                frange, end_of_identifier = find_start_brace(@view s[1:prevind(s, end)])
+                isinfix = Meta.parse(@view(s[frange[1]:end]), raise=false, depwarn=false) == ex.args[end]
+            end
+            if isinfix
+                ex = ex.args[end]
+            end
         end
     end
     append!(suggestions, complete_symbol(ex, name, ffunc, context_module))

stdlib/REPL/test/replcompletions.jl

@@ -25,6 +25,7 @@ let ex = quote
         (::Test_y)() = "", ""
         unicode_αβγ = Test_y(1)
 
+        Base.:(+)(x::Test_x, y::Test_y) = Test_x(Test_y(x.xx.yy + y.yy))
         module CompletionFoo2
 
         end
@@ -2069,3 +2070,17 @@ end
     # If this last test starts failing, that's okay, just pick a new example symbol:
     @test !Base.isexported(Base, :ispublic)
 end
+
+# issue #51194
+for (s, compl) in (("2*CompletionFoo.nam", "named"),
+                   (":a isa CompletionFoo.test!1", "test!12"),
+                   ("-CompletionFoo.Test_y(3).", "yy"),
+                   ("99 ⨷⁻ᵨ⁷ CompletionFoo.type_test.", "xx"),
+                   ("CompletionFoo.type_test + CompletionFoo.Test_y(2).", "yy"),
+                   ("(CompletionFoo.type_test + CompletionFoo.Test_y(2)).", "xx"),
+                   ("CompletionFoo.type_test + CompletionFoo.unicode_αβγ.", "yy"),
+                   ("(CompletionFoo.type_test + CompletionFoo.unicode_αβγ).", "xx"),
+                   ("foo'CompletionFoo.test!1", "test!12"))
+    c, r = test_complete(s)
+    @test only(c) == compl
+end