The use of the word "may" suggests that an implementation isn't required to diagnose +; if it's in a template, but this is not true. Diagnostics are always required for syntax errors.

• +; is not a valid statement; therefore
• { +; } is not a valid compound-statement and not a valid function-body; therefore
• void g() { +; } is not a valid function-definition and not a valid member-declaration; therefore
• the definition of X is not a valid template-declaration and not an element of the declaration-seq of the TU; therefore
• the entire translation-unit is invalid

As you can see, a syntax error inevitably "bubbles upwards" in the grammar, and also means that any containing template isn't a template in the first place. If there is no containing template, and there isn't even a valid translation-unit, then diagnostics are required because syntax errors fall under the umbrella of diagnosable rules.

Unless "no diagnostic required" was explicitly stated, the following applies:

[...] if a program contains a violation of any diagnosable rule or an occurrence of a construct described in this document as “conditionally-supported” when the implementation does not support that construct, a conforming implementation shall issue at least one diagnostic message.

- [intro.compliance.general] p2.3