Fix InexactError in peek_behind_pos when skipping nested trivia nodes (#604)

Author: KristofferC

src/core/parse_stream.jl

@@ -635,8 +635,14 @@ function peek_behind_pos(stream::ParseStream; skip_trivia::Bool=true,
     while node_idx > 0
         node = stream.output[node_idx]
         if kind(node) == K"TOMBSTONE" || (skip_trivia && is_trivia(node))
-            node_idx -= 1
             byte_idx -= node.byte_span
+            # If this is a non-terminal node, skip its children without
+            # subtracting their byte_spans, as they're already included in the parent
+            if is_non_terminal(node)
+                node_idx -= (1 + node.node_span)
+            else
+                node_idx -= 1
+            end
         else
             break
         end

test/parse_stream.jl

@@ -156,3 +156,13 @@ end
     @test ParseStream(y) isa ParseStream
     @test parsestmt(Expr, y) == parsestmt(Expr, "1")
 end
+
+@testset "peek_behind_pos with negative byte index" begin
+    # Test that peek_behind_pos doesn't cause InexactError when byte_idx goes negative
+    # This can happen when parsing certain incomplete keywords like "do"
+    # where trivia skipping walks back past the beginning of the stream
+    @test_throws JuliaSyntax.ParseError parseall(GreenNode, "do")
+    @test_throws JuliaSyntax.ParseError parseall(GreenNode, "do ")
+    @test_throws JuliaSyntax.ParseError parseall(GreenNode, " do")
+    @test_throws JuliaSyntax.ParseError parseall(GreenNode, "do\n")
+end