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00e1a62ab4
compiler/det_report.m:
In some cases described in the new test case, the error message
we print for a missing switch inside an arm of a larger switch
is misleading unless we print *which* arm of the larger switch
we are complaining about, so print that information.
tests/invalid/require_complete_nested_switch.{m,err_exp}:
A test case derived from the real-life error that motivated this diff.
tests/invalid/Mmakefile:
Enable the new test case.
113 lines
3.8 KiB
Mathematica
113 lines
3.8 KiB
Mathematica
%---------------------------------------------------------------------------%
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% vim: ft=mercury ts=4 sw=4 et
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%---------------------------------------------------------------------------%
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%
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% This is a regression test. Versions of the compiler before 2023 april 19
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% generated a quite misleading error message for the bug in test_pred.
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%
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% The bug is that the switch on Str has an arm that is explicitly for
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% "Str = k", but this test is *also* present in the first switch arm,
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% which is now for a, b and k. (This can come about naturally when
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%
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% - k used to be treated the same way as a and b;
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% - the programmer decides to give k its own switch arm, but
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% - then forgets to delete the "Str = k" from its original switch arm.
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%
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% (That is exactly what happened with the original code that inspired
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% this cut-down test case.)
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%
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% The original version of the compiler generated two errors for this bug:
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%
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% Warning: this disjunct will never have any solutions.
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% Error: the goal inside the require_complete_switch [Str] scope is
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% not a switch on `Str'.
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%
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% The contexts were the position of the first Str = k in the code
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% (the line marked below as THE PROBLEM LINE), and the position of the
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% first require_complete_switch scope respectively. These are confusing
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% because
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%
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% - there is not supposed to be a disjunct at THE PROBLEM LINE, since
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% switch detection is supposed to transform that disjunct into a switch arm,
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% and
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% - the body of the require_complete_switch [Str] scope IS in fact a switch
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% on Str.
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%
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% The reason why these error messages are misleading is that despite
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% the contexts, they do not refer to that piece of code at all. They refer
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% to a *copy* of that code. When switch detection sees that there are
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% *two* switch arms for Str =k, it creates a separate arm of the switch
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% on Str for the Str = k case, and sets the code of this arm to be
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% a two-way disjunction containing the code that came after the two
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% occurrences of Str = k. This meant that the second disjunct was
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% "Msg = arg_k", and the first disjunct was this:
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%
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% BaseMsg = "arm_ab_",
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% require_complete_switch [Str]
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% (
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% Str = "a",
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% Msg = BaseMsg ++ "arm_a"
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% ;
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% Str = "b",
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% Msg = BaseMsg ++ "arm_b"
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% )
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%
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% The compiler recognized that *none* of those two switch arms of this *inner*
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% switch is actually reachable in the Str = k branch of the *outer* switch
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% on Str, so it replaced this switch with fail, which meant that this disjunct
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% couldn't succeed (hence the first error message). The fact that it replaced
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% the switch with fail but kept the require_complete_switch scope led to the
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% second error message.
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%
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% So both the error messages the compiler generates are justified, including
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% the line numbers attached to them. What made them misleading was the lack
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% of any reference to the fact that the error occurs only inside the Str = k
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% arm of the outer switch. The fix for that is to print this context.
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:- module require_complete_nested_switch.
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:- interface.
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:- import_module io.
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:- pred main(io::di, io::uo) is det.
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:- implementation.
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:- import_module list.
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:- import_module string.
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main(!IO) :-
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( if test_pred("abc", Msg) then
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io.format("Success: %s\n", [s(Msg)], !IO)
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else
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io.format("Failure\n", [], !IO)
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).
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:- pred test_pred(string::in, string::out) is semidet.
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test_pred(Str, Msg) :-
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(
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( Str = "a"
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; Str = "b"
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; Str = "k" % THE PROBLEM LINE
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),
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BaseMsg = "arm_ab_",
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require_complete_switch [Str]
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(
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Str = "a",
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Msg = BaseMsg ++ "arm_a"
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;
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Str = "b",
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Msg = BaseMsg ++ "arm_b"
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)
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;
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Str = "k",
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Msg = "arg_k"
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;
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( Str = "x"
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; Str = "y"
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),
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Msg = "arg_x_y"
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).
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