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<h3>
Marge Bot pushed to branch master
at <a href="https://gitlab.haskell.org/ghc/ghc">Glasgow Haskell Compiler / GHC</a>
</h3>
<h4>
Commits:
</h4>
<ul>
<li>
<strong><a href="https://gitlab.haskell.org/ghc/ghc/-/commit/a0d8e92e9c9b67426aa139d6bc46363d8940f992">a0d8e92e</a></strong>
<div>
<span>by Ryan Scott</span>
<i>at 2020-03-29T17:33:20-04:00</i>
</div>
<pre class="commit-message" style="white-space: pre-wrap; margin: 0;">Run checkNewDataCon before constraint-solving newtype constructors
Within `checkValidDataCon`, we used to run `checkValidType` on the
argument types of a newtype constructor before running
`checkNewDataCon`, which ensures that the user does not attempt
non-sensical things such as newtypes with multiple arguments or
constraints. This works out in most situations, but this falls over
on a corner case revealed in #17955:
```hs
newtype T = Coercible () T => T ()
```
`checkValidType`, among other things, peforms an ambiguity check on
the context of a data constructor, and that it turn invokes the
constraint solver. It turns out that there is a special case in the
constraint solver for representational equalities (read: `Coercible`
constraints) that causes newtypes to be unwrapped (see
`Note [Unwrap newtypes first]` in `TcCanonical`). This special case
does not know how to cope with an ill formed newtype like `T`, so
it ends up panicking.
The solution is surprisingly simple: just invoke `checkNewDataCon`
before `checkValidType` to ensure that the illicit newtype
constructor context is detected before the constraint solver can
run amok with it.
Fixes #17955.
</pre>
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<h4>4 changed files:</h4>
<ul>
<li class="file-stats">
<a href="#bdfa0cc139f61d1236abc1598c5d684b43c357f9">
compiler/typecheck/TcTyClsDecls.hs
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</li>
<li class="file-stats">
<a href="#4053234192fa36a93a1f485cf7758c95f7100954">
<span class="new-file">
+
testsuite/tests/typecheck/should_fail/T17955.hs
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</a>
</li>
<li class="file-stats">
<a href="#d2f3eb0369e995800c7e3cc3f5ea200114d2a063">
<span class="new-file">
+
testsuite/tests/typecheck/should_fail/T17955.stderr
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</li>
<li class="file-stats">
<a href="#b9e86d897a62509b0114d81d5e736630d6f68f43">
testsuite/tests/typecheck/should_fail/all.T
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</li>
</ul>
<h4>Changes:</h4>
<li id="bdfa0cc139f61d1236abc1598c5d684b43c357f9">
<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/a0d8e92e9c9b67426aa139d6bc46363d8940f992#bdfa0cc139f61d1236abc1598c5d684b43c357f9"><strong>compiler/typecheck/TcTyClsDecls.hs</strong></a>
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<td class="diff-line-num unfold js-unfold new_line" data-linenumber="3992" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>
<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -3992,9 +3992,6 @@ checkValidDataCon dflags existential_ok tc con</td>
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3992
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3992
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<pre style="margin: 0;"> <span id="LC3992" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Reason: it's really the argument of an equality constraint</span></span>
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3993
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3993
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<pre style="margin: 0;"> <span id="LC3993" class="line" lang="haskell"> <span class="p">;</span> <span class="n" style="color: #333;">checkValidMonoType</span> <span class="n" style="color: #333;">orig_res_ty</span></span>
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3994
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3994
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<pre style="margin: 0;"> <span id="LC3994" class="line" lang="haskell"></span>
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3995
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<pre style="margin: 0;">-<span id="LC3995" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Check all argument types for validity</span></span>
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3996
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<pre style="margin: 0;">-<span id="LC3996" class="line" lang="haskell"> <span class="p">;</span> <span class="n" style="color: #333;">checkValidType</span> <span class="n" style="color: #333;">ctxt</span> <span class="p">(</span><span class="n" style="color: #333;">dataConUserType</span> <span class="n" style="color: #333;">con</span><span class="p">)</span></span>
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3997
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<pre style="margin: 0;">-<span id="LC3997" class="line" lang="haskell"></span>
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3998
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3995
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<pre style="margin: 0;"> <span id="LC3995" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- If we are dealing with a newtype, we allow levity polymorphism</span></span>
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3999
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3996
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<pre style="margin: 0;"> <span id="LC3996" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- regardless of whether or not UnliftedNewtypes is enabled. A</span></span>
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4000
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3997
