remove unnecessary true && part from at-view macro (#53064)

The modification that expands `@view A[i]` to `true && view(A, i)`
appears to go back as far as #20247. However, I'm not entirely sure why
this is necessary. Considering that just expanding it to `view(A, i)`
still seems to pass the base test suite, I wonder if it might be just
better to get rid of that part.

base/views.jl

@@ -123,20 +123,21 @@ julia> A
 ```
 """
 macro view(ex)
+    Meta.isexpr(ex, :ref) || throw(ArgumentError(
+        "Invalid use of @view macro: argument must be a reference expression A[...]."))
+    ex = replace_ref_begin_end!(ex)
+    # NOTE We embed `view` as a function object itself directly into the AST.
+    #      By doing this, we prevent the creation of function definitions like
+    #      `view(A, idx) = xxx` in cases such as `@view(A[idx]) = xxx.`
     if Meta.isexpr(ex, :ref)
-        ex = replace_ref_begin_end!(ex)
-        if Meta.isexpr(ex, :ref)
-            ex = Expr(:call, view, ex.args...)
-        else # ex replaced by let ...; foo[...]; end
-            if !(Meta.isexpr(ex, :let) && Meta.isexpr(ex.args[2], :ref))
-                error("invalid expression")
-            end
-            ex.args[2] = Expr(:call, view, ex.args[2].args...)
-        end
-        Expr(:&&, true, esc(ex))
+        ex = Expr(:call, view, ex.args...)
+    elseif Meta.isexpr(ex, :let) && (arg2 = ex.args[2]; Meta.isexpr(arg2, :ref))
+        # ex replaced by let ...; foo[...]; end
+        ex.args[2] = Expr(:call, view, arg2.args...)
     else
-        throw(ArgumentError("Invalid use of @view macro: argument must be a reference expression A[...]."))
+        error("invalid expression")
     end
+    return esc(ex)
 end
 
 ############################################################################

test/subarray.jl

@@ -937,3 +937,21 @@ end
     v = view(1:2, r)
     @test v == view(1:2, collect(r))
 end
+
+# https://github.com/JuliaLang/julia/pull/53064
+# `@view(A[idx]) = xxx` should raise syntax error always
+@test try
+    Core.eval(@__MODULE__, :(@view(A[idx]) = 2))
+    false
+catch err
+    err isa ErrorException && startswith(err.msg, "syntax:")
+end
+module Issue53064
+import Base: view
+end
+@test try
+    Core.eval(Issue53064, :(@view(A[idx]) = 2))
+    false
+catch err
+    err isa ErrorException && startswith(err.msg, "syntax:")
+end