handle "part_of_explicit_cast" + "cannot overflow"

[tgulacsi]

And better panic message (use %q to separate regexp from line).

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This change is 

[codecov]

Codecov Report

Merging #818 into master will increase coverage by <.01%.
The diff coverage is 88.88%.

@@            Coverage Diff            @@
##           master    #818      +/-   ##
=========================================
+ Coverage    79.9%   79.9%   +<.01%     
=========================================
  Files         169     169              
  Lines        9793    9794       +1     
=========================================
+ Hits         7825    7826       +1     
  Misses       1503    1503              
  Partials      465     465

Impacted Files	Coverage Δ
ast/ast.go	98.34% <0%> (ø)	⬆️
ast/implicit_cast_expr.go	100% <100%> (ø)	⬆️
ast/unary_operator.go	100% <100%> (ø)	⬆️

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[elliotchance]

Reviewed 3 of 3 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @tgulacsi)

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ast/implicit_cast_expr.go, line 21 at r1 (raw file):

		 '(?P<type>[^']*?)'
		(:'(?P<type2>[^']*?)')?
		 <(?P<kind>[^>]*)>( part_of_explicit_cast)?`,

Did you hit a panic for these cases? It would be great if you could put those panics as unit tests.