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<pre style="margin: 0;"> <span id="LC3997" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- later check in checkNewDataCon handles this, producing a</span></span>
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<td class="diff-line-num unfold js-unfold old_line" data-linenumber="4002" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>
<td class="diff-line-num unfold js-unfold new_line" data-linenumber="3999" style="width: 35px; color: rgba(0,0,0,0.3); border-right-width: 1px; border-right-color: #f0f0f0; border-right-style: solid; padding: 0 5px;" align="right" bgcolor="#fafafa">...</td>
<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -4002,9 +3999,16 @@ checkValidDataCon dflags existential_ok tc con</td>
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4002
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<pre style="margin: 0;"> <span id="LC3999" class="line" lang="haskell"> <span class="p">;</span> <span class="n" style="color: #333;">unless</span> <span class="p">(</span><span class="n" style="color: #333;">isNewTyCon</span> <span class="n" style="color: #333;">tc</span><span class="p">)</span></span>
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4003
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<pre style="margin: 0;"> <span id="LC4000" class="line" lang="haskell"> <span class="p">(</span><span class="n" style="color: #333;">mapM_</span> <span class="p">(</span><span class="n" style="color: #333;">checkForLevPoly</span> <span class="n" style="color: #333;">empty</span><span class="p">)</span> <span class="p">(</span><span class="n" style="color: #333;">dataConOrigArgTys</span> <span class="n" style="color: #333;">con</span><span class="p">))</span></span>
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4004
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4001
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<pre style="margin: 0;"> <span id="LC4001" class="line" lang="haskell"></span>
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4005
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<pre style="margin: 0;">-<span id="LC4005" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Extra checks for newtype data constructors</span></span>
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<pre style="margin: 0;">+<span id="LC4002" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Extra checks for newtype data constructors. Importantly, these</span></span>
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<pre style="margin: 0;">+<span id="LC4003" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- checks /must/ come before the call to checkValidType below. This</span></span>
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4004
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<pre style="margin: 0;">+<span id="LC4004" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- is because checkValidType invokes the constraint solver, and</span></span>
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4005
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<pre style="margin: 0;">+<span id="LC4005" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- invoking the solver on an ill formed newtype constructor can</span></span>
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4006
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<pre style="margin: 0;">+<span id="LC4006" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- confuse GHC to the point of panicking. See #17955 for an example.</span></span>
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<pre style="margin: 0;"> <span id="LC4007" class="line" lang="haskell"> <span class="p">;</span> <span class="n" style="color: #333;">when</span> <span class="p">(</span><span class="n" style="color: #333;">isNewTyCon</span> <span class="n" style="color: #333;">tc</span><span class="p">)</span> <span class="p">(</span><span class="n" style="color: #333;">checkNewDataCon</span> <span class="n" style="color: #333;">con</span><span class="p">)</span></span>
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<pre style="margin: 0;"> <span id="LC4008" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC4009" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Check all argument types for validity</span></span>
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<pre style="margin: 0;">+<span id="LC4010" class="line" lang="haskell"> <span class="p">;</span> <span class="n" style="color: #333;">checkValidType</span> <span class="n" style="color: #333;">ctxt</span> <span class="p">(</span><span class="n" style="color: #333;">dataConUserType</span> <span class="n" style="color: #333;">con</span><span class="p">)</span></span>
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<pre style="margin: 0;">+<span id="LC4011" class="line" lang="haskell"></span>
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<pre style="margin: 0;"> <span id="LC4012" class="line" lang="haskell"> <span class="c1" style="color: #998; font-style: italic;">-- Check that existentials are allowed if they are used</span></span>
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<pre style="margin: 0;"> <span id="LC4013" class="line" lang="haskell"> <span class="p">;</span> <span class="n" style="color: #333;">checkTc</span> <span class="p">(</span><span class="n" style="color: #333;">existential_ok</span> <span class="o" style="font-weight: 600;">||</span> <span class="n" style="color: #333;">isVanillaDataCon</span> <span class="n" style="color: #333;">con</span><span class="p">)</span></span>
