Add opportunistic warnings for shadowed map patterns #8600
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The compiler can emit so-called *opportunistic warnings*, which are
warnings emitted when the compiler notices something suspicious during
optimization or code generation. The trouble with opportunistic
warnings is that there is no guarantee that the compiler will notice
all kind of suspicious code. For example, the compiler in Erlang/OTP
27 and earlier would not emit any warnings for the following
functions:
mm_1(#{}) ->
a;
mm_1(#{first := First}) ->
{b,First}.
mm_2(#{first := First}) ->
{b,First};
mm_2(#{first := First, second := Second}) ->
{c,First,Second}.
Since it is easy to arrange clauses in the wrong order and put the
matching for any map before other map patterns, it seems worthwhile
to catch at least some of the map patterns that will shadow subsequent
patterns.
With this commit, the compiler will generate the following warnings:
t.erl:6:1: Warning: this clause cannot match because a previous clause at line 4 matches the same pattern as this clause
% 6| mm_1(#{first := First}) ->
% | ^
t.erl:11:1: Warning: this clause cannot match because a previous clause at line 9 matches the same pattern as this clause
% 11| mm_2(#{first := First, second := Second}) ->
% | ^
Note that these warnings are still opportunistic and there is no
guarantee that the compiler will notice all kind of shadowed map
patterns.
Resolves erlang#8558
bjorng
added
team:VM
CT Test Results 2 files 324 suites 10m 17s ⏱️ Results for commit 8a8ea04. ♻️ This comment has been updated with latest results. To speed up review, make sure that you have read Contributing to Erlang/OTP and that all checks pass. See the TESTING and DEVELOPMENT HowTo guides for details about how to run test locally. Artifacts// Erlang/OTP Github Action Bot |
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Just one more time here, thanks a lot for taking the time to implement this. It's a great improvement! 💚
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The compiler can emit so-called opportunistic warnings, which are warnings emitted when the compiler notices something suspicious during optimization or code generation. The trouble with opportunistic warnings is that there is no guarantee that the compiler will notice all kind of suspicious code. For example, the compiler in Erlang/OTP 27 and earlier would not emit any warnings for the following functions:
Since it is easy to arrange clauses in the wrong order and put the matching for any map before other map patterns, it seems worthwhile to catch at least some of the map patterns that will shadow subsequent patterns.
With this pull request, the compiler will generate the following warnings:
Note that these warnings are still opportunistic and there is no guarantee that the compiler will notice all kind of shadowed map patterns.
Resolves #8558