Fix BSV parsing of naked expressions

in statement and case-expression contexts (Issue 646)

src/comp/Parser/BSV/CVParser.lhs

@@ -2261,7 +2261,12 @@ if both fail, the error messages will come from the more likely alternative.
 >                       pImperativeLet decl_atts bind_atts flags pos)
 >                 <|> (pKeyword SV_KW_match >> noAttrs_ >>
 >                       pImperativePatternDecl flags)
->                 <|> try pVarTypeCases
+>                 <|> try (do
+>                      -- require a match of the whole statement, to avoid parsing just
+>                      -- the start of a naked expression that begins with a variable
+>                      s <- pVarTypeCases
+>                      lookAhead pSemi
+>                      return s)
 >                 <|> try (do
 >                      when (not $ allowNakedExpr flags) (fail "naked expression")
 >                      e <- pExpression
@@ -3391,6 +3396,8 @@ The same, but reads an output port name instead of a type
 >        ((pImperativeWithCallRegWrite flags pos primy >>= noBindAttrs)
 >         <|> (pImperativeWithCallEq flags primy >>= noBindAttrs)
 >         <|> pImperativeWithCallBind bind_atts flags primy
+>         -- Naked expressions are accepted here, so that a helpful error
+>         -- for that situation can be given (EForbiddenNakedExprInExprBlock).
 >         <|> (pImperativeWithExprNakedExpr flags pos primy >>= noBindAttrs))
 > pImperativeWithVarCall bind_attrs flags vars =
 >     failWithErr (getIdOrTuplePosition vars, EForbiddenTuple)

testsuite/bsc.syntax/bsv05/NakedExpr_CaseExpr.bsv

@@ -0,0 +1,14 @@
+typedef enum {Foo, Bar, Foobar} FooNum deriving(Bits, Eq);
+
+module mkCase(Empty);
+  Reg#(int) r <- mkReg(0);
+  Reg#(FooNum) r_foo <- mkReg(Foo);
+
+  rule change_foo;
+    r_foo <= case (r_foo) matches
+                Foo: Bar;
+                Bar: r == 42 ? Foobar : Bar;
+                Foobar: Foobar;
+             endcase;
+  endrule
+endmodule

testsuite/bsc.syntax/bsv05/NakedExpr_Stmt.bsv

@@ -0,0 +1,11 @@
+module mkTest (Empty);
+  int x = 1;
+  int y = 2;
+  x == y ? mkSub1 : mkSub2;
+endmodule
+
+module mkSub1 (Empty);
+endmodule
+
+module mkSub2 (Empty);
+endmodule

testsuite/bsc.syntax/bsv05/bsv05.exp

@@ -402,7 +402,6 @@ compile_pass ImperativeVariableBind_NewLetType.bsv
 
 # -----
 
-
 # Tests for decoding UTF8 and erroring out on non-UTF8 files
 compile_pass UTF8Code.bsv
 compile_pass UTF8LineComment.bsv
@@ -422,3 +421,21 @@ compile_fail_error UTF8BadCons1.bsv P0005
 compile_fail_error UTF8BadCons2.bsv P0005
 compile_pass 風呂敷.bsv
 
+# -----
+
+# GitHub Issue #646
+#
+# In a statement context that allows naked expressions,
+# the parser should accept the full expression and not
+# stop early on a prefix that also happens to be an
+# expression.  For example, "x == y ? e1 : e2" should
+# parse and not stop early on "x" or "x == y".
+
+compile_pass NakedExpr_Stmt.bsv
+
+# The arms of case expressions are parsed as statements
+# allowing naked expression, so test there, too
+#
+compile_pass NakedExpr_CaseExpr.bsv
+
+# ---------------