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<pre style="margin: 0;"> <span id="LC4014" class="line" lang="haskell"> <span class="p">(</span><span class="n" style="color: #333;">badExistential</span> <span class="n" style="color: #333;">con</span><span class="p">)</span></span>
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<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/a0d8e92e9c9b67426aa139d6bc46363d8940f992#4053234192fa36a93a1f485cf7758c95f7100954"><strong>testsuite/tests/typecheck/should_fail/T17955.hs</strong></a>
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<pre style="margin: 0;">+<span id="LC1" class="line" lang="haskell"><span class="cp" style="color: #999; font-weight: 600;">{-# LANGUAGE FlexibleContexts #-}</span></span>
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<pre style="margin: 0;">+<span id="LC2" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">module</span> <span class="nn" style="color: #555;">T17955</span> <span class="kr" style="font-weight: 600;">where</span></span>
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<pre style="margin: 0;">+<span id="LC3" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC4" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">import</span> <span class="nn" style="color: #555;">Data.Coerce</span></span>
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<pre style="margin: 0;">+<span id="LC5" class="line" lang="haskell"></span>
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<pre style="margin: 0;">+<span id="LC6" class="line" lang="haskell"><span class="kr" style="font-weight: 600;">newtype</span> <span class="kt" style="color: #458; font-weight: 600;">T</span> <span class="o" style="font-weight: 600;">=</span> <span class="kt" style="color: #458; font-weight: 600;">Coercible</span> <span class="nb" style="color: #0086b3;">()</span> <span class="kt" style="color: #458; font-weight: 600;">T</span> <span class="o" style="font-weight: 600;">=></span> <span class="kt" style="color: #458; font-weight: 600;">T</span> <span class="nb" style="color: #0086b3;">()</span></span></pre>
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<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/a0d8e92e9c9b67426aa139d6bc46363d8940f992#d2f3eb0369e995800c7e3cc3f5ea200114d2a063"><strong>testsuite/tests/typecheck/should_fail/T17955.stderr</strong></a>
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<pre style="margin: 0;">+<span id="LC2" class="line" lang="plaintext">T17955.hs:6:13: error:</span>
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<pre style="margin: 0;">+<span id="LC3" class="line" lang="plaintext"> • A newtype constructor cannot have a context in its type</span>
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<pre style="margin: 0;">+<span id="LC4" class="line" lang="plaintext"> T :: Coercible () T => () -> T</span>
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<pre style="margin: 0;">+<span id="LC5" class="line" lang="plaintext"> • In the definition of data constructor ‘T’</span>
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<pre style="margin: 0;">+<span id="LC6" class="line" lang="plaintext"> In the newtype declaration for ‘T’</span></pre>
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<a href="https://gitlab.haskell.org/ghc/ghc/-/commit/a0d8e92e9c9b67426aa139d6bc46363d8940f992#b9e86d897a62509b0114d81d5e736630d6f68f43"><strong>testsuite/tests/typecheck/should_fail/all.T</strong></a>
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<td class="line_content match " style="padding-left: 0.5em; padding-right: 0.5em; color: rgba(0,0,0,0.3);" bgcolor="#fafafa">@@ -561,3 +561,4 @@ test('T17566c', normal, compile_fail, [''])</td>
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561
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<pre style="margin: 0;"> <span id="LC561" class="line" lang="perl"><span class="nv" style="color: #008080;">test</span><span class="p">('</span><span class="s1" style="color: #d14;">T17773</span><span class="p">',</span> <span class="nv" style="color: #008080;">normal</span><span class="p">,</span> <span class="nv" style="color: #008080;">compile_fail</span><span class="p">,</span> <span class="p">[''])</span></span>
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<pre style="margin: 0;"> <span id="LC562" class="line" lang="perl"><span class="nv" style="color: #008080;">test</span><span class="p">('</span><span class="s1" style="color: #d14;">T17021</span><span class="p">',</span> <span class="nv" style="color: #008080;">normal</span><span class="p">,</span> <span class="nv" style="color: #008080;">compile_fail</span><span class="p">,</span> <span class="p">[''])</span></span>
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563
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563
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<pre style="margin: 0;"> <span id="LC563" class="line" lang="perl"><span class="nv" style="color: #008080;">test</span><span class="p">('</span><span class="s1" style="color: #d14;">T17021b</span><span class="p">',</span> <span class="nv" style="color: #008080;">normal</span><span class="p">,</span> <span class="nv" style="color: #008080;">compile_fail</span><span class="p">,</span> <span class="p">[''])</span></span>
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564
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<pre style="margin: 0;">+<span id="LC564" class="line" lang="perl"><span class="nv" style="color: #008080;">test</span><span class="p">('</span><span class="s1" style="color: #d14;">T17955</span><span class="p">',</span> <span class="nv" style="color: #008080;">normal</span><span class="p">,</span> <span class="nv" style="color: #008080;">compile_fail</span><span class="p">,</span> <span class="p">[''])</span></span></pre>
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